Global patterns and drivers of nitrogen isotope signal in modern marine sediments

Domain: earth_science
Published: 2025-11-25
Editorial outcome: major_revision

Abstract
Marine nitrogen cycle, shaped by nitrogen input and loss processes, regulates ocean productivity and leaves a distinct imprint on the ratio of stable nitrogen isotopes, δ15N. However, quantifying spatial patterns and centennial-scale variations in marine nitrogen cycling through δ15N fingerprints remains poorly constrained. Here, we present a global database of δ15N in marine sediments, comprising over 8400 seafloor samples, and create 148 time series from sediment cores spanning the past century. Our findings reveal that bulk δ15N in global marine sediments predominantly records organic matter, despite the non-negligible role of inorganic nitrogen. A widespread centennial δ15N increase, averaging 1.3‰ (interquartile range: 0.6–3.3‰) per 100 years, coinciding with total nitrogen enrichment, indicates external nitrogen inputs, productivity, and accelerated denitrification across global coastal oceans. However, in river-influenced continental margins, terrestrial sedimentary processes can obscure the 15N-enriched signal, as riverine export of 15N-depleted refractory organic matter likely lowers sedimentary δ15N.

Introduction
The nitrogen (N) cycle is essential to ocean biogeochemistry1. The limited availability of nitrogen in bioavailable forms, primarily as nitrate, poses a significant nutritional constraint on phytoplankton growth, thereby influencing autotrophic carbon dioxide (CO2) fixation and marine productivity2,3. Over geological timescales, these interactions have modulated atmospheric CO2 levels and global climate patterns4–6. However, the natural marine nitrogen cycle has been profoundly altered during the Anthropocene, a period marked by escalating human impacts such as industrial agriculture, fossil fuel combustion, and land-use changes7–9. Global riverine nitrogen export to coastal systems roughly doubled from ~20 Tg N yr−1 to ~42 Tg N yr−1 between 1900 and 201010,11. The human-induced fraction increased from ~26% to ~43%12, primarily driven by agricultural intensification, land-use change, and sewage input12,13. This excess nitrogen has substantially altered coastal ecosystem structure and function, driving shifts in community composition (e.g., macrophytes to fast-growing algae, diatoms to flagellates), reducing water quality, depleting oxygen, and fostering harmful algal blooms and eutrophication3,14–17. Paradoxically, river damming and sediment retention have simultaneously reduced the flux of fluvial particulates, including nitrogen, to global coastal oceans by 49% between the 1950s and 2010s9. Although eutrophication and deoxygenation can enhance carbon preservation, long-term sedimentary carbon burial is closely tied to sediment delivery18. A decline in global fluvial particulate input reduces marine sediment accumulation9, increases oxygen exposure and organic matter remineralization19,20, and consequently diminishes sedimented carbon storage, potentially enhancing CO2 emissions to the atmosphere and generating negative climate feedbacks. These dual pressures underscore the urgent need to unravel how marine nitrogen cycling is adapting to anthropogenic perturbations and to assess the cascading effects on marine biodiversity, carbon storage, and global climate feedbacks.

Marine biogeochemical processes fractionate nitrogen isotopes (15N to 14N), generating measurable differences in the 15N/14N ratio, expressed as δ15N ([(15N/14N)sample / (15N/14N)ref –1] × 1,000‰)21. These differences arise from the production of nitrogen-bearing organic matter in the surface ocean and its subsequent burial in sediments. Nitrogen fixation produces nitrate with slightly depleted δ15N values (−4‰ to 0‰) due to minor negative isotopic fractionation relative to atmospheric N2 5,22. In contrast, denitrification and anammox processes preferentially release isotopically light 14N into the atmosphere21,23, leaving behind a 15N-enriched oceanic nitrate pool with δ15N elevated by 10–25‰5. As a result, δ15N of sinking organic matter is preserved in marine sediments, providing critical insights into past nutrient utilization, atmospheric CO2 concentration, and nitrogen source dynamics4,6,23–25. However, traditional paleoceanographic archives from deep-sea sediment cores—often characterized by low sedimentation rates—primarily resolve glacial–interglacial or millennial-scale variability, offering limited temporal resolution for studying Anthropocene-era changes confined to surface sediment layers4,6,25–28. Consequently, global reconstructions of decadal to centennial δ15N shifts in modern marine sediments remain sparse, hindering our ability to assess how anthropogenic perturbations are altering nitrogen cycling in today’s rapidly changing oceans.

Recent studies reveal that primary nitrogen isotopic signals related to surface ocean productivity are often obfuscated in bulk sedimentary δ15N of organic matter29,30. This obfuscation may arise from early diagenesis, such as organic matter remineralization during sinking and burial31,32, and contributions from both organic and inorganic nitrogen (IN) derived from terrestrial or distal marine sources, particularly in continental margins33–35. These processes modify the nitrogen signal exported to and preserved in sediments, biasing interpretations of past nitrogen utilization, budgets, and cycling dynamics. The complexity of marine nitrogen sources and cycling complicates the use of bulk δ15N as a robust tracer of modern global nitrogen biogeochemical cycles.

In this study, we conducted a global-scale analysis of seafloor sediments, expanding pioneering observations to include modern sediment records from the past century. Our database comprises over 8400 globally distributed sediment samples—including 3833 surface sediments and 189 radiometrically dated cores—collected from latitudes 69°S to 90°N and depths <10m to >7000m, encompassing coastal to abyssal zones (Supplementary Table 1). We incorporated data on inorganic and organic nitrogen to identify the sources of sedimentary total nitrogen and the drivers of spatial variability in bulk δ15N. Subsequently, we constructed 148 δ15N and 120 total nitrogen time series from dated marine sediment cores to investigate their responses to environmental perturbations. To disentangle the confounding effects of heterogeneous terrigenous inputs, sedimentation dynamics, and biological production, we prioritize the East China Sea as a representative continental shelf system (Supplementary Fig. 1). Our investigation aims to address three key questions: (i) What spatiotemporal patterns characterize sedimentary δ15N in modern oceans over the past century? (ii) How have anthropogenic forcing and climate change reshaped marine nitrogen cycling, as evidenced by sedimentary δ15N trends? (iii) What mechanisms govern sedimentary δ15N shifts across diverse marine settings? Here, we present the global assessment of centennial-scale changes in sedimentary δ15N and nitrogen content in modern marine sediments. We begin with a synoptic overview of sedimentary nitrogen species and their isotopic composition, followed by a quantitative analysis of potential environmental drivers shaping the spatial and temporal patterns of δ15N. By synthesizing this unprecedented dataset, we provide critical insights into the drivers of sedimentary δ15N variability and the role of anthropogenic perturbations in altering marine nitrogen cycling.

Results
Global distributions of nitrogen isotopes in seafloor sediments
By synthesizing δ15N measurements from previously published studies of modern seafloor sediment samples, including seabed surface sediments and core tops, we generated a high-resolution global spatial pattern of δ15N (Fig. 1a). Our findings regarding the geographical characteristics of sedimentary δ15N closely align with prior simulations from ocean-biogeochemical models36 and matches observed δ15N patterns in surface nitrate and particulate organic matter26,37–39. Globally, sedimentary δ15N exhibits statistically (Kruskal–Wallis test) significant latitudinal variation at 1° bins (P<0.05) and discrete zonal differences across ocean regions (P<0.05), indicating regional heterogeneity despite minimal overall longitudinal trends (Fig. 1b). Pronounced isotopic gradients are confined to dynamic frontal zones26,40, reflecting sharp transitions between biogeochemical regimes. Latitudinal variability in δ15N is generally consistent across ocean basins, reflecting large-scale gradients in nutrient utilization associated with phytoplankton uptake25. Nevertheless, regional anomalies occur in regions influenced by strong upwelling, oxygen minimum zones, and riverine inputs29,35,41. Low-latitude marginal regions (30°N–30°S) under equatorial upwelling influence display elevated δ15N (4.4–11.3‰), with maxima in high-nutrient, suboxic systems of the subtropical Southeastern Pacific and Arabian Sea—key zones of water-column denitrification and influenced by nitrogen sources such as nitrogen from deep-water nitrate and/or horizontally advected nitrate37,41,42. In contrast, high-latitude regions such as the Antarctic/subantarctic (south of 45°S) and the western Pacific (~45°N) display 15N-depleted signatures. Although this pattern appears consistent with previous reports of intense N2 fixation in phosphorus- and iron-limited waters1,43–45, it more likely reflects the mixing of low-δ15N nitrate, terrestrial inputs, and variable nutrient utilization (e.g., limited N2 fixation in the Southern Ocean)46,47. In the Arctic (>75°N), subarctic (~55°N), and mid-latitude transition zones (30–45°S), δ15N values are elevated compared to neighboring latitudes and display greater variability. High-latitude systems exhibit distinct isotopic partitioning that primarily reflects the degree of nitrate utilization37.

These spatial patterns underscore that nitrogen loss via denitrification and anammox primarily drives modern bulk sedimentary δ15N values, yielding a global median of 5.5‰ (interquartile range, IQR: 4.3–6.8‰, N=8260; Fig. 1a)—higher than oceanic nitrate δ15N (~5.0‰)48,49. Latitudinal δ15N trends align closely with surface suspended particulate nitrogen isotope, even within Pacific-specific data constraints37, indicating sedimentary preservation of water-column nitrogen cycling signals. This linkage is reinforced by a positive δ15N–δ13Corg correlation (R=0.377, P<0.001, N=4631; Fig. 1c), suggesting that nitrogen-rich sinking particulate organic matter effectively transfers surface ocean isotopic signatures to sediments. While δ13Corg and δ15N are not always correlated, covariation can occur in marine systems where both are influenced by overlapping sources and processes. δ13Corg reflects the balance between marine and terrestrial organic matter, and δ15N records nutrient cycling in the surface ocean50. Enhanced marine productivity can simultaneously enrich δ13Corg and δ15N, whereas greater terrestrial input tends to lower both50–52. Such a moderate correlation suggests secondary variability, with additional processes—such as differences in organic matter and nitrogen sources30, diagenetic alteration31, and regional variations in nutrient utilization25—likely contribute to the scatter in the δ15N–δ13Corg relationship.

Modern sedimentary δ15N measurements reveal a globally non-normal distribution, with a median value ~20% lower than prior mean estimates (~7‰) based on limited datasets5,40. We note that over 90% of the analyzed samples originate from coastal regions (water depth <2000m)53, where median δ15N (5.8‰), δ13Corg (−22.5‰), and nitrogen content (0.13%) differ markedly from deep-sea (>2000m) samples (δ15N=7.2‰, δ13Corg =−20.0‰, TN=0.14%) (Fig. 1a, d). These disparities highlight the potential for geographical heterogeneity and terrestrial influences on sedimentary nitrogen dynamics to be overlooked when relying on global mean δ15N values.

To assess the extent to which geographical disparities and associated terrestrial processes control the global distribution of δ15N in seabed sediments, we evaluated water depths and sedimentation rates as indicators for marine environment conditions. We identified statistically significant correlations (P<0.001) between sedimentary δ15N and both water depth (positive, Fig. 1d) and modern sedimentation rates (negative, Fig. 1e), suggesting that, in addition to marine processes, terrestrial influences may contribute substantially to global δ15N variability. High sedimentation rates also occur in upwelling regions with minimal terrestrial influence, where elevated sedimentary δ15N reflects the deposition of marine organic matter sustained by upwelling54. Notably, the mobilization of IN (primarily clay-bound NH4+) from terrestrial sources has been recognized as a non-negligible contributor to marine sedimentary nitrogen burial31,33–35,55. However, interpretations of δ15N in marine sediments remain challenging, as they typically assume invariant mineral-associated nitrogen source signatures and neglect spatial heterogeneity in post-depositional alteration processes30.

To elucidate the role of IN in shaping sedimentary nitrogen signals, we quantified its content in marine sediments using both seafloor surface and core samples. While data derive predominantly from the Western Pacific, Circum-Arctic, Antarctic, and Eastern Atlantic regions, our dataset spans a bathymetric range (200–5000m). Challenging previous assumptions that IN impacts are largely restricted to coastal systems receiving illite-rich clays, such as river-dominated margins30, our analysis reveals pronounced regional variability in IN within marine sediments (0–0.11%; median ~0.03%), accounting for ~32.4% (IQR: 22.1–43.0%; N=810) of total nitrogen (Fig. 2a). Although elevated IN concentrations occur in coastal zones influenced by direct fluvial and glacial inputs (e.g., Spitsbergen continental margin, South China Sea, and Arctic fjords)34,35,56, substantial contributions in deep-sea environments (e.g., western subarctic Pacific) likely reflect aeolian dust deposition and lateral advection of terrigenous clays through ocean currents33,57.

Comparative analysis of nitrogen forms indicates that δ15N values of IN (δ15NIN) in marine sediments (median: 3.9‰, IQR: 3.2–4.8‰; non-normal distributions) are systematically 1.6‰ lower than those of δ15N of organic nitrogen (δ15NON; 5.5‰, 4.9–6.1‰) and δ15Nbulk (Fig. 2b–d). This implies that terrigenous IN input may dilute δ15Nbulk signal towards low values, especially in IN-rich continental margin sediments (Supplementary Fig. 2a). Furthermore, increasing IN contributions correlate with a negative δ15NIN shift (Fig. 2e, f), likely driven by the amplified influence of 13C-depleted refractory terrigenous organic matter in organic-poor sediment fractions30,31,57.

In continental margins influenced by terrestrial inputs, such as the East China Marginal Seas affected by rivers and high-latitude fjords, organic nitrogen can be interpreted as a binary mixture of terrestrial-derived and marine production. We propose that the delivery of 15N-depleted terrigenous organic matter drives the low δ15NON values in marine sediments, supported by a positive correlation between water depth and δ15NON (R=0.683, P<0.0001, N=134, Supplementary Fig. 2b). Although diagenetic alteration—specifically, the preferential loss of 14N during remineralization of sedimentary organic matter, enhanced in deep, oxygenated sediments—may bias this interpretation32,58, the lack of correlation between ON content and δ15NON suggests that diagenesis does not dominantly influence these isotopic signals (Supplementary Fig. 3).

Our findings contribute to mounting evidence that terrestrial influences—both inorganic and organic—must be considered alongside established processes (e.g., denitrification, N2 fixation, and internal nitrate cycling by phytoplankton)1,26 to fully interpret sedimentary δ15Nbulk and δ15NON signals in marine systems. δ15NON closely tracks δ15Nbulk (R=0.796, P<0.001, N=578; Fig. 2g), with deviations likely driven by clay-bound inorganic nitrogen30, terrigenous organic nitrogen33, and organic nitrogen mineralization32. These patterns suggest that inorganic nitrogen effects are eclipsed by marine productivity and water column denitrification. Consequently, δ15Nbulk remains a robust archive for reconstructing phytoplankton nitrate consumption in regions minimally impacted by terrigenous inputs4,23,58–60. While the global synthesis establishes a baseline of spatial variability in sedimentary δ15N, the extent to which these signals have evolved or remained stable in response to anthropogenic and climatic forcing remain poorly constrained.

Decadal to centennial changes in global marine sedimentary δ15N
Reconstructing historical nitrogen cycling using δ15N preserved in modern marine sediments provides an alternative approach to evaluate global-scale drivers of δ15N variability. Our database comprises 148 sedimentary δ15N records, spanning decadal to centennial timescales (Fig. 3). Core sites are globally distributed, predominantly along continental margins (water depth <2000m), where high sedimentation rates (mean = 0.82 cm yr⁻¹; range: 0.03–5.22 cm yr⁻¹) prevail. These records encompass margins of North and South America, East Asia, the North Indian Ocean, Australia, and the Southeast Atlantic, capturing modern climatic conditions and levels of anthropogenic influence. Elevated sedimentation rates in these coastal regions likely minimize diagenetic alteration27,59,60, enhancing the reliability of sedimentary δ15N as a tracer of nitrogen cycle dynamics.

Analysis of an extensive set of high-resolution sediment cores (AD 1900–present) reveals that no coherent global signal in sedimentary δ15N, suggesting minimal consistent decadal-to-centennial changes in marine nitrogen cycling. Of 148 sites analyzed, 97 (65%) exhibited significant temporal trends (P<0.05), with 63 sites showing increases and 34 decreases, while the remaining 51 (35%) showed no significant change (P>0.05). Notably, significant increasing trends outnumbered decreasing trends by nearly 2:1. Sedimentary δ15N increased at a rate of 1.5‰ per century (range: 1.2–2.2‰ per century) in sites with positive trends, compared to a decline of −1.2‰ per century (range: −2.3 to −0.9‰ per century) in sites with negative trends. This increase in sedimentary δ15N aligns with a doubling of riverine nitrogen export to the oceans (1900–2010)10,11, a 4.6% increase in net primary productivity in coastal oceans (1960–2010)61, and a 2.19% yr−1 rise in global coastal phytoplankton bloom frequency (2003–2020)62.

Climatic and anthropogenic factors have jointly influenced marine sedimentary δ15N trends over the past century. Sites with opposing δ15N trajectories cluster in northern tropical and temperate zones, while Southern Hemisphere data remain sparse (Fig. 3). Notably, nitrogen-rich regions with low sediment accumulation rates predominantly exhibit rising δ15N trends toward the present (Supplementary Fig. 4). High-latitude northern sites show consistent δ15N increases, contrasting with global lacustrine records that report declines in cold, nitrogen-limited regions63. This divergence suggests that climatic variables such as latitude and temperature inadequately explain marine δ15N trends, implicating additional controls.

Coastal zones under intense anthropogenic pressure—characterized by population growth, industrialization, and nutrient discharge—exhibit significant increases in sedimentary δ15N over recent decades. Hotspots of rising δ15N trends are clustered along the continental margins of Europe (North-Baltic Sea, Portuguese estuaries, Spitsbergen fjords), North America (Gulf of Mexico, Chesapeake Bay, Delaware), and South America (Brazilian coast, Comau Fjord). These regions coincide with coastal systems receiving excess nutrients from riverine inputs, wastewater discharge, groundwater infiltration, and atmospheric deposition11,64,65. Supporting for this hypothesis, our analysis of 120 global marine sediment cores reveals a century-long enrichment trend in sedimentary N burial in marine sediments (Fig. 4). Of these, 104 sites (~87%) exhibited increasing trends in N content or accumulation rates over time. Among these, 81 sites showed statistically significant trends (P<0.05), with an average rate of 1.55% per century. These spatial patterns of enhanced sedimentary N burial align closely with globally observed increases in sedimentary δ15N values during the same period. Thus, the observed 15N enrichment can be attributed to elevated inputs of high-δ15N nitrate (10–20‰) from wastewater65,66 and isotopic fractionation during phytoplankton nutrient uptake and enhanced denitrification in response to eutrophication67.

Carbon isotopic data indicate that dramatic δ15N shifts coincide with elevated primary productivity (higher δ13Corg, Supplementary Fig. 5a), reflecting increased phytoplankton biomass and likely eutrophication61,62,64. Given widespread core coverage, increased riverine nitrogen inputs10,11, enhanced coastal productivity61, and frequent phytoplankton blooms62 over the past century, this trend may partly reflect a global link between δ15N and marine productivity68,69. Additionally, the marked rise in sedimentary δ15N can be partially attributed to the preferential degradation of fresh organic matter during sedimentation or burial, despite high sedimentation rates in coastal zones. This hypothesis is supported by significant (P<0.001) correlations between δ15N and water depth (positive) and total nitrogen content (negative) [Supplementary Fig. 5b, c]. Enhanced remineralization of exported organic matter elevates microbial respiration and oxygen demand, aligning δ15N increases with coastal hypoxic zones62,64, phytoplankton biomass expansion (post-1950s)69, and deoxygenation trends70.

Centennial δ15N declines prevail in mid- to low-latitude river-influenced continental margins (e.g., Amazon River estuary, western North America, Bohai Sea, East and South China Seas), with 76% of decreasing trends (44 of 58) occurring at 20°–50°N (Supplementary Fig. 6). Although a significant positive δ13Corg–δ15N correlation exists (P<0.001), limited alteration in δ13Corg (~2.0‰) fails to account for high variability in δ15N (Supplementary Fig. 7). The absence of δ15N enriched signatures derived from marine production may be attributed to preferential degradation of labile marine organic carbon and nitrogen in dynamic margins, where preservation efficiency is <1%71. As a result, continental margins exhibiting declining sedimentary δ15N trends are typically characterized by reduced N content or no significant increases in N burial rates (Fig. 4). Sediments in these dynamic areas experience repeated cycles of sediment reworking and deposition20, reflected by thick seafloor sediment mixed layers caused by physical perturbation and bioturbation72,73. Radiocarbon and biomarker analyses indicate that buried organic carbon in these continental margin sediments is predominantly 13C-depleted, pre-aged materials74,75, sourced from terrestrial materials and aged through sediment reworking and lateral transport76. Thus, the disconnect between riverine inputs and δ15N trends cannot be ignored.

Variations in riverine sediment supply modulate the quantity and quality of terrigenous organic matter and mineral particles, potentially reshaping marine sedimentary nitrogen dynamics, with terrestrial organic and inorganic nitrogen inputs potentially overshadowing changes in marine nitrate utilization. Despite a ~49% decline in fluvial particulate transport (1950–2010)9—notably in European and Asian regions77—expected reductions in terrestrial organic matter contributions and enhanced diagenesis due to decreased sedimentation rates20 are not reflected in δ15N trends. This paradox challenges hypotheses of increasing ¹⁵N-depleted terrestrial inputs and points to unresolved controls, such as shifts in the chemical nature of terrestrially exported nitrogen.

Terrigenous sediment transport modulates δ15N dynamics in large river-dominated continental margins
To unravel drivers of the centennial-scale decline in sedimentary δ15N, we focus on the East China Sea (ECS)—a Yangtze River-influenced system and one of the world’s largest river-dominated continental shelves (Supplementary Fig. 1). Sedimentary archives from the ECS reveal a striking divergence: while δ13Corg exhibits an increasing trend, 10 of 13 cores show coherent declines in δ15N from the early 1900s to present, with an average rate of −1.09 ± 0.48‰ per century (Supplementary Figs. 8 and 9a–b). Notably, cores from ECS constitute ~21% (10 of 58) of global sedimentary records with statistically significant δ15N declines (P<0.05). Additionally, the Yangtze, like many other large rivers (e.g., Mississippi, Amazon, Ganges and Yellow) has experienced considerable increases in nutrient loading alongside declining sediment fluxes over the last half-century9,68,78,79. This dual trajectory positions the ECS as a natural laboratory to disentangle the competing effects of nutrient enrichment associated production and sediment reduction on δ15N dynamics in coastal marine sediments.

In river-dominated continental margins, terrestrial organic carbon burial rates are tightly coupled to riverine lithogenic influx19. Anthropogenic reductions in particulate load diminish sedimentation rates, limiting organic carbon burial and enhancing oxygen exposure, thereby accelerating remineralization in oxic coastal sediments20,71. Paradoxically, despite increased marine productivity (elevated δ13Corg) and remineralization, sedimentary δ15N exhibits a sustained decline—a trend inconsistent with conventional nutrient cycling models. We posit that anthropogenic perturbations may alter the composition, rather than the bulk flux of terrigenous materials, modulating nitrogen isotope signatures through selective organic matter preservation.

Dam construction and land-use changes in the Yangtze River basin have reduced sediment discharge to ~20% of pre-dam levels (Supplementary Fig. 9c), triggering riverbed erosion and grain-size coarsening in both suspended and seabed sediments80. This trajectory mirrors global patterns observed in the Mississippi81, Colorado82, and Yellow River83 systems. Across the ECS, 86% of records (6 of 7) declining fine clay content (<4 μm) since 1900, with 71% (5 of 7) showing statistically significant trends (P<0.05; Supplementary Fig. 9d). These mineralogical shifts correlate strongly with δ15N depletion, indicating that terrigenous particle size distributions regulate organic nitrogen sequestration efficiency, with finer sediments enhancing preservation of 15N-enriched organic matter (primarily for marine-derived OC).

To disentangle the effects of sediment supply and hydrodynamic sorting on the marine nitrogen signal, we analyzed sedimentary δ15N against clay content and terrestrial endmember δ15N signatures (Fig. 5a, b; Supplementary Table 2). Coarse sediments predominantly host 15N-depleted woody debris and fossil-related carbon, while fine clay fractions retain 15N-enriched soil and marine OC84. Particulate δ15N remains consistent (P>0.05) from riverine inputs to seafloor deposits (Fig. 5b), indicating minimal isotopic alteration during rapid transport and burial in high-sedimentation-rate margins.

Size-fractionated analyses in core DH3-1 from the inner shelf of the East China Sea reveal divergent δ15N trajectories: the <63μm fraction shows increasing δ15N, whereas the >63μm fraction exhibits declining values, paralleling bulk sediment trends (Supplementary Fig. 10 and Table 3). Despite the dominance of fine sediments, 15N-depleted organic matter in coarse refractory materials persists and is unlikely to be offset by the 15N-enriched fine minerals that have incorporated marine carbon. We attribute this to anthropogenic mobilization of 15N-depleted refractory OC (e.g., vascular plant detritus, fossil carbon) into coastal zones via dam releases and petroleum inputs85–87, driving centennial δ15N declines.

To isolate refractory OC’s role, we applied a chemical oxidation approach to remove labile OC (Methods). This analysis allows us to reveal the signals and contribution of nitrogen associated with recalcitrant, degraded sedimentary organic matter88,89 and assess its historical changes (Supplementary Tables 4–7). This chemical oxidation eliminated ~80% of the total OC on average but only ~20% of nitrogen, doubling the N/C ratio (Fig. 5c). Near-identical intercepts for bulk and refractory C-N regressions (0.019% vs. 0.022%) exclude inorganic nitrogen as a major influence. We find that nitrogen associated with refractory organic carbon strongly correlates with bulk nitrogen across cores (P<0.05; Supplementary Fig. 11), suggesting that variance in bulk nitrogen abundance is primarily controlled by nitrogen associated with refractory OC. Compared to bulk OC, the depleted isotope signatures in refractory, with a reduction of ~2.6‰ in δ13Corg (median = −26.3‰, ranging from −28‰ to −23.5‰) and ~2.3‰ in δ15N (median = 2.1‰, ranging from −12‰ to 6.5‰), aligning with terrigenous C3 plant and petrogenic sources (Fig. 5d). Due to their chemical recalcitrance, these materials can preserve depleted δ15N signatures during transport and burial in marine sediments. Supporting this interpretation, the low C/N ratios of refractory OC likely reflect 13C-depleted terrestrial inputs, particularly petrogenic OC derived from sedimentary rocks in the Yangtze basin (e.g., shale, sandstone/siltstone, marlstone, limestone), which inherently exhibit low C/N ratios (2–6; Supplementary Table 8), consistent with previously reported bedrock data from Taiwan90.

Two mechanisms linked to the supply and sequestration of 15N-depleted refractory terrestrial OC may account for the broad declines in sedimentary δ15N observed over the past 1–2 centuries. Although terrestrial particulate load has decreased overall, anthropogenic perturbations—including land-use changes and fossil fuel combustion—appear to enhance the mobilization of refractory riverine OC87,89. This increased flux of 15N-depleted terrestrial OC into marine sediments could dilute sedimentary δ15N values30,31, a hypothesis corroborated by concurrent increases in the burial flux and contribution of 13C-depleted refractory OC in sedimentary records. Notably, the preferential degradation of labile organic matter, which typically enriches residual δ15N37, may fail to offset the 15N-depleted signature of refractory OC under these conditions. We propose that declining sedimentation rates, coupled with intensified remineralization of labile OC during particle sinking, amplify the influence of 15N-depleted organic matter on sedimentary nitrogen pools. This mechanism explains the absence of rising δ15N trends in most studied ECS sediment cores.

Globally, sedimentary δ15N records from the past century exhibit no consistent directional shift. In coastal regions, however, high-resolution sedimentary records reveal enrichment in both sedimentary δ15N and N content, likely driven by processes such as anthropogenic nitrogen inputs, enhanced productivity, water-column denitrification, and hypoxia10–12,61,62,64,69,91. Conversely, in river-dominated continental margins, human activities enhance the delivery and burial of 15N-depleted, refractory terrigenous OC, overwhelming signals of nitrate utilization and driving δ15N declines in modern sediments. This divergence between coastal and river-dominated systems has rapidly reshaped global sedimentary δ15N patterns, with nitrogen sources in river-influenced regions shifting from marine production to terrestrial dominance.

The heterogeneity in δ15N trends underscores the necessity of investigating local sedimentary processes on continental margins. Recent changes in marine nitrogen cycling cannot be attributed solely to inventory-altering (e.g., N2 fixation) or internal-cycling (e.g., denitrification) fractionation processes1,26,36. Unlike deep-sea environments, terrestrial inputs, isotopic fractionation during transport, and sediment dynamics may collectively govern δ15N evolution in modern continental margin sediments.

Additionally, our compiled global dataset includes both acidified and untreated marine sediment samples. Although acidification can introduce unintended shifts in TN and δ15N—primarily linked to carbonate characteristics, the isotopic composition of associated nitrogen, and the susceptibility of different N-containing compounds to acid treatment92–96—these effects are minor relative to the spatial and temporal variability of sedimentary nitrogen and do not compromise large-scale patterns relevant to the objectives of this work. Future work spanning latitudinal and environmental gradients, from coastal zones to the deep sea, will be essential to further constrain the effects of different acidification methods and refine interpretations of nitrogen dynamics under future ocean acidification scenarios.

Sedimentary δ15N remains a robust archive of past nitrogen cycle dynamics, and has constrained how the evolution of nitrogen burial in modern marine sediments in response to recent environmental changes. Centennial-scale trends suggest that anthropogenic perturbations and climate change may drive oceanic nitrogen cycling further from steady-state equilibria, accelerating rates of change. To refine δ15N interpretations, future work must prioritize: constraining terrigenous nitrogen sources and isotopic alterations during organic matter transport; expanding spatial coverage of centennial δ15N records, particularly in understudied deep-sea sediments; compound-specific and fossil-bound δ15N analyses29,97; and integration with isotopic models to disentangle biogeochemical drivers36,38. Addressing these gaps is critical for resolving uncertainties in the marine nitrogen budget and tracing the depositional fate of nitrogen in global marine sediments.

Methods
Data sources
Our database was compiled from more than 160 peer-reviewed publications and complemented by newly generated datasets, providing a framework to investigate the spatial and temporal variability of nitrogen characteristics in marine sediments at regional to global scales. The compilation includes seafloor surface sediments and primarily 210Pb/137Cs dated sediment cores, with parameters selected to capture both nitrogen cycling and sedimentary context. Key variables include total, organic, and inorganic nitrogen contents and isotopes, together with water depth, sedimentation rate, grain size, OC content, and δ13Corg. Note that all datasets outside the Yangtze River basin and the East China Sea are derived from previously published sources.

To better constrain the drivers of modern sedimentary δ15N signals in river-dominated continental margins, we present nitrogen measurements from six sediment cores (C0702, C0803, DH3-1, DH3-2, DH5-1, and DH6-1) recovered from the East China Sea. These include bulk sediment analyses, size-fractionated samples from core DH3-1, and refractory OC fractions from cores C0803, CJK, and DH3-1. In addition, we conducted paired analyses on terrestrial samples (sedimentary rocks and river bed sediments) and marine samples (seabed surface sediments, sediment cores, size-fractionated fractions, and suspended particulate matter) to assess the effects of acidification and, for rocks only, chemical oxidation on nitrogen signals. An overview of both literature-derived and newly generated datasets is provided in Supplementary Table 1.

Literature dataset compilation
Data on sample location, nitrogen content, bulk δ15N, δ13Corg, and chronology for seabed surface and sediment cores were extracted directly from tables or obtained via correspondence with authors. Where original data files were unavailable, values were digitized from published figures using PlotDigitizer (v. 1.3). All δ15N values are reported relative to atmospheric N2.

A global compilation of 8260 bulk δ15N measurements from modern marine sediments was assembled to map spatial trends. To evaluate the influence of inorganic nitrogen (primarily lattice-bound NH4+ in clay minerals) on marine sedimentary nitrogen pool, additional datasets were incorporated, including inorganic-to-organic nitrogen ratios (N=760) and their isotopic compositions (δ15NIN, N=512; δ15NON, N=560). The contribution and isotopic composition of ON were determined by indirect mass–balance calculations from TN and IN isotopic compositions after ON removal with KOBr–KOH solution31,33,34, and by direct δ15N measurements following organic matter isolation with HF–HCl digestion and polymer columns35,98.

Regional δ15N trends were assessed using 148 sediment cores spanning the past ~120 years, selected based on availability of water depth, modern sedimentation rates, total N content, bulk δ15N, and δ13Corg data. Evolution of N burial (content or accumulation rate) in marine sediments was reconstructed using 120 globally distributed sediment cores. Chronologies were established primarily using 210Pb models and 137Cs time markers, partly constrained by 239+240Pu, 14C, or other tracers. The choice of a age model was based on the goodness of fit, local fallout of 137Cs, recommendations from the original published studies, and the authors’ interpretation. Our reconstruction primarily applies to continental margin sediments with high sedimentation rates, and it poorly resolves low-sedimentation deep-sea sediment deposits. If the data come from sediment cores sampled during different seasons at the same location, we averaged the values of carbon and nitrogen content and isotope composition, considering these averages to be representative of temporal variability within that system.

Despite >90% of modern δ15N records originating from continental margins, spatial resolution remains insufficient to resolve small-scale features (e.g., estuaries, fjords, and continental shelves). To mitigate local heterogeneity (e.g., sediment focusing, terrestrial inputs, and energetic marine forcing), δ15N time series are expressed as relative centennial changes (Δ15N) from core-specific baselines. Data were grouped by geographic zones to assess coherent trends. Sediments with accumulation rates <0.01 cm yr−1 (e.g., deep-sea deposits) were excluded due to poor chronological resolution. Site-averaged δ15N values were assumed representative of system-wide trends, though single-core records may retain stochastic variability smoothed by regional grouping.

Sample collection and treatment
To examine terrestrial influences on sedimentary nitrogen isotope signals in river-dominated margins and their response to anthropogenic perturbations over decadal to centennial timescales, we generated high-resolution δ15N records from six marine sediment cores in the East China Sea. These cores, spanning one of the world’s largest continental shelves, were dated using 210Pb and/or 137Cs chronologies. Summaries, descriptive information, age data, models, and physical properties data were reported previously84 and references therein. Detailed site information is available in Supplementary Fig. 1.

To evaluate hydrodynamic sorting of deposited particles with distinct δ15N signatures, we performed grain-size fractionation on core DH3-1. Sediments samples were wet-sieved using stainless steel mesh (63 μm) and ultrapure water to isolate coarse (>63 μm) and fine (<63 μm) fractions. Elemental concentrations (C, N) and isotopic compositions (δ15N, δ13C) were measured for both fractions. To quantify the contribution of refractory organic matter to bulk δ15N, we applied a chemical oxidation protocol to remove labile organic carbon in four sediment cores (C0702, C0803, CJK, and DH3-1). Residual materials were analyzed for geochemical composition (OC, TN, δ15N, and δ13Corg).

To constrain the end-member of nitrogen associated with refractory OC and to evaluate the effects of acidification and chemical oxidation, we collected twenty-four sedimentary rock samples from the northeastern lower Yangtze region (Supplementary Fig. 1), part of the South China Block, which exposes strata from the Upper Precambrian to the Middle Triassic99. The sampled sedimentary rocks include marlstone, siliceous shale, shale, carbonaceous shale, sandstone, lignite, siltstone, fine sandstone, and limestone, broadly representing the main types of outcropped sedimentary rocks in the region. Detailed information on the sampling sites and sedimentary rock compositions is provided in Supplementary Table 8.

Elemental and isotopic analyses
Freeze-dried sediment samples were homogenized in an agate mortar, and inorganic carbon was removed via acid treatment following established protocols84. Briefly, decarbonation was performed using 4M HCl, followed by rinsing with deionized water and drying. Acidified aliquots were analyzed for organic carbon (OC) and total nitrogen (TN) content, as well as δ13Corg and δ15N values, using an elemental analyzer (Elementar Analysensysteme GmbH, Germany) coupled to an isotope ratio mass spectrometer (IsoPrime 100). Isotopic ratios are reported in δ notation (‰) relative to the Vienna PeeDee Belemnite (V-PDB) standard for δ13Corg and atmospheric N2 for δ15N. Calibration and quality control utilized certified standards (L-histidine, D-glutamic acid, glycine, acetanilide) and the USGS 40 reference material. Analytical precision (1σ), determined from replicate analyses of USGS 40 (N=12) and samples, was ±0.02% (OC), ±0.006% (TN), ±0.2‰ (δ15N), and ±0.3‰ (δ13Corg).

To directly evaluate the potential influence of acid pretreatment on nitrogen measurements, we analyzed 160 paired samples with and without 4M HCl treatment, spanning terrestrial (Yangtze River basin) and marine (East China Sea) environments (Supplementary Figs. 1 and 12). These included marine seabed surface sediments (N=40), five sediment cores (N=67), suspended particulate matter (N=12), grain size-fractionated (>63 μm and <63 μm) sediments (N=13), river bed materials (N=4), and sedimentary rocks (N=24), representing a wider range of TN contents and δ¹⁵N values (Supplementary Tables 8–9).

Across this diverse dataset, differences in ΔTN (TNacidfied – TNuntreated; mean = 0.01% and median = 0.007%) and Δδ15N (δ15Nacidfied – δ15Nuntreated; mean = –0.183‰, median = –0.266‰) between bulk and acidified marine samples remain well within analytical precision (0.01% for TN and 0.3‰ for δ¹⁵N) [Supplementary Fig. 12a–d] Importantly, acidification did not alter overall trends in either terrestrial or marine samples, as evidenced by highly significant (P<0.0001) and robust (R>0.91) correlations between pre- and post-treatment values (Supplementary Fig. 12e–h). Nevertheless, acid treatment can introduce unintended alterations in both ΔTN and Δδ15N values: most marine (~79%, N=104) and terrestrial (~82%, N=23) samples showed positive shifts in TN content after treatment, whereas δ15N tended to decrease in a substantial proportion of marine samples (~64%, N=84) compared with terrestrial samples (~39%, N=11) [Supplementary Fig. 13]. We attribute these patterns to a combination of factors, including the potential chemical transformation or loss of nitrogen (e.g., carbonate-associated), which may cause fractionation (posited but unknown)92–96, and the enrichment of nitrogen resulting from the removal of carbonate minerals.

Our results align with previous studies showing that acidified treatments generally yield TN and δ15N values indistinguishable from—or within the analytical precision of—untreated marine sediments, as differences between bulk and decarbonated samples typically fall within precision limits27,93,95,96. Nonetheless, we note that stronger acid treatments, particularly for N-poor samples, may induce larger deviations under certain conditions96. Applying identical procedures to sediment samples from the same core helps minimize analytical artifacts and ensure that relative trends in TN and δ15N remain robust27. Finally, whether the acid effect is considered significant depends on the specific research question93, as the shifts in both δ15N and TN induced by acid treatment are often minor relative to the large-scale patterns and variability of nitrogen signals in global marine sediments (Figs. 1, 3, and 4).

Refractory organic carbon analysis
To characterize nitrogen associated with refractory organic matter in marine sediments, we isolated chemical oxidation-resistant OC using sodium persulfate (Na2S2O8), a method simulating microbially mediated oxidative degradation88. Freeze-dried bulk samples (~0.5 g) were dispersed in 250 mL distilled water, reacted with 20 g Na2S2O8 (MACKLIN) and 22 g NaHCO3 for 48 h at 80°C, and rinsed to collect residual refractory OC. OC, TN, δ13Corg, and δ15N were measured in the residual fraction to identify terrestrial refractory OC composition and contributions. Analytical precision (1σ) for repeated measurements of oxidized residues was ±0.02% (OC), ±0.008% (TN), ±0.43‰ (δ13Corg), and ±0.24‰ (δ15N).

To evaluate potential analytical artifacts, eleven replicate blank analyses were conducted using the same protocols as for sediment samples. OC and TN contents in the aqueous phase were measured with an elemental analyzer (TOC-L, Shimadzu, Japan). The mean blank values (±1σ) were 0.408 ± 0.04 ppm for OC and 0.004 ± 0.003 ppm for TN, the latter being close to the analytical uncertainty (~0.004 ppm). These correspond to 0.023 ± 0.01% and 0.001 ± 0.0004% of typical contents in refractory marine sediments (N=75; Fig. 5c, d), respectively, confirming that blank contributions are negligible and do not affect δ13C or δ15N measurements.

The chemical oxidation approach was designed to remove labile, 13C- and 15N-enriched OC in marine sediments while preserving most refractory terrestrial carbon that is typically 13C- and 15N-depleted. To test this, paired oxidation experiments were conducted on sedimentary rock samples from the Yangtze River basin (N=24), dominated by graphitic refractory OC largely resistant to oxidation100. Chemically oxidized and unoxidized samples yielded comparable mean values for OC (1.2% vs. 1.5%), TN (0.121% vs. 0.129%), δ13Corg (–27.1‰ vs. –27.0‰), and δ15N (2.5‰ vs. 2.3‰) [Supplementary Fig. 14]. Although minor labile fractions may be removed, no significant differences were observed (P>0.05, Wilcoxon test), and isotopic values consistently fell within the terrestrial end-member range50–52.

Together, these results confirm that the oxidation treatment effectively distinguishes refractory from labile pools while minimizing isotopic artifacts. The isotopic shifts observed in marine sediments therefore reflect genuine compositional differences in organic matter rather than analytical bias. Paired datasets for bulk (unoxidized) and refractory (oxidized) rock samples are provided in Supplementary Table 10.

Statistical analysis and plotting
Data distributions were assessed for normality using the D’Agostino-Pearson test (GraphPad Prism v9.5.1) to determine the most reliable measure of central tendency. For normally distributed data, we report the mean with the 95% confidence intervals (CI). For non-normally distributed data, we instead report the median with the interquartile range (IQR, 25–75%) as a more appropriate descriptor of central tendency101,102. Group comparisons employed Student’s t-tests (two groups) or analysis of variance (ANOVA; >2 groups), with statistical significance thresholded at P<0.05. The Wilcoxon test, a nonparametric method, was used to compare two paired groups, while the Kruskal–Wallis test, also nonparametric, was applied to assess differences among three or more independent groups.

For temporal δ15N trends, linear regressions between sediment age and δ15N values were performed on each core spanning the past 100–120 years. To standardize records, δ15N time series were expressed as deviations from core-specific baselines (Δ15N). Regional patterns were inferred by grouping 148 globally distributed cores into geographic sets, accounting for bathymetric variability. Bootstrapped 95% confidence intervals of linear fits for δ15N were generated from 10,000 iterations using Acycle103. The global distribution map of data points was created and downloaded using the open-access online platform ChiPlot (https://www.chiplot.online/)104,105.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "General Comments:\nSun et al. have presented a wide-reaching review and synthesis of nitrogen signals contained in marine sediments and, for\nthe most part, provide solid data analysis and interpretations. In addition to my line comments, I have a few more general\ncomments:\n1. I am concerned that the signals in the global dataset are impacted by acid pretreatment. I provide further rationale at the\nend of my line comments, but, in brief, a significant fraction (~30%) of Supplementary Data 2 have been acid pre-treated and\nmay compromise the fidelity of practically all the data and inferences within the manuscript.\n2. On a similar topic, I am extremely skeptical of the insights based on the refractory digestion in the later portions of the\npaper and, chiefly, in Figure 6. All organic nitrogen compounds contain carbon and the impacts of this digestion on nitrogen\ncomposition are, as far as I can tell, unconstrained. The original application of this refractory digestion was to establish the\nradiocarbon age of the residual pool and it seems like careful experimentation would be needed to confidently apply and\ninterpret its effects on total nitrogen and nitrogen stable isotopes. I also suspect its impacts on carbon stable isotopes are\nalso poorly constrained.\nThis manuscript and its data collation effort have significant merit but also have some serious hurdles to overcome before\nbeing suitable for publication.\nIf the authors need any clarification, they can contact me (Jack Hutchings) directly if desired at jackh@wustl.edu.\nLine/Specific Comments:\nLine 103-105: What is meant by “significant”? I assume, statistically, given the p-value, but then there is non-specific\nlanguage about lateral (zonal) continuity. Using whatever test was done for latitudinal variation, does zonal variability fail\nthat test of significance?\nLine 115: Revered = reversed?\nLine 122: What is the scientific significance of defining the 95% C.I. for the global data set? I recognize the desire to provide\nerror, but I am wondering if we can infer anything specifically from that range.\nLine 127 / Figure 1 c,d,e: The figure shows “R”, but do you mean R-squared? If R, shouldn’t Fig 1e have a negative R value?\nLine 126-129: It is not immediately obvious to me why we would expect d13C and d15N to be correlated with one another.\nWhy?\nFig 1d: I think log-transforming your depth prior to this analysis would have it be more sensible.\nFig 1e: There appears to be a lot of overplotting at specific sedimentation rates. Are these all samples from a given core or\njust a rounding artefact of reported sedimentation rates? E.g., the set of values at a sed. rate of ~5.2 cm / yr … if these are\nfrom a single core, wouldn’t we want to average them prior to the analysis?\nLine 158: What is the intended meaning/implication of “unignored” here?\nSupplementary Fig 2a: Similar to Fig 1d, depth should probably be log-transformed here.\nLine 195: Do you show this lack of correlation somewhere? If not, I would have it in the supplement.\nLine 203: Isn’t this somewhat tautological? Most of d15N_bulk *is* d15N_organic…\nLine 206: If you mass-balance d15N_inorganic and d15N_organic by their respective amounts, how faithfully would d15N\n_bulk be recomputed?\nLine 280-283: Is there any way via mass balance to back out the percent contribution of runoff with enriched d15N_nitrate\nfrom these trends? I suspect a first-order estimate is possible.\nLine 317: Module = modulate?\nFigure 6c: The C:N of refractory OC is exceptionally low… I guess this is an artefact of the chemical digestion, but I cannot\nthink of a “real” sizeable pool of organic matter whose C:N is 3-4.\nTable S3 / Supplementary Data 1: I see that we should have paired before/after refractory digestion experiment data here\nthat I believe forms the basis of Fig 6d, however Table S3 has ages rounded to the nearest year whereas Supp. Data 1\nincludes decimals that makes pairing those data (if they represent paired measurements) hard/impossible.\nSupplementary Fig 10: I am pretty unconvinced by this plot. It is fairly well known that acid treatment of any kind alters both\nthe TN and d15N composition of samples. We can see it in your plot and I can see it by quickly plotting up some of your\nSupplementary Data File 2 and grouping by either untreated or acid-treated:\n(attached)\nThe left set of plots contains all of the data and the right set of plots includes only data within two standard deviations of the\nmedian. In all cases, I’ve plotted standard boxplots (median center bar) and a gaussian kernel density function. I’d argue we\ncan cleanly see some deviation between treatments. Now, it is possible that the two\ngroupings (untreated and acidified) come from distinct locations on the planet and the\ndifferences are real, but your sample sizes are quite large and I suspect we are seeing\nartefacts related to acidification. Besides, there is a rich literature on the impacts of acidification pretreatment\non nitrogen values during elemental analysis. For example:\nSchlacher, T. A., & Connolly, R. M. (2014). Effects of acid treatment on carbon and nitrogen stable isotope ratios in\necological samples: A review and synthesis. Methods in Ecology and Evolution, 5(6), 541–550. https://doi.org/10.1111/2041-\n210X.12183\nWalthert, L., Graf, U., Kammer, A., Luster, J., Pezzotta, D., Zimmermann, S., & Hagedorn, F. (2010). Determination of organic\nand inorganic carbon, δ13C, and nitrogen in soils containing carbonates after acid fumigation with HCl. Journal of Plant\nNutrition and Soil Science, 173(2), 207–216. https://doi.org/10.1002/jpln.200900158\nBrodie, C. R., Leng, M. J., Casford, J. S. L., Kendrick, C. P., Lloyd, J. M., Yongqiang, Z., & Bird, M. I. (2011). Evidence for bias\nin C and N concentrations and δ13C composition of terrestrial and aquatic organic materials due to pre-analysis acid\npreparation methods. Chemical Geology, 282(3–4), 67–83. https://doi.org/10.1016/j.chemgeo.2011.01.007\nKennedy, P., Kennedy, H., & Papadimitriou, S. (2005). The effect of acidification on the determination of\norganic carbon, total nitrogen and their stable isotopic composition in algae and marine sediment. Rapid Communications in Mass Spectrometry,\n19(8), 1063–1068. https://doi.org/10.1002/rcm.1889", "reviewer_2": "Sun et al present a comprehensive global synthesis of nitrogen isotope (δ¹⁵N) data from modern marine sediments, including\nover 8,200 surface and core-top samples and 148 dated sediment cores spanning the past century. The authors identify\nspatial patterns and centennial-scale trends in δ¹⁵N, linking them to anthropogenic nitrogen inputs, productivity changes, and\nsediment dynamics. The study highlights the dual role of marine and terrestrial processes in shaping sedimentary δ¹⁵N\nsignals, with particular emphasis on the contrasting trends observed in coastal versus river-dominated continental margins.\nThis is an important and timely contribution to our understanding of how anthropogenic and climatic drivers have reshaped\nthe marine nitrogen cycle over the past century. The global scope, high-resolution temporal data, and integration of inorganic\nand organic nitrogen forms provide a robust framework for interpreting δ¹⁵N variability. However, the manuscript would\nbenefit from streamlining and clearer separation between the introduction, results, and discussion sections. The figures are\ngenerally clear and informative, though their large number makes it challenging for readers to navigate and identify the\ncentral message.\nI recommend this paper for publication in Nature Communications with major revisions.\nGeneral comments:\nThe manuscript would benefit from a concise methods summary and a table outlining which datasets are newly generated\nand which are compiled from previous studies. This would help readers quickly grasp the scope and novelty of the work.\nThe structure of the manuscript feels somewhat fragmented. The first part (global compilation) appears disconnected from\nthe second and third parts, which follow a more logical progression, first examining centennial δ15N trends and then\nfocusing on a case study of the Yangtze River system. The authors may consider whether the manuscript could be split into\ntwo complementary papers: one focused on global patterns and drivers of δ15N, and the other on historical changes under\nanthropogenic influence.\nGiven the broad readership of Nature Communications, the introduction could be expanded to better contextualize the role of\nrivers in the nitrogen cycle, how these systems have changed over the past century, and what the key drivers of those\nchanges are.\nOne of the key outputs of this study is the extensive δ15N dataset. I encourage the authors to consider also publishing the\ndata compilation, as it would be a valuable resource for the community.\nComments Within Text:\nLine 19–24 (Abstract): The phrase “coinciding with nitrogen content enrichment” could be clarified. Does this refer to\nincreased organic nitrogen, inorganic nitrogen, or both?\nLine 35–41: Given the broad readership of Nature Communications, can the authors elaborate on the\npressures/consequences of increase in N-export to coastal systems, as well as state a consequence for reduced flux of\nfluvial particulates with respect to marine biodiversity, C-storage and climate feedbacks?\nLine 73-75: “such unresolved challenges” and “to address this knowledge gap” sounds a bit vague.\nLine 83: “to assess environmental perturbations”. Are the authors looking for improving the robustness of bulk sedimentary\nd15N or investigate environmental perturbations or both?\nLine 98: do yo mean seabed?\nLine 99: it is unclear to the reader if the data was generated by the authors or if this is it a data compilation from existing\ndata?\nLine 102: also add nitrate data from Fripiat et al., 2021 (Nature Geoscience)\nLine 103–107: The authors mention latitudinal variability in δ¹⁵N but could clarify whether this variability is consistent across\ndifferent ocean basins or if regional anomalies exist.\nLine 113: Southern Ocean is not an area of N-fixation. The N-isotope gradient is due to different nutrient uptake efficiency by\nphytoplankton and the amount of nutrients available across the (sub)polar front as mentioned further down in the text, add\nref. (Sigman et al., 2000, Journal of Geophysical Research Oceans)\nLine 117: increase or decrease? depends on from which direction we make this observation.\nLine 122: The median cannot logically fall outside the reported confidence interval. If the CI is 5.7–5.8‰, then the median\nmust be at least 5.7‰, not 5.5‰, was that a typo?\nLine 126: The positive correlation between d15N and d13C is statistically robust, though moderate in strength. The authors\ncould consider discussing the implications of this moderate correlation, i.e., what other processes might contribute to the\nscatter in the relationship.\nLine 141: to avoid confusion use the term sedimentary d15N, which is also used in other literature.\nLine 146: Sentence is a bit hard to follow as a reader, consider to rearrange the deep-sea numbers in the brackets to make it\nmore intuitive to follow.\nLine 150: The distinction between coastal and deep-sea δ¹⁵N values is important. Consider referencing Fig. 1a and 1d\ndirectly here to support the discussion of depth-related trends. Do you assume that terrestrial processes occur in places of\nlow water depth and high sedimentation rate? what about high sedimentation rate places which are not driven by terrestrial\nprocesses? for example upwelling areas.\nLine 162: add reference.\nLine 164: bed —> seafloor?\nLine 167: I disagree that the IN data has a broad latitudinal range. The data is heavily skewed towards the Arctic Ocean and\nI would defer from making bold global statements in the following text about this and rather focus on regional trends.\nLine 176: give example for region?\nLine 179: ON was not defined in previous text.\nLine 195: can the authors reference a figure for that? Suppl. Fig. 4?\nLine 222: This feels a little bit disconnected from the first part. In the introduction the authors state they are interested in\nstudying anthropogenic influence on N-cycle. The motivation here is now backwards, looking at marine sediments to\nevaluate global-scale drivers of d15N. A more streamlined introduction part in all three parts of the manuscript will help the\nreader to follow why the authors chose to study those samples.\nLine 243–244: The authors report a 1.5‰ per century increase in δ¹⁵N at some sites. It would be helpful to contextualize this\nrate relative to known historical changes in nitrogen loading or productivity. Is this considered a large change?\nLine 284–285: The link between δ¹³Corg and productivity is implied but not fully explained. Consider briefly elaborating on\nthe mechanisms (e.g., increased phytoplankton biomass, eutrophication). Also, briefly mention if this is a global\nphenomenon or restricted to one particular area.\nLine 319: I question the use of “alternatively” here. Using “alternatively” implies these are two separate explanations, when\nin reality, the second sentence is more like a consequence or elaboration of the first.\nLine 343–344: The phrase “coherent declines in δ¹⁵N” could be strengthened by specifying the magnitude of decline or the\nrange observed across the ECS cores. In both Figure 5 and Supplementary Fig. 7 it is hard to see the declines in d15N. The\nauthors could consider to make an additional panel in Supplementary Fig. 7 to focus only on the sites where the decline has\nbeen observed.\nLine 396: in which core? add information in the main text (DH3-1 and where the core is located more or less).\nLine 417–418: The δ¹⁵N values of refractory OC are reported as low as −12‰. This is quite depleted—could the authors\ncomment on the potential sources or preservation mechanisms of such low values?\nLine 446: what kind of data? nitrate d15N?\nMethods:\nA table with the collected data type, number of samples and references would be useful for the reader to see what part of the\ndata is from previously published records and what data was newly generated for this study.\nLine 477: bed —> seabed or seafloor?\nLine 491: … on marine sedimentary nitrogen poor --> nitrogen pool?\nFigures:\nFigure 1:\npanel (c): what are the grey dots? indicate in figure caption.\npanel (d&e): figure caption says “environmental drivers influencing d15N bulk signals”, however, the correlation is relatively\nweak for the data set. Consider rewording to “potential environmental drivers…”\nFigure 6:\nPanel (b) includes a wide range of δ¹⁵N values. Consider adding a shaded band to indicate typical marine vs. terrestrial\nranges for context.\nwhat do the letters at the top stand for? consider removing.\nPanel (c): indicate which data is from the river basin, shelf and seafloor. refractory OC is clearly indicated.\nSuppl. Figure 7:\nTo be consistent with the other figures and to help the reader navigate through the numerous figures, I suggest to pivot the\ngraph to read the time series from left to right, similar to Fig. 3,4 and 5."}, "author_response": "Point-by-point Response to Reviewers’ Comments\nManuscript: Global patterns and drivers of nitrogen isotope signal in modern marine sediments\nAuthors: Xueshi Sun*, Dejiang Fan, Limin Hu, Houjie Wang\nReviewer comments are shown in black, and our responses are given in blue.\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nGeneral Comments:\nSun et al. have presented a wide-reaching review and synthesis of nitrogen signals\ncontained in marine sediments and, for the most part, provide solid data analysis and\ninterpretations. In addition to my line comments, I have a few more general comments:\nResponse: We thank you for the careful evaluation of our manuscript and for the recognition of\nthe breadth and rigor of our data analysis and interpretations. We grateful for the constructive\ncomments and suggestions, which have greatly helped us improve the manuscript. We have\ncarefully considered your general comments, as well as the line-by-line suggestions, and have\nrevised the manuscript accordingly to improve clarity, flow, and overall readability. In particular,\nwe have conducted additional verification experiments to compare the effects of acidification\nand oxidation on the nitrogen signal. These experiments not only help us better constrain our\ndata compilation but also provide new insights into the underlying mechanisms driving these\neffects.\nSpecific responses to each point are provided below.\n1. I am concerned that the signals in the global dataset are impacted by acid\npretreatment. I provide further rationale at the end of my line comments, but, in brief,\na significant fraction (~30%) of Supplementary Data 2 have been acid pre-treated and\nmay compromise the fidelity of practically all the data and inferences within the\nmanuscript.\nResponse: We sincerely thank you for raising this important global concern and providing these\ninsightful comments. We fully agree with your view and acknowledge that acid pretreatment\nmay influence nitrogen content and isotopic composition, and the extent of such effects remains\na subject of ongoing discussion in previous versions of the manuscript. While ~30% of sediment\nsamples in Supplementary Data 2 are acid-treated, these samples originate from diverse global\nregions, limiting direct paired comparisons.\nAccording to your valuable comments, to address this, we conducted additional analyses\non 160 paired samples with and without acid treatment (4 M HCl), spanning both terrestrial\n(Yangtze River basin) and marine (East China Sea) environments (Supplementary Fig. 1). The\nmarine dataset comprises seafloor surface sediments (N = 40), five sediment cores (N = 67),\nmarine suspended particulate matter (N = 12), and grain-size–fractionated sediments (N = 13),\nwhile the terrestrial dataset comprises riverbed sediments (N = 4) and sedimentary rock (N =\n24) samples dominated by refractory OC and associated nitrogen. Collectively, these samples\nrepresent a broad range of TN content and δ¹⁵N values, from N-poor, 15N-depleted terrigenous\nsedimentary rocks to N-rich, 15N-enriched marine suspended particles (Supplementary Tables\n8–9).\nAs shown in the following figure (now presented as Supplementary Fig. 12), we compared\nthe distributions of TN content and δ¹⁵N, as well as their differences, between the untreated and\nacidified marine and terrestrial samples (Fig. S12 a–d). We found that, compared with untreated\nsamples (i.e., non-normally distributed), the median differences in △TN (TN –TN )\nacidfied untreated\ncontent (IQR, 25%–75%) for acidified marine samples and terrestrial samples were 0.010%\n(IQR, 0.002–0.016%; N = 132) and 0.008% (IQR, 0.003–0.019%; N = 28), respectively. The\ncorresponding median differences in △ δ¹⁵N (δ15N –δ15N ) were –0.266‰ (IQR, –\nacidfied untreated\n0.561 to 0.142‰) and 0.133‰ (IQR, –0.557 to 0.469‰), respectively (Fig. S12 e–h).\nImportantly, acid treatment did not alter the trends of TN or δ¹⁵N, despite some deviations from\nthe regression fit, as evidenced by their significant (P < 0.0001) and strong (R > 0.910)\ncorrelations before and after acidified treatment. We agree with your comment and are\ncognizant of the possibility that decarbonation of sediment samples can significantly impact\ntheir absolute TN and δ15N values (P < 0.05, not for δ15N in studied terrigenous sediments),\nyet\nour studies have documented that the differences in TN (average = 0.01%, median = 0.007%)\nand δ15N (average = –0.183‰, median = –0.266‰) values between bulk and decarbonated\nmarine samples generally fall within the analytical precision (0.01% for TN and 0.3‰ for δ15N).\n(New) Supplementary Figure 12 | Comparison of total nitrogen (TN) content and δ15N between acid-treated\nand untreated samples. (a–b) Boxplots show the median (central line), mean (square), and interquartile range (box:\n25th–75th percentile) for TN and δ15N in marine (a, b) and terrestrial (c, d) samples before and after acid treatment.\n(e–h) Relationships between TNacidfied and TNuntreated, and δ15Nacidfied and δ15Nuntreated for marine (e, f) and terrestrial\n(g, h) samples, with differences represented as a color map. Insets show frequency distributions of ΔTN (TNacidified–\nTNuntreated) and Δδ15N (δ15Nacidfied–δ15Nuntreated); Data are presented as median with interquartile range (non-normally\ndistributed). Marine sediment samples comprise sediment cores (circles), seafloor surface sediments (rightward-\ntriangles), suspended particles (squares), and size-fractionated sediments (>63 μm, upward-triangles; <63 μm,\ndownward-triangles) from the East China Sea. Terrestrial samples include sedimentary rocks (circles) and riverbed\nsediments (triangles) from the Yangtze River basin. In panels (a–d), letters a and b indicate statistical groupings\n(Wilcoxon test), with different letters denoting significant differences (P < 0.05). Solid black lines show fitted\nregressions, and dashed blue lines indicate 95% confidence intervals. Detailed sampling information and the\ncorresponding data sources are provided in Supplementary Figure 1 and Tables 8–9.\nFor our studied samples, we note that ~64% of marine sediment samples (N = 84) exhibited\nmore negative △δ¹⁵N values after acid treatment. In principle, if acidification reduces mean\nTN content, it should generate a positive isotopic shift in △δ15N due to the preferential loss of\n15N-depleted 𝑁𝐻(cid:2878) adsorbed on, or fixed within, the fine clay mineral matrix (e.g., Robinson\n(cid:2872)\net al., 2012). However, a substantial proportion of marine samples (~79%, N = 104) showed\nhigher TN after treatment, contrary to the expected N loss during acidification process, despite\nthe observed negative relationship between inorganic carbon content (IC) and changes in △TN\ncontent (now Supplementary Fig. 13). In contrast, terrestrial samples displayed more consistent\npositive shifts after acidification, with increases in δ15N (~61%, N = 17) and TN (~82%, N =\n23). This indicates that acid treatment can introduce unintended alterations in both TN and δ15N,\nwith terrestrial samples (refractory OC and associated N) responding differently from marine\nsamples (dominated by labile carbon and associated N).\nWe propose that the △TN enrichment observed after acid treatment reflects the removal\nof a dilution effect caused by the carbonate minerals. Supporting this interpretation, carbonate\nmineral-rich (>90% IC of total carbon) sedimentary rocks such as limestone and marlstone (N\n= 6) also showed TN enrichment after acidification, pointing to the dilution from terrigenous\ncarbonate minerals. In marine sediment samples, marine biogenic-derived CaCO precipitation\n3\nmay also contribute: it typically incorporates organic matter (e.g., glycoproteins synthesized to\nfacilitate calcification; e.g., Ingalls et al., 2004), and the removal of such 15N-enriched organic\nmatter during acid treatment likely increase δ15N values relative to untreated marine samples\n(Schlacher and Connolly, 2014).\nTaken together, our data suggest that carbonate minerals likely dilute %TN content. Acid\ntreatment can increase TN by removing this dilution effect; however, it may also lower TN by\nremoving 15N-enriched biogenic/organic N and 15N-depleted inorganic or clay-fixed N, leading\nto opposite trends in sedimentary δ¹⁵N depending on which fraction is preferentially affected.\n(New) Supplementary Figure 13 | Effects of inorganic carbon (IC, carbonate) removal on total nitrogen (TN)\nand δ¹⁵N in marine (a) and terrestrial (b) samples. ΔTN and Δδ¹⁵N (color-filled) represent the differences in\nacidified samples relative to untreated samples. Solid lines show linear regressions, with 95% confidence intervals\n(CI) indicated by grey shading. Dashed red lines indicate the direction of acidification-induced changes,\ndistinguishing positive (>0) from negative (<0) TN values. Negative IC observed in sedimentary rock samples before\nand after acidification likely reflect their inherently low IC content and sample heterogeneity. In panel (b), the dashed\nellipse highlights six marlstone and limestone samples with exceptionally high CaCO₃ contents (>90%), which\nexhibit TN enrichment after acidification; no linear fitting was applied to these data points.\nFor both our new study and compiled datasets of global marine sediment samples, acid\ntreatments using 1–6 M HCl have generally shown limited effects on nitrogen measurements.\nPrevious studies reported no significant differences in mean TN content (for 1 M, 2 M, and 6\nM HCl) or δ¹⁵N values (for 1 M and 2 M treatments) between acid-treated and untreated marine\nsediment samples, with the mean offset for 6 M HCl remaining below 0.2‰ (Kennedy et al.,\n2005). Compiled data from marine sediments, suspended particulate organic matter, and marine\nbiological materials further support this, showing that most δ15N differ by <0.5‰ following\nacid treatment, with an average shift of only 0.16‰ (Schlacher and Connolly, 2014). Moreover,\ncomparing δ15N and TN within the same core, analyzed using identical procedures, helps\nminimize potential biases from analytical artifacts and ensures that relative patterns in TN and\nδ15N remain reliable (Meyers et al., 2019). Finally, whether the acid effect is considered\n‘significant’ depends on the specific research question (Schlacher and Connolly, 2014), as the\nshifts in both δ15N and TN induced by acid treatment are often minor relative to the large-scale\npatterns and variability of nitrogen signals in global marine sediments (Figs. 1, 3, and 4).\nDespite the number of paired analyses conducted, we acknowledge that our dataset does\nnot allow us to fully resolve the combined effects of carbonate characteristics (i.e., terrigenous\nversus marine biogenic origins), the isotopic composition of associated organic and inorganic\nnitrogen, and the susceptibility of different N-containing compounds to acid treatment (e.g.,\nKennedy et al., 2005; Walthert et al., 2010; Brodie et al., 2011; Schlacher and Connolly, 2014).\nThese processes likely interact in complex ways, influencing both the magnitude and direction\nof shifts in TN content and δ¹⁵N values. Future work incorporating compound-specific isotope\nanalyses, metal–nitrogen associations, and controlled experiments of different marine settings\nwith well-characterized carbonate mineral (terrigenous & marine) endmembers will be essential\nto disentangle these mechanisms and better constrain the extent to which carbonate-related\ndilution, isotopic fractionation, and nitrogen volatility contribute to the observed variability.\nOverall, these results reinforce the broader conclusion that, although acidification can alter\nsample chemistry through carbonate removal or alteration of nitrogen-containing compounds,\nthe magnitude of isotopic shifts is generally minor and unlikely to compromise the reliability\nof bulk TN and δ¹⁵N measurements in 15N-enriched marine sediments. After considering your\nvaluable comments, we have added a detailed description of the procedures (Lines 625–658),\nassociated figures (Supplementary Figs. 12 and 13), and the original data (%C, %TN, δ¹³C, and\nδ¹⁵N) from paired analyses of acid-treated and untreated marine and terrestrial samples to the\nSupporting Information (Supplementary Tables 8 and 9). In the main text, we have also clarified\nthe constraints of our study based on combined datasets from both acidified and untreated\nanalyses (Lines 520–530). We believe these additions ensure full transparency, allowing\nresearchers to directly assess the effects of acidification on elemental and isotopic compositions\nacross different sample types, and provide further insight into the mechanisms by which acid\ntreatment influences nitrogen in diverse marine settings.\n2. On a similar topic, I am extremely skeptical of the insights based on the refractory\ndigestion in the later portions of the paper and, chiefly, in Figure 6. All organic nitrogen\ncompounds contain carbon and the impacts of this digestion on nitrogen composition\nare, as far as I can tell, unconstrained. The original application of this refractory\ndigestion was to establish the radiocarbon age of the residual pool and it seems like\ncareful experimentation would be needed to confidently apply and interpret its effects\non total nitrogen and nitrogen stable isotopes. I also suspect its impacts on carbon\nstable isotopes are also poorly constrained.\nResponse: Many thanks for this valuable comment. As you noted, the chemical oxidation\n(refractory digestion) was originally developed to isolate residual carbon pools for radiocarbon\ndating, and that its direct application to nitrogen and stable isotope systematics is less well\nconstrained. In this study, we employed the method in an operational sense—consistent with\nits broad use in soils, riverine materials, and rocks—to differentiate refractory OC from more\nlabile marine C pools and thus to quantify the nitrogen associated with refractory organic matter.\nTo address your concern, we first conducted eleven replicate blank analyses under\nidentical protocols to those applied to refractory sediment samples. Total OC and nitrogen\nconcentrations in aqueous solution were quantified using an elemental analyzer (TOC-L,\nSHIMADZU, Japan). The mean blank values (±1σ) were 0.408 ± 0.04 ppm for OC and 0.004\n± 0.003 ppm for nitrogen, the latter close to analytical uncertainty (~0.004 ppm). These\ncorrespond to only 0.023 ± 0.01% and 0.001 ± 0.0004% of the mean OC and TN contents in\nmeasured refractory marine sediment samples (N = 75; Fig. 5c and d), respectively—orders of\nmagnitude smaller than sample values and well below the analytical uncertainties of particulate\nOC and TN. This demonstrates that blank-derived OC and TN have no significant influence on\nδ13C and δ15N measurements.\nSecondly, it is important to note that the chemical oxidation approach is designed to\ndifferentiate 13C- and 15N-depleted refractory terrestrial OC from the relatively 13C- and 15N-\nenriched labile fraction of marine origin. Consistent with this principle, our results show\nsignificant (P < 0.05) decreases in both δ15N and δ13C values for the refractory OC in marine\norg\nsediments (Fig. 5d). In other words, when the target of oxidation is inert terrestrial material, the\ntreatment is not expected to alter the isotopic composition of OC or nitrogen sediment samples,\nkeeping it within the terrestrial end-member range.\nTo further verify this assumption, we conducted comparative paired chemical oxidation\nexperiments on rock samples (N = 24) from the Yangtze River basin (Supplementary Table 10),\ndominated by graphitic refractory OC that is largely resistant to oxidation (Galy et al., 2008).\nWe found that the chemically oxidized samples exhibit comparable average values relative to\nunoxidized counterparts: OC (1.2% vs. 1.5%), TN (0.121% vs. 0.129%), δ13C (–27.1‰ vs. –\norg\n27.0‰), and δ15N (2.5‰ vs. 2.3‰). As shown in the following figure (now Supplementary Fig.\n14), although oxidation may remove a minor labile fraction, the OC and TN content, as well as\nδ13C and δ15N values of these refractory rock samples, remained statistically unchanged (P >\norg\n0.05, Wilcoxon test) and consistently fall within the terrestrial range (–25‰ to –30‰; δ15N <\n+4‰; Peterson and Howard, 1987; Fogel and Cifuentes, 1993; Goñi et al., 2008). These results\nsupport the robustness of the oxidation approach in distinguishing refractory from labile pools\nwhile minimizing isotopic artifacts. They also provide confidence that the isotopic shifts\nobserved in marine sediment samples are not analytical artifacts of the procedure, but instead\nreflect genuine differences in composition of refractory versus labile organic matter.\nAlthough this approach is applied here for the first time to disentangle the association\nbetween N and refractory OC, we argue that chemical oxidation provides a novel approach for\npartitioning the nitrogen–refractory OC coupling and their isotopic composition. Our results\nshow that the N is significantly (P < 0.001) correlated with refractory OC in marine sediments\nfrom the East China Sea (Fig. 5a), likely also in other river-influenced continental margins,\nwhich together account for majority of sedimentary total N (~68%; Supplementary Fig. 11).\nThis perspective opens new opportunities for tracing how terrestrial inputs and burial processes\nregulate the balance between labile and refractory nitrogen pools, and for assessing their imprint\non long-term carbon–nitrogen burial in marine sediments.\nBased on your valuable comments, we have incorporated the discussion of blank and\npaired analyses into the revised Methods section (Lines 668–690) and added the following\nfigure to Supporting Information (Supplementary Fig. 14). Detailed information for the paired\ndatasets of OC, TN, δ13C , and δ15N for bulk (unoxidized) and refractory (oxidized) rock\norg\nsamples is provided in Supplementary Table 10.\n(New) Supplementary Figure 14 | Comparison of organic carbon (OC, a) and total nitrogen (TN, b), and\nisotope δ13Corg (c) and δ15N (d) in bulk and refractory sedimentary rocks after chemical oxidation. Twenty-\nfour sedimentary rock samples from the Yangtze River Basin (Supplementary Fig.1), representing varied lithologies,\nwere analyzed. NS indicates no significant difference (P > 0.05, Wilcoxon test) between bulk (green circles) and\nrefractory (grey circles) samples. Box plots indicate the median (line), mean (square), interquartile range (box),\nand\nwhiskers (5th–95th percentiles). Detailed sampling information and paired datasets are provided in Supplementary\nTable 10.\nThis manuscript and its data collation effort have significant merit but also have some\nserious hurdles to overcome before being suitable for publication.\nIf the authors need any clarification, they can contact me (Jack Hutchings) directly if\ndesired at jackh@wustl.edu.\nResponse: Dr. Hutchings, we sincerely thank you for your evaluation of our manuscript and for\nrecognizing the significance of our data collation effort. We also greatly appreciate the specific\nsuggestions regarding acidification and oxidation treatments. These recommendations not only\nstrengthen our confidence in the robustness of the current study but also provide valuable\nguidance for future research directions. These suggestions will enable us to gain new insights\ninto unresolved, globally relevant questions, including the mechanisms (e.g., mineral types of\nmarine and terrigenous carbonate, and potential diluted effects) by which acidification affects\nnitrogen and δ15N in sediments.\nLine/Specific Comments:\nLine 103-105: What is meant by “significant”? I assume, statistically, given the p-value,\nbut then there is non-specific language about lateral (zonal) continuity. Using whatever\ntest was done for latitudinal variation, does zonal variability fail that test of significance?\nResponse: Thank you for pointing this out. In the manuscript, “significant” refers to statistical\nsignificance. When global sedimentary δ¹⁵N values are grouped into 1° latitude bins, a Kruskal–\nWallis test (the non-normal distributions) identifies significant latitudinal differences (P < 0.05),\nreflecting regional variability. Although overall longitudinal trends are negligible, applying the\nsame test to 1° longitude bins also reveals significant zonal differences (P < 0.05), confirming\nregional heterogeneity along both dimensions. Based on your suggestion, we have clarified the\nstatistical test used and revised the sentence as follows (Lines 120–123):\n“Globally, sedimentary δ¹⁵N exhibits statistically (Kruskal–Wallis test) significant latitudinal\nvariation at 1° bins (P < 0.05) and discrete zonal differences across ocean regions (P < 0.05),\nindicating regional heterogeneity despite minimal overall longitudinal trends (Fig. 1b).”\nLine 115: Revered = reversed?\nResponse: Based on your and other reviewers’ valuable comments, we acknowledge that the\nlatitudinal trend of δ¹⁵N depends on the direction of observation. To avoid misunderstanding,\nwe have removed “Revered” and rewritten the sentence for clarity (Lines 134–136).\nLine 122: What is the scientific significance of defining the 95% C.I. for the global data\nset? I recognize the desire to provide error, but I am wondering if we can infer anything\nspecifically from that range.\nResponse: Thank you for this insightful comment. In the previous version, the 95% C.I. values\nwere included solely to indicate the central tendency of the measured δ¹⁵N in marine seafloor\nsediments. Based on your suggestion and that of another reviewer, we have now conducted a\nstatistical assessment (D’Agostino & Pearson test) to determine whether the dataset follows a\nnormal distribution. For normally distributed data, we report the mean along with the 95% C.I.;\nfor non-normally distributed data, we instead report the median and the interquartile range (IQR,\n25–75%) as the most appropriate descriptors of central tendency (Saderne et al., 2019; Liu et\nal., 2021). Since the global marine sedimentary δ¹⁵N dataset is non-normally distributed (P <\n0.05), we have replaced the 95% C.I. with the IQR and revised this throughout the manuscript\n(Line 146). We have also clarified this in the Methods section and added the relevant references\n(Lines 694–696).\nReferences\nSaderne, V. et al. Role of carbonate burial in Blue Carbon budgets. Nat. Commun. 10, 1106 (2019).\nLiu, M., Zhang, Q., Maavara, T., Liu, S., Wang, X., & Raymond, P. A. Rivers as the largest source\nof mercury to coastal oceans worldwide. Nature Geosci. 14, 672–677 (2021).\nLine 127 / Figure 1 c,d,e: The figure shows “R”, but do you mean R-squared?\nIf R,\nshouldn’t Fig 1e have a negative R value?\nResponse: Thank you for pointing this out. The figure indeed shows “R,” and Figure 1e should\ndisplay a negative R value. We have corrected this error accordingly.\nLine 126-129: It is not immediately obvious to me why we would expect δ13C and δ15N\nto be correlated with one another. Why?\nResponse: We thank you for raising this important point. The correlation between δ¹³C and\norg\nδ¹⁵N is not expected in all cases, but it can arise in marine settings because both isotopes are\ninfluenced by overlapping source inputs and biogeochemical processes. δ¹³C largely reflects\norg\nthe relative contributions of marine versus terrestrial organic matter, while δ¹⁵N records nutrient\ncycling and utilization in the surface ocean (e.g., Fogel and Cifuentes, 1993). As the dominant\nnitrogen associated with organic matter, when these drivers act in concert, covariation between\nδ¹³C and δ¹⁵N may be observed. For instance, enhanced marine productivity can result in\norg\nisotopically heavier δ¹⁵N due to more complete nutrient utilization, while simultaneously\nshifting δ¹³C toward more marine-like enriched values (e.g., Peterson and Howarth, 1987;\norg\nFogel and Cifuentes, 1993). In contrast, greater terrestrial input tends to lower both δ¹³C and\norg\nδ¹⁵N values (Goni et al., 2008). Before this mentioned statement, we have clarified this rationale\nand added associated references in the revised manuscript to better guide the reader (Lines 153–\n158).\n“While δ¹³C and δ¹⁵N are not always correlated, covariation can occur in marine\norg\nsystems where both are influenced by overlapping sources and processes. δ¹³C reflects the\norg\nbalance between marine and terrestrial organic matter, and δ¹⁵N records nutrient cycling in the\nsurface ocean (Fogel and Cifuentes, 1993). Enhanced marine productivity can simultaneously\nenrich δ¹³C and δ¹⁵N, whereas greater terrestrial input tends to lower both (Peterson and\norg\nHowarth, 1987; Fogel and Cifuentes, 1993; Goni et al., 2008).”\nReferences\nFogel, M. L., & Cifuentes, L. A. Isotope fractionation during primary production, in Organic\nGeochemistry, edited by M. H. Engel and S. A. Macko, pp. 73–98, Springer, New York (1993).\nPeterson, B. J., & Howarth, R. W. Sulfur, carbon, and nitrogen isotopes used to trace organic matter\nflow in the salt‐marsh estuaries of Sapelo Island, Georgia 1. Limnol. Oceanogr. 32, 1,195–\n1,213 (1987).\nGoni, M. A. et al. Terrigenous organic matter in sediments from the Fly River delta‐clinoform system\n(Papua New Guinea). J. Geophys. Res. Earth Surf. 113, F01S10 (2008).\nFig 1d: I think log-transforming your depth prior to this analysis would have it be more\nsensible.\nResponse: We fully agree with your valuable advice and have log-transformed the water depth\nin revised Fig. 1d to present the results more effectively (now revised Fig. 1d, Supplementary\nFigs. 2 and 4b).\nFig 1e: There appears to be a lot of overplotting at specific sedimentation rates. Are\nthese all samples from a given core or just a rounding artefact of reported\nsedimentation rates? E.g., the set of values at a sed. rate of ~5.2 cm/yr … if these are\nfrom a single core, wouldn’t we want to average them prior to the analysis?\nResponse: We thank you for this insightful comment. As noted, the clustering in the previous\nversion reflects samples from the same core that share identical sedimentation rates. Following\nyour valuable suggestion, we have revised the data presentation by averaging the values within\neach core and displaying them mean ± SD. In addition, we have also indicated the number of\ncores and total data points in the figure to clarify the underlying dataset. Moreover, based on\nthis valuable comment, we have clarified the distinction between seafloor surface sediments\nand sediment cores in revised Fig. 1c and 1d, and Supplementary Figs. 2 and 3.\nLine 158: What is the intended meaning/implication of “unignored” here?\nResponse: We appreciate your comment. The term “unignored” was imprecise. Our intention\nwas to indicate that this process represents a non-negligible/important contributor to marine\nsedimentary nitrogen burial. We have therefore replaced “unignored” with “non-negligible”\nin the revised manuscript to improve clarity (Line 197).\nSupplementary Fig 2a: Similar to Fig 1d, depth should probably be log-transformed\nhere.\nResponse: Following your valuable suggestion, we have log-transformed the depth in revised\nthe Supplementary Fig. 2a to be consistent with Fig. 1d, which improves data visualization and\ncomparability. In addition, we have clarified that the datasets for seabed surface sediment and\ncore sediment samples are distinguished.\nLine 195: Do you show this lack of correlation somewhere? If not, I would have it in the\nsupplement.\nResponse: Thank you for this helpful suggestion. We have added the relevant figure illustrating\nthe lack of correlation to the Supporting Information (now Supplementary Fig. 3). In the revised\nmanuscript, we have also cited this figure at Line 236 to provide direct reference for readers.\nLine 203: Isn’t this somewhat tautological? Most of d15N_bulk *is* d15N_organic…\nResponse: We fully agree that δ¹⁵N largely reflects δ¹⁵N , and thus the correlation may\nbulk organic\nappear tautological at first glance. However, our intention was to quantitatively demonstrate\nthe extent of this relationship across a large global marine sediment dataset, and to highlight\nthe influence of additional nitrogen fractions (e.g., clay-bound inorganic N, terrigenous organic\nnitrogen, and post-depositional mineralization) that can cause deviations between δ¹⁵N and\nbulk\nδ¹⁵N . We have revised the sentence to clarify this point and avoid redundancy (Lines 241–\norganic\n243).\n“δ¹⁵N closely tracks δ¹⁵N (R = 0.796, P < 0.001, n = 578; Fig. 2g), with deviations\nbulk ON\nlikely driven by clay-bound inorganic nitrogen (Robinson et al., 2012), terrigenous organic\nnitrogen (Schubert and Calvert, 2001), and organic nitrogen mineralization (Möbius, 2013).”\nLine 206: If you mass-balance d15N_inorganic and d15N_organic by their respective\namounts, how faithfully would d15N _bulk be recomputed?\nResponse: In principle, if both the mass fractions and δ¹⁵N values of organic nitrogen (ON) and\ninorganic nitrogen (IN) are known, δ¹⁵N can be reconstructed as a mass-weighted average of\nbulk\nthe two components. In fact, to discern the contributions of IN and ON in marine sediment\nstudies, researchers have adopted various methodologies, both direct and indirect, to accurately\nquantify and analyze their content and isotopic compositions. The indirect methods included to\nmeasure the TN and δ15N , determining IN content and δ15N in residual sediment after ON\nbulk IN\nremoval by wet oxidation and thus estimates ON content and δ15N using mass and isotope\nON\nbalance equations (e.g., Schubert and Calvert, 2001; Freudenthal et al., 2001; Knies et al.,\n2007). Direct methods have also been used, for example, isolating organic matter with\npolystyrene divinylbenzene polymer columns during HF-HCl digestion to determine δ¹⁵N\nON\n(e.g., Ye et al., 2024; Feng et al., 2024). In practice, global datasets rarely provide the necessary\nquantitative partitioning of ON and IN for direct analysis at sufficient precision to perform full\nmass-balance reconstructions, and may be complicated by diagenetic transformations. Based\non your valuable comments, and to avoid any confusion, we have now clarified this approach\nin Method section for the compiled ON and δ¹⁵N datasets (Lines 561–565).\nON\n“The contribution and isotopic composition of ON were determined by indirect mass–\nbalance calculations from TN and IN isotopic compositions after ON removal with KOBr–KOH\nsolution (e.g., Schubert and Calvert, 2001; Freudenthal et al., 2001; Knies et al., 2007), and\nby direct δ¹⁵N measurements following organic matter isolation with HF–HCl digestion and\nON\npolymer columns (e.g., Ye et al., 2024; Feng et al., 2024)”\nLine 280-283: Is there any way via mass balance to back out the percent contribution\nof runoff with enriched d15N_nitrate from these trends? I suspect a first-order estimate\nis possible.\nResponse: We thank you for this insightful suggestion. In principle, a mass-balance approach\ncould provide a first-order estimate of the contribution of enriched δ¹⁵N from runoff. Here,\nnitrate\n15N-riched waste water from runoff, which also contributes to marine nutrient cycling, may\npartially explain the observed increase in sedimentary δ¹⁵N over the past century. However, our\ndataset lacks the quantitative constraints required to perform such a calculation, including\nabsolute nitrate inputs across coastal zones, their temporal variability, δ¹⁵N compositions, and\nthe relative contributions of other nitrogen sources (e.g., atmospheric deposition, N fixation, in\nsitu biological recycling, or terrigenous particles). Furthermore, diagenetic transformations and\nwater-column processes, such as phytoplankton uptake and denitrification, further complicate\nthe direct attribution of δ¹⁵N to specific runoff sources. Nevertheless, as the you correctly\nbulk\npoint out, a mass-balance approach may be feasible in more constrained marine systems where\nend-member δ¹⁵N signatures and source fluxes are well characterized. We acknowledge that\nthis valuable advice will guide future studies aiming to quantitatively disentangling the drivers\nand source contributions of δ¹⁵N variations in global marine sediments.\nLine 317: Module = modulate?\nResponse: The word “module” has been revised to “modulate”. (Line 362)\nFigure 6c: The C:N of refractory OC is exceptionally low… I guess this is an artefact of\nthe chemical digestion, but I cannot think of a “real” sizeable pool of organic matter\nwhose C:N is 3-4.\nResponse: Thank you for pointing this out. We agree with your observation that the refractory\nOC shows exceptionally low C:N ratios (wt. %), but we suggest that this is not an artefact of\nchemical digestion. Instead, it reflects contributions from 13C-depleted terrestrial sources,\nparticularly refractory OC derived from sedimentary rocks (petrogenic/fossil origin). To assess\nthe effects of acid treatment and oxidation, we analyzed bedrock samples from the Yangtze\nRiver basin, supporting for our hypothesis, which show consistently low C:N ratios (2–6; mean\n± SD = 3.6 ± 1.3; n = 16) across lithologies including shale, sandstone, siltstone, marlstone, and\nsiliceous limestone (now Supplementary Table 8). These values are comparable to previously\nreported results for bedrock samples from Taiwan (Hilton et al., 2009), including shale, black\nshale, sandstone, mudstone, schist, and slate, which also typically show lower C:N ratios in the\nrange of 2–8.\nWe thank you for this valuable comment, which allowed us to provide the direct evidence\nsupporting the contribution of refractory OC from terrestrial and petrogenic sources to the river-\ninfluenced continental margins. This clarification strengthens the interpretation why lower C/N\nratios observed in refractory OC and highlights the importance of accounting for such sources\nwhen assessing sedimentary nitrogen composition in coastal ocean settings. We have included\nthe comprehensive OC and N data of rocks in the Supporting Information (Supplementary Table\n8) and provided an explanation, supported by the reference, for the low C:N ratios observed in\nrefractory OC in marine sediments. (Lines 455–460)\n“Low C/N ratios in refractory OC likely reflect 13C-depleted terrestrial inputs, particularly\npetrogenic OC from the sedimentary rocks of the Yangtze River basin (shale, sandstone/siltstone,\nmarlstone, limestone), which inherently exhibit low C/N ratios (2–6; Supplementary Table 8),\nconsistent with previously bedrock data from Taiwan (Hilton et al., 2009).”\nReference\nHilton, R. G., Galy, A., Hovius, N., Horng, M. J., & Chen, H. The isotopic composition of particulate\norganic carbon in mountain rivers of Taiwan. Geochim. Cosmochim. Acta, 74, 3164-3181\n(2010).\nTable S3/Supplementary Data 1: I see that we should have paired before/after\nrefractory digestion experiment data here that I believe forms the basis of Fig 6d,\nhowever Table S3 has ages rounded to the nearest year whereas Supp. Data 1\nincludes decimals that makes pairing those data (if they represent paired\nmeasurements) hard/impossible.\nResponse: Thank you for pointing this out. Following your suggestion, we have revised Fig. 6d\n(now Fig. 5d) to clearly display the paired before/after digestion measurements. Regarding the\nage data, although decimals were originally retained to reflect the ages derived from ²¹⁰Pb\nand/or ¹³⁷Cs sedimentation rates, we recognize that this may cause confusion when comparing\ndatasets. To improve consistency and consider the dating approach precision at year-scale, we\nhave removed the decimal formatting from the updated Supplementary Data 1. In addition, to\nfacilitate direct comparison for refractory and bulk nitrogen sediment data, we have added the\ncorresponding bulk OC, total nitrogen content, and isotopes data to revised Tables S4–S7 for\nthe studied cores C0702, C0803, CJK, and DH3-1.\nSupplementary Fig 10: I am pretty unconvinced by this plot. It is fairly well known that\nacid treatment of any kind alters both the TN and d15N composition of samples. We\ncan see it in your plot and I can see it by quickly plotting up some of your\nSupplementary Data File 2 and grouping by either untreated or acid-treated:\n(attached)\nThe left set of plots contains all of the data and the right set of plots includes only data\nwithin two standard deviations of the median. In all cases, I’ve plotted standard\nboxplots (median center bar) and a gaussian kernel density function. I’d argue we\ncan cleanly see some deviation between treatments. Now, it is possible that the two\ngroupings (untreated and acidified) come from distinct locations on the planet and the\ndifferences are real, but your sample sizes are quite large and I suspect we are seeing\nartefacts related to acidification. Besides, there is a rich literature on the impacts of\nacidification pretreatment on nitrogen values during elemental analysis.\nResponse: We thank you for this insightful comment and for carefully re-plotting our data. We\nfully acknowledge that acid treatment can alter both TN and δ¹⁵N values, and we agree that\nsuch effects are well-documented in literature (e.g., Schlacher & Connolly, 2014; Walthert et\nal., 2010; Brodie et al., 2011; Kennedy et al., 2005). Our original intention with Supplementary\nFig. 10 was to provide a preliminary overview of paired data, but we agree that the presentation\nmay not fully convey the magnitude and nature of potential treatment effects. To address this\nconcern, we have undertaken the following steps in the revised manuscript:\nFirst, we have expanded paired analysis: We explicitly separate acid-treated and untreated\nsamples for all relevant sedimentary materials, including terrestrial (riverbed sediments and\nsedimentary rocks) and marine samples (seabed surface sediments, sediment cores, suspended\nparticulate matter, and grain size fractions). In total, 160 paired measurements are analyzed.\nSecond, we provide statistical tests (e.g., paired t-tests or Wilcoxon tests as appropriate) to\nevaluate systematic differences between treated and untreated samples for TN, %C, C/N, and\nδ¹⁵N. Supplementary Fig. 10 has been revised (now Supplementary Fig. 12) to include boxplots\nwith frequency distributions of △TN (TN −TN ) and △δ15N (δ¹⁵N −δ¹⁵N )\nacidifie untreated acidified untreated\nestimates, highlighting the median differences and variability across treatments (non-normal\ndistributions).\nWe found that acid treatments of terrestrial and marine samples did not lead to consistent\nincreases or decreases in TN content or δ¹⁵N values (Supplementary Fig. 13). This variability\nlikely results from multiple interacting factors, including the dilution effect of terrestrial\ncarbonates, the removal of 15N-enriched nitrogen associated with marine biogenic carbonates,\nand the loss of 15N-depleted nitrogen from clay-associated fractions. While some systematic\nsignificant shifts were observed for our studied samples, these effects were generally small\n(median differences <0.01% for TN and <0.3‰ for δ¹⁵N) and fall within analytical precision\ndespite large larger acid effects can occur, indicating that acid treatment introduces only\npredictable changes without compromising the overall reliability and direction of the data.\nOur results using 4 M HCl are consistent with those of Kennedy et al. (2005), who found\nno significant differences in mean TN between acid-treated (1M, 2M, and 6 M HCl) and non-\nacidified samples, also for δ15N in 1 M and 2 M HCl treatments, and only minimal changes in\nδ¹⁵N for 6 M HCl treatment (mean difference <0.2‰). Similarly, Schlacher and Connolly (2014)\ncompiled literature data from sediments, suspended particulate matter, and marine biological\nmaterials, concluding that in two-thirds of comparisons, δ¹⁵N differed by less than 0.5‰ (mean\nδ¹⁵N difference for all datasets <0.2‰). Additionally, Walthert et al. (2010) showed that\nalthough some individual samples fell slightly outside the defined precision bands, the overall\ndifferences in TN content were small for untreated (0.5–10.1 g kg⁻¹) versus acid-fumigated\nsamples (0.5–10.4 g kg⁻¹ for 8h and 0.5–10.7 g kg⁻¹ for 32h, under same 37% HCl). Although\ninconsistencies exist between acidified and non-acidified treatments of TN and δ¹⁵N in global\nmarine sediments, relying solely on non-treated datasets is insufficient to capture their full\nglobal distribution. While some biases may be introduced by different treatment methods (e.g.,\nSchlacher & Connolly, 2014; Walthert et al., 2010; Brodie et al., 2011; Kennedy et al., 2005),\nthe resulting errors are generally within an acceptable range relative to the spatial and temporal\nvariability of the sedimentary nitrogen signal and do not compromise the overall interpretation\nof large-scale TN and δ¹⁵N patterns.\nWe also acknowledge that our acidification control analyses primarily focused on limited\nterrestrial materials and estuarine and nearshore areas influenced by riverine inputs. However,\ncarbonate sources and composition vary substantially across marine regions, and the effects of\nterrestrial supply versus marine inputs, as well as their associated fractions and biogeochemical\nprocesses, on nitrogen are highly variable for different acidified treatment conditions. Future\nstudies should extend these investigations across latitudinal and environmental gradients, from\nthe coastal zone to the deep-sea setting. Such efforts would not only help validate the effects of\nacidification treatments on the marine nitrogen cycle but also provide a broader reference for\nunderstanding nitrogen dynamics under future ocean acidification scenarios.\nAccording to your valuable comments, we have added the paired analysis data of acidified\nand untreated samples, along with the corresponding figures, to revised Supporting Information\n(Supplementary Figs. 12–13 and Tables 8–9). In addition, we have incorporated a description,\nthe mentioned references and discussion of these results into the Methods section and the main\ntext of the revised manuscript for improved clarity (Lines 520–530, 625–658). We thank you\nonce again for your insightful comments.\nReferences\nSchlacher, T. A., & Connolly, R. M. Effects of acid treatment on carbon and nitrogen stable isotope\nratios in ecological samples: A review and synthesis. Methods Ecol. Evol. 5, 541–550 (2014).\nWalthert, L. et al. Determination of organic and inorganic carbon, δ13C, and nitrogen in soils\ncontaining carbonates after acid fumigation with HCl. J. Plant Nutr. Soil Sci. 173, 207–216\n(2010).\nBrodie, C. R. et al. Evidence for bias in C and N concentrations and δ13C composition of terrestrial\nand aquatic organic materials due to pre-analysis acid preparation methods. Chem. Geol. 282,\n67–83 (2011).\nKennedy, P., Kennedy, H., & Papadimitriou, S. The effect of acidification on the determination of\norganic carbon, total nitrogen and their stable isotopic composition in algae and marine\nsediment. Rapid Commun. Mass Spectrom. 19, 1063–1068 (2005).\nIngalls, A. E., Aller, R. C., Lee, C., & Wakeham, S. G. Organic matter diagenesis in shallow water\ncarbonate sediments. Geochim. Cosmochim. Acta. 68, 4363–4379 (2004).\nMeyers, P. A., Poli, M. S., & Thunell, R. C. Similar Glacial‐Interglacial δ15N Variations in Two MIS\n13–10 Sediment Sequences in the Western North Atlantic Ocean: Changes in Nitrogen Sources,\nDenitrification, or Diagenesis?. Paleoceanogr. Paleoclimatol. 34, 2171–2182 (2019).\nGaly, V., Beyssac, O., France-Lanord, C., & Eglinton, T. Recycling of graphite during Himalayan\nerosion: A geological stabilization of carbon in the crust. Science, 322, 943–945 (2008).\nReviewer #2 (Remarks to the Author):\nSun et al present a comprehensive global synthesis of nitrogen isotope (δ¹⁵N) data\nfrom modern marine sediments, including over 8,200 surface and core-top samples\nand 148 dated sediment cores spanning the past century. The authors identify\nspatial patterns and centennial-scale trends in δ¹⁵N, linking them to anthropogenic nitrogen\ninputs, productivity changes, and sediment dynamics. The study highlights the dual\nrole of marine and terrestrial processes in shaping sedimentary δ¹⁵N signals, with\nparticular emphasis on the contrasting trends observed in coastal versus river-\ndominated continental margins.\nThis is an important and timely contribution to our understanding of how anthropogenic\nand climatic drivers have reshaped the marine nitrogen cycle over the past century.\nThe global scope, high-resolution temporal data, and integration of inorganic and\norganic nitrogen forms provide a robust framework for interpreting δ¹⁵N variability.\nHowever, the manuscript would benefit from streamlining and clearer separation\nbetween the introduction, results, and discussion sections. The figures are generally\nclear and informative, though their large number makes it challenging for readers to\nnavigate and identify the central message.\nI recommend this paper for publication in Nature Communications with major revisions.\nResponse: Many thanks for your thoughtful and constructive evaluation of our manuscript.\nWe greatly appreciate the recognition of the significance of our study, including its global scope,\nhigh-resolution temporal data, and the integration of inorganic and organic nitrogen forms,\nas well as the emphasis on the dual role of marine and terrestrial processes in shaping sedimentary\nδ¹⁵N signals. Regarding the suggestions for improvement: We have revised the manuscript to\nimprove clarity and flow. Specifically, the Introduction now focuses on setting the context and\nresearch objectives. We have reorganized the figures by grouping related panels and moved\nsome less critical figures (Fig. 6) to the Supplementary Information to help readers focus on the\ncentral messages. Further detailed responses and modifications are provided in the following\ncomments.\nGeneral comments:\nThe manuscript would benefit from a concise methods summary and a table outlining\nwhich datasets are newly generated and which are compiled from previous studies.\nThis would help readers quickly grasp the scope and novelty of the work.\nResponse: We thank you for this helpful suggestion. We fully agree with your advice and have\nadded a concise summary of the methods and a new table (Supplementary Table 1) that clearly\ndistinguishes newly generated datasets from those compiled from previous studies. The table\nsummarizes the study regions, dataset types, water depth, main variables (e.g., sample/core ID,\ngeographic coordinates, C, N, and stable isotopes), number of data points (and core numbers),\nand data sources (literature data from this study), enabling readers to quickly grasp the scope\nand novelty of our work. To further clarify, all datasets beyond the Yangtze River basin and\nEast China Sea are derived entirely from previously published studies.\nFollowing your valuable suggestion, we have also added a Data Sources section providing\na brief overview of compiled literature and new measurements (Lines 532–551), followed by\ndetailed descriptions: “Data sources: Our database was compiled from more than 160 peer-\nreviewed publications and complemented by newly generated datasets, providing a framework\nto investigate the spatial and temporal variability of nitrogen characteristics in marine\nsediments at regional to global scales. The compilation includes seafloor surface sediments\nand primarily 137Cs/210Pb dated sediment cores, with parameters selected to capture both\nnitrogen cycling and sedimentary context. Key variables include total, organic, and inorganic\nnitrogen contents and isotopes, together with water depth, sedimentation rate, grain size, OC\ncontent, and δ¹³C . Note that all datasets outside the Yangtze River basin and the East China\norg\nSea are derived from previously published sources.\nTo better constrain the drivers of modern sedimentary δ¹⁵N signals in river-dominated\ncontinental margins, we present new nitrogen measurements from six sediment cores (C0702,\nC0803, DH3-1, DH3-2, DH5-1, and DH6-1) recovered from the East China Sea. These include\nbulk sediment analyses, size-fractionated samples from core DH3-1, and refractory OC\nfractions from cores C0803, CJK, and DH3-1. In addition, we conducted paired analyses on\nterrestrial samples (sedimentary rocks and riverbed sediments) and marine samples (seabed\nsurface sediments, sediment cores, size-fractionated fractions, and suspended particulate\nmatter) to assess the effects of acidification and, for rocks only, chemical oxidation on nitrogen\nsignals. An overview of both literature-derived and newly generated datasets is provided in\nSupplementary Table 1.”\nThe structure of the manuscript feels somewhat fragmented. The first part (global\ncompilation) appears disconnected from the second and third parts, which follow a\nmore logical progression, first examining centennial δ15N trends and then focusing on\na case study of the Yangtze River system. The authors may consider whether the\nmanuscript could be split into two complementary papers: one focused on global\npatterns and drivers of δ15N, and the other on historical changes under\nanthropogenic influence.\nResponse: We thank you for this thoughtful and constructive comment. We agree that the\nmanuscript consists of three distinct but interconnected components: (i) a global compilation\nof marine sedimentary δ¹⁵N data, (ii) an analysis of centennial-scale δ¹⁵N and nitrogen trends,\nand (iii) a detailed investigation of the Yangtze River–East China Sea system. Our rationale for\nintegrating these components into a single manuscript is to first establish a global baseline of\nδ¹⁵N patterns, then examine how these signals have evolved over the past century, and finally\ntest the mechanistic insights in a representative river-dominated continental margin.\nThe first component (i) was designed to document the global spatial distribution of δ¹⁵N\nand to evaluate its primary drivers (e.g., water depth, modern sedimentation rates, and marine\nproductivity). By comparing δ¹⁵N with organic and inorganic nitrogen fractions, we confirmed\nthat the large-scale patterns are predominantly controlled by marine processes. Building on this\nframework, we then reconstructed centennial time series of δ¹⁵N and N content from sediment\ncores (ii), which allowed us to investigate the imprint of anthropogenic perturbations on the\nsedimentary nitrogen signals. This analysis revealed the contrasting responses of sedimentary\nδ¹⁵N in river-dominated continental margins compared with other coastal systems.\nTo further resolve these contrasts, we examined the Yangtze River–East China Sea system\n(iii), one of the world’s largest river-dominated margins and thus an ideal natural laboratory for\ntesting the global patterns against local processes. This regional study enabled us to identify a\npreviously underappreciated mechanism: the increased supply of ¹⁵N-depleted refractory OC\nfrom terrestrial sources, rather than the exclusively marine processes such as N₂ fixation or\ndenitrification, as a key driver of the declining δ¹⁵N trends observed in river-influenced margins.\nAccording to your valuable comments, we have revised the manuscript to make this three-\npart structure clearer and to strengthen the logical connections between them. Specifically, we\nclarified the rationale for the integrated framework in Introduction (highlighting how coastal\nsystems have responded to historical anthropogenic perturbations, including altered riverine N\ninputs and marine N processes), streamlined transitions between these sections (e.g., from the\nglobal spatial patterns to centennial trends: “While the global synthesis establishes a baseline\nof spatial variability in sedimentary δ¹⁵N, the extent to which these signals have evolved or\nremained stable in response to anthropogenic and climatic forcing remains poorly\nconstrained.”), and reduced redundancy to improve readability. We believe that presenting\nthese components together provides a more comprehensive and impactful understanding of\nsedimentary nitrogen dynamics than if they were divided into two separate papers, as it directly\nlinks global-scale patterns to regional processes under anthropogenic influence.\nGiven the broad readership of Nature Communications, the introduction could be\nexpanded to better contextualize the role of rivers in the nitrogen cycle, how these\nsystems have changed over the past century, and what the key drivers of those\nchanges are.\nResponse: Thank you for this valuable suggestion. Considering the broad readership of Nature\nCommunications, we have expanded the Introduction to better contextualize the role of rivers\nin the nitrogen cycle, highlighting how riverine nitrogen export has changed over the past\ncentury and the key drivers of these changes. Global riverine nitrogen export to the coastal\nocean has approximately doubled since the pre-industrial period (from the 1900s), with human\ncontributions increasing mainly due to agricultural intensification, land-use change, and sewage\ninputs. These anthropogenic perturbations represent a major global challenge, significantly\nimpacting water quality and biodiversity. Elevated fluxes of ammonium, nitrate, and dissolved\norganic nitrogen have raised nutrient concentrations in coastal waters, often leading to hypoxia.\nAccordingly, the following text has been added to the Introduction (Lines 34–42):\n“Global riverine nitrogen export to coastal systems roughly doubled from ~20 Tg N yr⁻¹\nto ~42 Tg N yr⁻¹ between 1900 and 2010 (Green et al., 2004; Wang et al., 2024). The human-\ninduced fraction increased from ~26% to ~43% (Bouwman et al., 2005), primarily driven by\nagricultural intensification, land-use change, and sewage inputs (Bouwman et al., 2005;\nSchlesinger et al., 2006). This excess nitrogen has substantially altered coastal ecosystem\nstructure and function, driving shifts in community composition (e.g., macrophytes to fast-\ngrowing algae, diatoms to flagellates), reducing water quality, depleting oxygen, and fostering\nharmful algal blooms and eutrophication (Herbert, 1999; Howarth, 2008; Conley et al., 2009;\nVoss et al., 2011; Hutchins and Capone, 2022).”\nReferences\nGreen, P. A., Vörösmarty, C. J., Meybeck, M., Galloway, J. N., Peterson, B. J., & Boyer, E. W. Pre-\nindustrial and contemporary fluxes of nitrogen through rivers: a global assessment based on\ntypology. Biogeochemistry, 68, 71–105 (2004).\nWang, J., Bouwman, A. F., Vilmin, L. et al. Global inland-water nitrogen cycling has accelerated in\nthe Anthropocene. Nat Water 2, 729–740 (2024).\nBouwman, A. F., Van Drecht, G., Knoop, J. M., Beusen, A. H. W. & Meinardi, C. R. Exploring\nchanges in river nitrogen export the world’s oceans. Global Biogeochem. Cycles 19, GB1002\n(2005).\nSchlesinger, W. H., Reckhow, K. H., & Bernhardt, E. S. Global change: The nitrogen cycle and rivers.\nWater Resour. Res. 42，W03S06 (2006).\nHerbert, R. A. Nitrogen cycling in coastal marine ecosystems. FEMS Microbiol. Rev. 23, 563–590\n(1999).\nHowarth, R. W. Coastal nitrogen pollution: a review of sources and trends globally and regionally.\nHarmful Algae, 8, 14–20 (2008).\nConley, D. J. et al. Controlling eutrophication: nitrogen and phosphorus. Science 323, 10141015\n(2009).\nVoss, M et al. Nitrogen processes in coastal and marine systems. In The European nitrogen\nassessment (eds MA Sutton, CM Howard, JW Erisman, G Billen, A Bleeker, P Greenfelt, H Van\nGrinsven, B Grizzetti), Cambridge, UK: Cambridge University Press, pp. 147–176 (2011).\nHutchins, D. A., & Capone, D. G. The marine nitrogen cycle: new developments and global change.\nNat. Rev. Microbiol. 20, 401–414 (2022).\nOne of the key outputs of this study is the extensive δ15N dataset. I encourage the\nauthors to consider also publishing the data compilation, as it would be a valuable\nresource for the community.\nResponse: We appreciate your excellent suggestions. We agree that the extensive sedimentary\nδ¹⁵N dataset compiled in this study represents a valuable resource for the community. In the\nrevised submission, we have included the δ¹⁵N dataset as Supplementary Tables 3–10, and have\npublished the global δ¹⁵N dataset to Mendeley Data repository (Sun, X.S., “Supplementary Data\n| Global patterns and drivers of nitrogen isotope signal in modern marine sediments,”\nMendeley Data, https://data.mendeley.com/datasets/7tjynvbm58/2) which is cited in the Data\nAvailability section. The detailed metadata—including study region, core/site information,\nwater depth, variables, and references—are provided to facilitate reuse by other researchers.\nComments Within Text:\nLine 19–24 (Abstract): The phrase “coinciding with nitrogen content enrichment” could\nbe clarified. Does this refer to increased organic nitrogen, inorganic nitrogen, or both?\nResponse: Thank you for pointing this out. We agree that the original phrasing was ambiguous.\nWe have revised the sentence (Line 20) to clarify that the observed enrichment refers to “total\nnitrogen”, encompassing both organic and inorganic forms.\nLine 35–41: Given the broad readership of Nature Communications, can the authors\nelaborate on the pressures/consequences of increase in N-export to coastal systems,\nas well as state a consequence for reduced flux of fluvial particulates with respect to\nmarine biodiversity, C-storage and climate feedbacks?\nResponse: Thank you for this valuable comment. We have revised the text (Lines 38–42) to\nmore clearly explain the ecological and biogeochemical consequences of increased nitrogen\nexport and reduced fluvial particulate fluxes with respect to biodiversity, carbon storage, and\nclimate feedbacks. Specifically, we now describe how excess nitrogen alters coastal ecosystem\nstructure, reduces biodiversity, and promotes eutrophication and hypoxia; how these changes\naffect organic carbon burial dynamics; and how altered sediment delivery and remineralization\ninfluence CO₂ fluxes and climate feedbacks. It now reads as:\n“This excess nitrogen has substantially altered coastal ecosystem structure and function,\ndriving shifts in community composition (e.g., macrophytes to fast-growing algae, diatoms to\nflagellates), reducing water quality, depleting oxygen, and fostering harmful algal blooms and\neutrophication (Herbert, 1999; Howarth, 2008; Conley et al., 2009; Voss et al., 2011; Hutchins\nand Capone, 2022).”.\n“Although eutrophication and deoxygenation can enhance carbon preservation, long-term\nsedimentary carbon burial is closely tied to sediment delivery (Keil, 2017). A decline in global\nfluvial particulate input reduces marine sediment accumulation (Syvitski et al., 2022), increases\noxygen exposure and organic matter remineralization (Galy, et al., 2007; Blair & Aller, 2012),\nand consequently diminishes sedimented carbon storage, potentially enhancing CO₂ emissions\nto the atmosphere and generating negative climate feedbacks.” (Lines 44–50)\nReferences\nKeil, R. Anthropogenic forcing of carbonate and organic carbon preservation in marine sediments.\nAnn Rev Mar Sci. 9, 151–172 (2017).\nSyvitski, J. et al. Earth’s sediment cycle during the Anthropocene. Nat. Rev. Earth Environ. 3, 179–\n196 (2022).\nGaly, V., France-Lanord, C., Beyssac, O., Faure, P., Kudrass, H., & Palhol, F. Efficient organic\ncarbon burial in the Bengal fan sustained by the Himalayan erosional system. Nature, 450,\n407–410 (2007).\nBlair, N. E., & Aller, R. C. The fate of terrestrial organic carbon in the marine environment. Ann\nRev Mar Sci. 4, 401–423 (2012)\nLine 73-75: “such unresolved challenges” and “to address this knowledge gap” sounds\na bit vague.\nResponse: Thank you for this valuable advice. We have revised the sentence “such unresolved\nchallenges…” to read: “The complexity of nitrogen isotope cycling complicates the use of bulk\nδ¹⁵N as a robust tracer of modern global nitrogen biogeochemical cycles.”. (Lines 82–84). In\naddition, we have replaced “To address this knowledge gap” with the more concise and clear\nphrasing “In this study” (Line 85).\nLine 83: “to assess environmental perturbations”. Are the authors looking for improving\nthe robustness of bulk sedimentary δ15N or investigate environmental perturbations or\nboth?\nResponse: Thank you for pointing this out. In this study, we first determine the sources of total\nnitrogen by examining both organic and inorganic nitrogen fractions and their isotope\ncompositions, which allows us to establish that sedimentary δ¹⁵N predominantly reflects the\nsignal of organic nitrogen and the drivers of its spatial variability. Based on this framework, the\ntemporal patterns of δ¹⁵N in sediment cores can then be used to investigate environmental\nperturbations. To avoid any ambiguity, we have revised the sentence in Lines 90–95 as follows:\n“We incorporated data on inorganic and organic nitrogen to identify the sources of\nsedimentary total nitrogen and the drivers of spatial variability in bulk δ¹⁵N. Subsequently, we\nconstructed 148 δ¹⁵N and 120 total nitrogen time series from dated marine sediment cores to\ninvestigate their responses to environmental perturbations.”\nLine 98: do you mean seabed?\nResponse: Yes, we have revised “bed” to “seabed” for greater clarity. (Line 114)\nLine 99: it is unclear to the reader if the data was generated by the authors or if this is\nit a data compilation from existing data?\nResponse: We thank you for pointing this out. We realize that the original wording may have\nbeen unclear. In fact, the sedimentary δ¹⁵N data were almost entirely compiled from previously\npublished studies, rather than generated by the authors. To make this explicit, we have revised\nthe text (Lines 113–115) as follows:\n“By synthesizing δ¹⁵N measurements from previously published studies of modern seafloor\nsediments, …”\nLine 102: also add nitrate data from Fripiat et al., 2021 (Nature Geoscience).\nResponse: We fully agree and have incorporated the suggested reference to Fripiat et al. (2021)\nin the revised manuscript. (Line 119)\nReference\nFripiat, F. et al. Nitrogen isotopic constraints on nutrient transport to the upper ocean. Nat. Geosci.\n14, 855–861 (2021).\nLine 103–107: The authors mention latitudinal variability in δ¹⁵N but could clarify\nwhether this variability is consistent across different ocean basins or if regional\nanomalies exist.\nResponse: We fully agree with your view that clarification is needed. Latitudinal variability in\nδ¹⁵N is generally consistent across ocean basins, reflecting large-scale gradients in nutrient\nutilization associated with phytoplankton uptake (Altabet and Francois, 1994). Nevertheless,\nregional anomalies occur in regions influenced by strong upwelling (e.g., eastern boundary\ncurrents; Studer et al., 2021), oxygen minimum zones (e.g., eastern tropical Atlantic and Pacific;\nDeutsch et al., 2014), and riverine inputs (e.g., major river-dominated continental margins;\nFeng et al., 2024). We have revised the text (Lines 127–129) to explicitly indicate these regions\nand clarify the observed patterns.\nLine 113: Southern Ocean is not an area of N-fixation. The N-isotope gradient is due\nto different nutrient uptake efficiency by phytoplankton and the amount of nutrients\navailable across the (sub)polar front as mentioned further down in the text, add\nref. (Sigman et al., 2000, Journal of Geophysical Research Oceans).\nResponse: Thank you for pointing this out. We fully agree that the Southern Ocean is not a\nregion of N₂ fixation. Recent evidence (Gu et al., 2025) also supports limited N₂ fixation in this\narea. We also concur that the observed N-isotope gradient reflects differences in nutrient uptake\nefficiency by phytoplankton and nutrient availability across the (sub)polar front (Sigman et al.,\n2000). Following your valuable suggestion, we have added the references Sigman et al. (2000)\nand Gu et al. (2025) and revised the text for clarity (Lines 134–140):\n“In contrast, high-latitude regions such as the Antarctic/subantarctic (south of 45°S) and\nthe western Pacific (~45°N) display ¹⁵N-depleted signatures. Although this pattern appears\nconsistent with previous reports of intense N₂ fixation in phosphorus- and iron-limited waters\n(Deutsch et al., 2001; Pichevin et al., 2009; Sohm et al., 2001; Sherwood et al., 2014), it more\nlikely reflects the mixing of low-δ¹⁵N nitrate, terrestrial inputs, and variable nutrient utilization\n(e.g., limited N₂ fixation in the Southern Ocean; Sigman et al., 2000; Gu et al., 2025).”\nReferences\nSigman, D. M., Altabet, M. A., McCorkle, D. C., Francois, R., & Fischer, G. The δ15N of nitrate in\nthe Southern Ocean: Nitrogen cycling and circulation in the ocean interior. J. Geophys. Res.\nOceans, 105, 19,599–19,614 (2000).\nGu, S., Berthelot, H., Lin, Y., Tang, W., Robidart, J., Eren, A. M., ... & Cassar, N. Evidence of limited\nN fixation in the Southern Ocean. Commun. Earth Environ. 6, 264 (2025).\nobservation.\nResponse: Thanks! We agree that the original phrasing was ambiguous. Our intention was to\ndescribe that δ¹⁵N values are higher in the Arctic (>75°N), subarctic (~55°N), and mid-latitude\ntransition zones (30–45°S) relative to adjacent lower-latitude regions. To avoid any confusion,\nwe have revised the sentence as follows (Lines 140–142):\n“In the Arctic (>75°N), subarctic (~55°N), and mid-latitude transition zones (30–45°S),\nδ¹⁵N values are elevated compared to neighboring latitudes and display greater variability.”\nLine 122: The median cannot logically fall outside the reported confidence interval. If\nthe CI is 5.7–5.8‰, then the median must be at least 5.7‰, not 5.5‰, was that a typo?\nResponse: Thank you for carefully noting this inconsistency. You are correct that the median\ncannot logically fall outside the reported confidence interval. Upon revisiting the data, we\nrealized that the use of the 95% confidence interval of the mean was inappropriate, given that\nthe tested distribution of δ¹⁵N values is not normally distributed (the D'Agostino-Pearson test).\nTo provide a more robust and representative measure of central tendency, we have revised our\nreporting to use the median along with the interquartile range (IQR) 25th–75th percentiles\n(Saderne et al., 2019; Liu et al., 2021), rather than the 95% confidence interval of the mean.\nAccordingly, we have corrected the values to “…a global median of 5.5‰ (interquartile range,\nIQR: 4.3–6.8‰)”. (Line 146). We have also clarified this in the Methods section and added the\nrelevant references (Lines 694–696).\nReferences\nSaderne, V. et al. Role of carbonate burial in Blue Carbon budgets. Nat. Commun. 10, 1106 (2019).\nLiu, M., Zhang, Q., Maavara, T., Liu, S., Wang, X., & Raymond, P. A. Rivers as the largest source\nof mercury to coastal oceans worldwide. Nature Geosci. 14, 672–677 (2021).\nLine 126: The positive correlation between δ15N and δ13C is statistically robust, though\nmoderate in strength. The authors could consider discussing the implications of this\nmoderate correlation, i.e., what other processes might contribute to the scatter in the\nrelationship.\nResponse: We agree that the moderate positive correlation between δ¹⁵N and δ¹³C indicates\norg\nthat additional processes contribute to the observed variability. While this relationship can be\nbroadly interpreted as a mixture of ¹³C- and ¹⁵N-depleted terrigenous organic matter with ¹³C-\nand ¹⁵N-enriched marine-derived organic matter, it may also be modified by factors such as\nvariable nitrogen sources (e.g., inorganic inputs), cases where δ¹³C shows limited variability\norg\nwhile δ¹⁵N spans a wide range due to complex terrestrial and marine sources, and diagenetic\nalteration during deposition and burial, which tends to elevate δ¹⁵N more strongly than δ¹³C .\norg\nIn the revised manuscript, we have added the following clarification (Lines 158–162):\n“Such a moderate correlation suggests secondary variability, with additional processes—\nsuch as differences in organic matter and nitrogen sources (Robinson et al., 2012), diagenetic\nalteration (Freudenthal et al., 2001), and regional variations in nutrient utilization (Altabet et\nal., 1994)—likely contribute to the scatter in the δ¹⁵N–δ¹³C relationship.”\norg\nLine 141: to avoid confusion use the term sedimentary δ15N, which is also used in other\nliterature.\nResponse: To ensure consistency with the broader literature and avoid any confusion, we have\nrevised the terminology throughout the manuscript to use “sedimentary δ¹⁵N.” (Line 177)\nLine 146: Sentence is a bit hard to follow as a reader, consider to rearrange the deep-\nsea numbers in the brackets to make it more intuitive to follow.\nResponse: To improve clarity and readability, we have revised the sentence by specifying the\ndeep-sea values in the brackets. The updated text now reads: “…differ markedly from deep-sea\n(>2,000 m) samples (δ¹⁵N = 7.2‰, δ¹³Cₒᵣ = −20.0‰, TN = 0.14%) (Fig. 1a, d)”. (Lines 182–\ng\n183)\nLine 150: The distinction between coastal and deep-sea δ¹⁵N values is important.\nConsider referencing Fig. 1a and 1d directly here to support the discussion of depth-\nrelated trends. Do you assume that terrestrial processes occur in places of low water\ndepth and high sedimentation rate? what about high sedimentation rate places which\nare not driven by terrestrial processes? for example upwelling areas.\nResponse: We have revised the text to explicitly reference Fig. 1a and 1d when discussing the\ndepth-related δ¹⁵N trends (Line 183). Regarding the processes involved, we agree that high\nsedimentation rates are not exclusively linked to terrestrial inputs. While shallow-water, high-\nsedimentation regions are often influenced by terrestrial processes, other environments—such\nas upwelling zones—can also exhibit high sedimentation rates driven by enhanced marine\nproductivity (Deutsch et al. 2014). Notably, these upwelling regions typically exhibit elevated\nsedimentary δ¹⁵N, reflecting their substantial inputs of marine organic matter. After considering\nyour valuable comments, we have added the following sentence for clarify (Lines 193–195):\n“High sedimentation rates also occur in upwelling regions with minimal terrestrial\ninfluence, where elevated sedimentary δ¹⁵N reflect the deposition of marine organic matter\nsustained by upwelling (Deutsch et al., 2014).”\nReference\nDeutsch, C. et al. Centennial changes in North Pacific anoxia linked to tropical trade winds.\nScience 345, 665–668 (2014).\nLine 162: add reference.\nResponse: The relevant reference, Robinson et al. (2012), has now been added at this point in\nthe revised manuscript. (Lines XX–XX)\nReference\nRobinson, R. S. et al. A review of nitrogen isotopic alteration in marine sediments.\nPaleoceanography 27, PA4203 (2012).\nLine 164: bed —> seafloor?\nResponse: The term “bed” has been replaced with “seafloor” accordingly. (Line 203)\nLine 167: I disagree that the IN data has a broad latitudinal range. The data is heavily\nskewed towards the Arctic Ocean and I would defer from making bold global\nstatements in the following text about this and rather focus on regional trends.\nResponse: We agree that the available sedimentary IN dataset is limited and heavily skewed\ntoward the Arctic Ocean and other well-studied regions, and that broad global statements could\nbe misleading. In the revised manuscript, we have focused on regional trends and depth-related\npatterns where data coverage is robust. Accordingly, we have now removed the terms “a broad\nlatitudinal range” “globally” and replaced “globally widespread” with “pronounced regional\nvariability”. (Lines 204–208)\nLine 176: give example for region?\nResponse: An example region, the “western subarctic Pacific” has now been added in the\nrevised manuscript. (Lines 213–214)\nLine 179: ON was not defined in previous text.\nResponse: Thank you for your careful review. Organic nitrogen (ON) has now been defined in\nthe manuscript. (Lines 218–219)\nLine 195: can the authors reference a figure for that? Suppl. Fig. 4?\nResponse: We have addressed this suggestion by adding the (new) Supplementary Fig. 3 to\nillustrate the lack of correlation between ON content and δ¹⁵N in studied marine sediments.\nON\nA reference to Supplementary Fig. 3 has also been added at this point (Line 236).\nLine 222: This feels a little bit disconnected from the first part. In the introduction the\nauthors state they are interested in studying anthropogenic influence on N-cycle. The\nmotivation here is now backwards, looking at marine sediments to evaluate global-\nscale drivers of δ15N. A more streamlined introduction part in all three parts of the\nmanuscript will help the reader to follow why the authors chose to study those samples.\nResponse: We agree that the motivation could be more clearly linked across the manuscript. In\nthe revised version, we have streamlined the introduction and relevant sections to better connect\nour study of marine sediments with the overarching goal of assessing anthropogenic influences\non the nitrogen cycle. In the first part, we focus on the spatial patterns of sedimentary δ¹⁵N in\nbulk, inorganic, and organic nitrogen, identifying the dominant sources of organic nitrogen that\nregulate total nitrogen changes and assessing the drivers of environmental gradients (e.g., water\ndepth, sedimentation rates, and marine productivity as indicated by δ¹³C ). Based on these\norg\nresults, this section was designed to evaluate whether δ¹⁵N remains stable or responds to\nanthropogenic perturbations. Following your valuable advice, we have added text introducing\nhow human activities have altered riverine nitrogen loading and marine productivity over the\npast century. To clearly convey our intention at the end of this section, we have added the\nfollowing sentence: “While the global synthesis establishes a baseline of spatial variability in\nsedimentary δ¹⁵N, the extent to which these signals have evolved or remained stable in response\nto anthropogenic and climatic forcing remains poorly constrained.” (Lines 247–250)\nLine 243–244: The authors report a 1.5‰ per century increase in δ¹⁵N at some sites.\nIt would be helpful to contextualize this rate relative to known historical changes in\nnitrogen loading or productivity. Is this considered a large change?\nResponse: We thank the reviewer for this suggestion. The observed 1.5‰ per century increase\nin δ¹⁵N at certain sites can be contextualized relative to known historical changes in nitrogen\nloading and marine productivity. This trend aligns with a doubling of riverine total nitrogen\nexport to the oceans (from 20 to 42(cid:3031)Tg(cid:3031)N(cid:3031)yr⁻¹ between 1900 and 2010; Green et al., 2004; Wu\net al., 2024), a 4.6% increase in net primary productivity in coastal oceans (1960–2010; Liu et\nal., 2021), and a 2.19%(cid:3031)yr⁻¹ rise in global coastal phytoplankton bloom frequency (2003–2020;\nDai et al., 2023). Although moderate compared with extreme localized nutrient inputs,\nthis\nlong-term increase reflects a systematic alteration of nitrogen cycling in these regions over the\npast century. According to your valuable suggestions, we have added the following sentences\nin the main text to contextualize this rate (Lines 285–288):\n\"This increase in sedimentary δ¹⁵N aligns with a doubling of riverine total nitrogen export\nto the oceans (1900–2010; Green et al., 2004; Wu et al., 2024), a 4.6% increase in net primary\nproductivity in coastal oceans (1960–2010; Liu et al., 2021), and a 2.19%(cid:2271)yr⁻¹ rise in global\ncoastal phytoplankton bloom frequency (2003–2020; Dai et al., 2023).\"\nReferences\nGreen, P. A., Vörösmarty, C. J., Meybeck, M., Galloway, J. N., Peterson, B. J., & Boyer, E. W. Pre-\nindustrial and contemporary fluxes of nitrogen through rivers: a global assessment based on\ntypology. Biogeochemistry, 68, 71–105 (2004).\nWang, J., Bouwman, A. F., Vilmin, L. et al. Global inland-water nitrogen cycling has accelerated in\nthe Anthropocene. Nat Water 2, 729–740 (2024).\nLiu, X., Stock, C. A., Dunne, J. P., Lee, M., Shevliakova, E., Malyshev, S. and Milly, P. C. D.\nSimulated global coastal ecosystem responses to a half-century increase in river nitrogen loads.\nGeophys. Res. Lett. 48, e2021GL094367 (2021).\nDai, Y., Yang, S., Zhao, D., Hu, C., Xu, W., Anderson, D. M., ... & Feng, L. Coastal phytoplankton\nblooms expand and intensify in the 21st century. Nature, 615, 280–284 (2023).\nLine 284–285: The link between δ¹³C and productivity is implied but not fully\norg\nexplained. Consider briefly elaborating on the mechanisms (e.g., increased\nphytoplankton biomass, eutrophication). Also, briefly mention if this is a global\nphenomenon or restricted to one particular area.\nResponse: Thanks. We have expanded our explanation of the mechanisms linking δ¹³C to\norg\nmarine productivity. Specifically, higher productivity (e.g., increased phytoplankton biomass\nand costal eutrophication) can enhance organic carbon burial and the preferential uptake of ¹²C,\nresulting in elevated δ¹³C values (Peterson and Howarth, 1987; Fogel and Cifuentes, 1993;\norg\nGoni et al., 2006). Additionally, considering the increase in global riverine nitrogen inputs to\nthe coastal oceans (Green et al., 2004; Wu et al., 2024), enhanced primary productivity (Liu et\nal., 2021), and the frequent occurrence of coastal phytoplankton blooms (Dai et al., 2023) over\nthe past century and several decades, this trend may in part reflect a global-scale link between\nδ¹⁵N and enhanced marine productivity. Following your valuable suggestions, we have briefly\nelaborated the mechanisms (e.g., increased phytoplankton biomass and eutrophication) and\nadded supported references. Given that these studied cores collected from global coastal ocean\nand these trends consistent with historical changes in river nitrogen loading, productivity and\ncoastal phytoplankton blooms. We believe this is likely a global phenomenon. These sentences\nhave been rewritten for clarify (Lines 328–334):\n“Carbon isotopic data indicate that dramatic δ¹⁵N shifts coincide with elevated primary\nproductivity (higher δ¹³C , Supplementary Fig. 4a), reflecting increased phytoplankton\norg\nbiomass and likely eutrophication (Peterson and Howarth, 1987; Fogel and Cifuentes, 1993;\nGoni et al., 2006). Given widespread core coverage, increased riverine nitrogen inputs (Green\net al., 2004; Wu et al., 2024), enhanced coastal productivity (Liu et al., 2021), and frequent\nphytoplankton blooms (Dai et al., 2023) over the past century, this trend may partly reflect a\nglobal link between δ¹⁵N and marine productivity.”\nReferences\nFogel, M. L., & Cifuentes, L. A. Isotope fractionation during primary production, in Organic\nGeochemistry, edited by M. H. Engel and S. A. Macko, pp. 73–98, Springer, New York (1993).\nPeterson, B. J., & Howarth, R. W. Sulfur, carbon, and nitrogen isotopes used to trace organic matter\nflow in the salt‐marsh estuaries of Sapelo Island, Georgia 1. Limnol. Oceanogr. 32, 1,195–\n1,213 (1987).\nGoni, M. A. et al. Terrigenous organic matter in sediments from the Fly River delta‐clinoform system\n(Papua New Guinea). J. Geophys. Res. Earth Surf. 113, F01S10 (2008).\nLine 319: I question the use of “alternatively” here. Using “alternatively” implies these\nare two separate explanations, when in reality, the second sentence is more like a\nconsequence or elaboration of the first.\nResponse: We fully agree with your point that the term “alternatively” implied two independent\nexplanations, whereas the second sentence is a consequence of the first. To clarify this, we have\nnow removed “alternatively” and combined the sentences to convey a causal relationship. The\nrevised text now reads (Lines 362–365):\n“Variations in riverine sediment supply modulate the quantity and quality of terrigenous\norganic matter and mineral particles, potentially reshaping marine sedimentary nitrogen\ndynamics, with terrestrial organic and inorganic nitrogen inputs potentially overshadowing\nchanges in marine nitrate utilization.”\nLine 343–344: The phrase “coherent declines in δ¹⁵N” could be strengthened by\nspecifying the magnitude of decline or the range observed across the ECS cores. In\nboth Figure 5 and Supplementary Fig. 7 it is hard to see the declines in δ15N. The\nauthors could consider to make an additional panel in Supplementary Fig. 7 to focus\nonly on the sites where the decline has been observed.\nResponse: To strengthen our description, we have now specified the magnitude of δ¹⁵N declines\nobserved across the ECS cores, with an average rate of −1.09 ± 0.48‰ per century (Line 390).\nAdditionally, to improve the visualization, we have revised Supplementary Fig. 7 (now as the\nSupplementary Fig. 8) by (i) separating the cores showing δ¹⁵N declines from those showing\nincreases, (ii) adding two new panels (Supplementary Fig. 8e and 8f) to highlight the sediment\ncores with δ¹⁵N declines (with further distinction between literature data and new data), and (iii)\nenlarging the y-axis scale for clearer display of temporal changes. Also, given that your valuable\ncomments, we have moved Figure 5 to Supplementary Figure 9 to avoid presenting excessive\ninformation for this local case study. This East China Sea is primarily included to illustrate the\ndecline in sedimentary δ¹⁵N signals in river-dominated continental margins and to thus highlight\nthe underlying drivers from terrestrial processes, such as nitrogen associated with refractory\norganic carbon.\nLine 396: in which core? add information in the main text (DH3-1 and where the core\nis located more or less).\nResponse: We thank you for this helpful comment. We have revised the main text to specify\nthat these patterns were observed in core DH3-1, recovered from the inner shelf of the East\nChina Sea. The revised text (Lines 429–432) now reads: “Size-fractionated analyses in core\nDH3-1 from the inner shelf of the East China Sea reveal divergent δ¹⁵N trajectories: the <63\nμm fraction shows increasing δ¹⁵N, whereas the >63 μm fraction exhibits declining values,\nparalleling bulk sediment trends (Supplementary Fig. 10 and Table 3).”\nLine 417–418: The δ¹⁵N values of refractory OC are reported as low as −12‰. This is\nquite depleted—could the authors comment on the potential sources or preservation\nmechanisms of such low values?\nResponse: Thank you for pointing this out. The low δ¹⁵N values of refractory OC (after removal\nof the predominantly ¹⁵N-enriched marine organic matter) likely reflect residual of refractory\nterrestrial OC, particularly soured from ¹⁵N-depleted vascular plant detritus and sedimentary\nrocks. Owing to their chemical recalcitrance, these materials can preserve their depleted δ¹⁵N\nsignatures during transport and burial, thereby exerting a considerable influence on the isotopic\ncomposition of the sedimentary refractory OC pool. Supporting this interpretation, the low C/N\nratios of refractory OC likely reflect 13C-depleted terrestrial inputs, particularly petrogenic OC\nderived from sedimentary rocks in the Yangtze basin (e.g., shale, sandstone/siltstone, marlstone,\nlimestone), which inherently exhibit low C/N ratios (2–6; Supplementary Table 8), consistent\nwith previously reported bedrock data from Taiwan (Hilton et al., 2009). According to your\nvaluable suggestion, we have clarified this in the revised manuscript (Lines 455–460).\nReference\nHilton, R. G., Galy, A., Hovius, N., Horng, M. J., & Chen, H. The isotopic composition of particulate\norganic carbon in mountain rivers of Taiwan. Geochim. Cosmochim. Acta, 74, 3164-3181\n(2010).\nLine 446: what kind of data? nitrate δ15N?\nResponse: We thank the reviewer for this comment. To avoid any ambiguity, we have clarified\nin the revised text that the data refer to sedimentary δ¹⁵N and N content, rather than nitrate δ¹⁵N.\nThe revised sentences (Lines 490–493) now read:\n“In coastal regions, however, high-resolution sedimentary records reveal enrichment in\nboth sedimentary δ¹⁵N and N content, likely driven by processes such as anthropogenic nitrogen\ninputs (Green et al., 2004; Bouwman et al., 2005; Wu et al., 2024), enhanced productivity (Liu\net al., 2021; Y. Dai et al., 2023), water-column denitrification (Deutsch et al., 2014), and\nhypoxia (M. Dai et al., 2023).”\nMethods:\nA table with the collected data type, number of samples and references would be useful\nfor the reader to see what part of the data is from previously published records and\nwhat data was newly generated for this study.\nResponse: To improve clarity, according to your valuable suggestions, we have added a new\ntable (now Supplementary Table 1) summarizing the collected data, including the type of data,\nthe number of samples, variables and associated references. This table distinguishes between\npreviously published datasets and new data generated in this study, thereby allowing readers to\nclearly see the source and scope of all data used.\nLine 477: bed —> seabed or seafloor?\nResponse: We appreciate this comment. The term “bed” has been revised to “seabed” for clarity,\nand we have ensured consistency throughout the revised manuscript. (Line 553)\nLine 491: … on marine sedimentary nitrogen poor --> nitrogen pool?\nResponse: Thank you for catching this mistake. We have corrected the text to “…nitrogen pool”\nto improve clarity. (Line 559)\nFigures:\nFigure 1:\npanel (c): what are the grey dots? indicate in figure caption.\npanel (d&e): figure caption says “environmental drivers influencing d15N bulk signals”,\nhowever, the correlation is relatively weak for the data set. Consider rewording to\n“potential environmental drivers…”\nResponse: Many thanks for your careful review, for panel (c) we have clarified that “Grey dots\nindicate data points for which nitrogen content is not available and thus are not represented by\nthe colored symbols.” in the revised figure caption. For panels (d) and (e), we acknowledge that,\nalthough significant correlations (P < 0.001) exist between sedimentary δ¹⁵N and environmental\nfactors such as water depth and sedimentation rates, the correlation coefficients are indeed weak\nto moderate, as you noted. Accordingly, we have revised the figure caption to read “potential\nenvironmental drivers…” to better reflect this.\nFigure 6:\nPanel (b) includes a wide range of δ¹⁵N values. Consider adding a shaded band to\nindicate typical marine vs. terrestrial ranges for context.\nwhat do the letters at the top stand for? consider removing.\nPanel (c): indicate which data is from the river basin, shelf and seafloor. refractory OC\nis clearly indicated.\nResponse: According to your valuable advice, based on these compiled δ¹⁵N data of the Yangtze\nRiver basin and previously reported end-member range of terrestrial and marine organic matter\n(e.g., Peterson and Howarth, 1987; Fogel and Cifuentes, 1993; Goni et al., 2008). We have\nclarified that “Panel (b) shows the typical compositional ranges of terrigenous organic matter\n(light brown shading) and marine algal sources (light blue shading) (Peterson and Howarth,\n1987; Fogel and Cifuentes, 1993; Goni et al., 2008).”\nFor panel (b), in the previous version the letters indicated statistically distinct δ¹⁵N groups (P <\n0.05). Following your valuable suggestions, to avoid any confusion, we have now replaced the\nletters with “NS” (not significant, P > 0.05, ANOVA) to indicate the absence of significant\ndifferences. “NS” is omitted when variables are significantly different. For example, no\nsignificant δ¹⁵N differences were found among riverine, marine suspended matter, and seafloor\nsediments, nor between rock and refractory samples. The meaning of “NS” has been clarified\nin the revised figure caption.\nFor panel (c), we have rewritten this figure caption for clarify: “Correlations between OC and\nnitrogen (N) content in bulk and refractory samples from sediment samples from sediment cores CJK, C0702,\nC0803 and DH3-1 in the East China Sea”.\nSuppl. Figure 7:\nTo be consistent with the other figures and to help the reader navigate through the\nnumerous figures, I suggest to pivot the graph to read the time series from left to right,\nsimilar to Fig. 3,4 and 5.\nResponse: We agree with your valuable advice; Supplementary Figure 7 (now Supplementary\nFigure 8) has been revised by pivoting the graph and adjusting the axes so that the time series\nnow reads from left to right, consistent with other mentioned figures. In addition, we have\nseparated the decreased δ¹⁵N data from the literature (panel e) and from this study (panel f) to\nbetter illustrate these trends. \n\nPoint-by-point Response to Reviewers’ Comments\nManuscript: Global patterns and drivers of nitrogen isotope signal in modern marine sediments\nAuthors: Xueshi Sun*, Dejiang Fan, Limin Hu, Houjie Wang\nReviewer comments are shown in black, and our responses are given in blue.\nREVIEWERS' COMMENTS\nReviewer #1 (Remarks to the Author):\nThe revised manuscript has very carefully addressed both the suggestions and\nconcerns in my initial review. I am rarely this pleased with how seriously the authors\nhave taken the review process. The improvement in this manuscript between the initial\nsubmission and this version is an excellent example of the peer review process\nworking successfully. Great work!\nBest,\nJack Hutchings\njackh@wustl.edu\nResponse: We thank Dr. Jack Hutchings, for the encouraging comments and positive evaluation.\nWe are delighted that the revisions have successfully addressed the reviewer’s concerns and\nimproved the overall quality of the manuscript. We greatly appreciate the time and effort you\ndedicated to reviewing our work.\nR2:\nSun et al. did a wonderful job in going through each of the reviewers' comments. I am\nsatisfied with the revisions made in response to my comments. The manuscript has\nimproved significantly in readability and structure, particularly through the streamlining\nof the introduction and discussion sections and the reduction in figure number. The\nauthors have addressed all critical points I raised, and the narrative now flows more\nclearly.\nWhile I cannot evaluate the technical aspects of the acidification and oxidation\ntreatments raised by Dr. Hutchings, I appreciate the authors’ extensive paired analyses\nand statistical evaluations. Their response appears robust and transparent, and I defer\nto Dr. Hutchings for final judgment on these more technical matters.\nI also commend the authors for publishing their extensive δ15N dataset to Mendeley\nData. This is a valuable resource for the community and enhances the transparency\nand reusability of the study.\nOne minor suggestion:\nTo improve the manuscript’s conclusion, I recommend ending with the more general\noutlook paragraph: “Sedimentary δ15N remains a robust archive of past nitrogen cycle\ndynamics, and has constrained how the evolution of nitrogen burial in modern marine\nsediments in response to recent environmental changes…” and moving the more\ntechnical sentence: “Additionally, our compiled global dataset includes both acidified\nand untreated…” above it, so that the manuscript closes with a broader perspective.\nIf Dr. Hutchings is satisfied with the technical corrections and additional analyses, I am\nhappy to recommend this study for publication.\nResponse: We sincerely thank the reviewer for the insightful feedback and for approving our\nrevised manuscript. Following your valuable suggestion, we have revised the conclusion part\naccordingly. Specifically, the broader outlook paragraph on sedimentary δ¹⁵N has been moved\nto the end of the section, while the technical sentence concerning our compiled global dataset\nhas been placed above it (Lines 448-471). This reorganization ensures that the conclusion ends\nwith a broader perspective, as recommended."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The revised manuscript has very carefully addressed both the suggestions and concerns in my initial review. I am rarely this pleased with how seriously the authors have taken the review process. The improvement in this manuscript between the initial submission and this version is an excellent example of the peer review process working successfully. Great work!\nBest,\nJack Hutchings\njackh@wustl.edu", "reviewer_2": "Sun et al. did a wonderful job in going through each of the reviewers' comments. I am satisfied with the revisions made in response to my comments. The manuscript has improved significantly in readability and structure, particularly through the streamlining of the introduction and discussion sections and the reduction in figure number. The authors have addressed all critical points I raised, and the narrative now flows more clearly.\nWhile I cannot evaluate the technical aspects of the acidification and oxidation treatments raised by Dr. Hutchings, I appreciate the authors’ extensive paired analyses and statistical evaluations. Their response appears robust and transparent, and I defer to Dr. Hutchings for final judgment on these more technical matters.\nI also commend the authors for publishing their extensive d15N dataset to Mendeley Data. This is a valuable resource for the community and enhances the transparency and reusability of the study.\nOne minor suggestion:\nTo improve the manuscript’s conclusion, I recommend ending with the more general outlook paragraph:\n“Sedimentary d15N remains a robust archive of past nitrogen cycle dynamics, and has constrained how the evolution of nitrogen burial in modern marine sediments in response to recent environmental changes…”\nand moving the more technical sentence:\n“Additionally, our compiled global dataset includes both acidified and untreated…”\nabove it, so that the manuscript closes with a broader perspective.\nIf Dr. Hutchings is satisfied with the technical corrections and additional analyses, I am happy to recommend this study for publication."}, "author_response": "Point-by-point Response to Reviewers’ Comments\nManuscript: Global patterns and drivers of nitrogen isotope signal in modern marine sediments\nAuthors: Xueshi Sun*, Dejiang Fan, Limin Hu, Houjie Wang\nReviewer comments are shown in black, and our responses are given in blue.\nREVIEWERS' COMMENTS\nReviewer #1 (Remarks to the Author):\nThe revised manuscript has very carefully addressed both the suggestions and\nconcerns in my initial review. I am rarely this pleased with how seriously the authors\nhave taken the review process. The improvement in this manuscript between the initial\nsubmission and this version is an excellent example of the peer review process\nworking successfully. Great work!\nBest,\nJack Hutchings\njackh@wustl.edu\nResponse: We thank Dr. Jack Hutchings, for the encouraging comments and positive evaluation.\nWe are delighted that the revisions have successfully addressed the reviewer’s concerns and\nimproved the overall quality of the manuscript. We greatly appreciate the time and effort you\ndedicated to reviewing our work.\nR2:\nSun et al. did a wonderful job in going through each of the reviewers' comments. I am\nsatisfied with the revisions made in response to my comments. The manuscript has\nimproved significantly in readability and structure, particularly through the streamlining\nof the introduction and discussion sections and the reduction in figure number. The\nauthors have addressed all critical points I raised, and the narrative now flows more\nclearly.\nWhile I cannot evaluate the technical aspects of the acidification and oxidation\ntreatments raised by Dr. Hutchings, I appreciate the authors’ extensive paired analyses\nand statistical evaluations. Their response appears robust and transparent, and I defer\nto Dr. Hutchings for final judgment on these more technical matters.\nI also commend the authors for publishing their extensive δ15N dataset to Mendeley\nData. This is a valuable resource for the community and enhances the transparency\nand reusability of the study.\nOne minor suggestion:\nTo improve the manuscript’s conclusion, I recommend ending with the more general\noutlook paragraph: “Sedimentary δ15N remains a robust archive of past nitrogen cycle\ndynamics, and has constrained how the evolution of nitrogen burial in modern marine\nsediments in response to recent environmental changes…” and moving the more\ntechnical sentence: “Additionally, our compiled global dataset includes both acidified\nand untreated…” above it, so that the manuscript closes with a broader perspective.\nIf Dr. Hutchings is satisfied with the technical corrections and additional analyses, I am\nhappy to recommend this study for publication.\nResponse: We sincerely thank the reviewer for the insightful feedback and for approving our\nrevised manuscript. Following your valuable suggestion, we have revised the conclusion part\naccordingly. Specifically, the broader outlook paragraph on sedimentary δ¹⁵N has been moved\nto the end of the section, while the technical sentence concerning our compiled global dataset\nhas been placed above it (Lines 448-471). This reorganization ensures that the conclusion ends\nwith a broader perspective, as recommended."}]

Decision letter