Warming-induced retreat of West Antarctic glaciers weakened carbon sequestration ability but increased mercury enrichment

Domain: earth_science
Published: 2025-02-20
Editorial outcome: major_revision

Abstract
The Southern Ocean, one of Earth’s most productive areas, is widely recognized as a major sink for atmospheric carbon and mercury, tightly coupling primary production with the sedimentary sequestration of these elements. The impacts of climate warming on these processes, however, remain unclear. Here, we utilize 20 sediment cores from the Ross Sea, a representative ice-shelf sea in West Antarctica, to examine how Holocene warming and extensive glacial retreat influenced carbon and mercury sequestration. We find that organic carbon (OC) burial has been relatively constant over the past 12,000 years, whereas mercury burial in the Ross Embayment and open ocean exhibited three- and eightfold increases, respectively. Carbon isotopes and accumulation profiles suggest warming boosted glacial- and terrestrial-derived OC inputs to the ocean, while trace elements and biomarkers reveal a declining contribution offshore. Biomarker ratios further indicate greater remineralization of this OC in the open ocean. Consequently, enhanced OC degradation, coupled with rising external mercury inputs, drives mercury enrichment in marine sediments before reaching the seafloor. These findings imply that ongoing warming could trigger a positive feedback loop, accelerating OC degradation into CO2 and amplifying the impacts of anthropogenic mercury on Southern Ocean ecosystems.

Introduction
Carbon dioxide (CO2) is the primary greenhouse gas driving global warming. Marine organic carbon (OC) burial is the dominant natural mechanism for long-term carbon sequestration1. The marine biological pump—involving photosynthesis, particle sinking, and remineralization—along with the physical carbon pump, exports surface ocean carbon to the seafloor as particulate OC2. Meanwhile, terrestrial OC can also be laterally transported to the ocean and seafloor via fluvial and aeolian inputs3. These processes impact marine biodiversity, ocean food web structures, and global biogeochemical cycles of elements like oxygen, nitrogen, silicon, iron, and heavy metals, including mercury (Hg)4–6. Hg is a global anthropogenic pollutant that affects human and ecosystem health, with anthropogenic emissions, primarily from fuel combustion, being 5 to 10 times greater than natural levels7. Inorganic Hg is transported long distances through the circulation of air and ocean water8. In aquatic systems, Hg can be transformed into the neurotoxin methylmercury (MeHg), which forms primarily in seawater and benthic sediments and bioaccumulates in food webs9,10. Because of Hg’s strong affinity for organic matter (OM), it can bind to OM and be transported laterally to the ocean6. Additionally, Hg in the euphotic layer can be scavenged and transported to the seafloor via the marine biological pump and sinking terrestrial particles11. Furthermore, OC remineralization drives Hg distribution and its methylation in the ocean water column12–14. The OC-Hg relationship is key for understanding Hg removal from the upper ocean and its subsequent sediment deposition and burial8,14. However, due to limited observations, a comprehensive understanding of the magnitude and spatial distribution of marine OC and Hg burial, particularly in relatively pristine polar ecosystems, remains elusive.

Ice core and sediment records indicate that atmospheric Hg deposition remained stable during the early Holocene but has more than tripled since industrialization15,16, closely aligning with the rise in CO2 emissions. The Southern Ocean plays an important role in the global carbon cycle and climate by sequestering CO2, making it a key factor in the global ocean carbon sink15,16. This is driven by CO2 removal during the cooling of southward-transported subtropical waters and the formation of Antarctic bottom water (AABW), which form mode waters17. These waters are dynamic in terms of primary productivity, sinking particle remineralization, and upwelling of carbon-rich water masses18. The Southern Ocean may also be a major sink for modern global atmospheric Hg, where Hg gets scavenged through the biological pump19. Moreover, anthropogenic warming has accelerated glacial retreat in the region, creating extensive new open water areas, triggering phytoplankton blooms, and potentially increasing carbon and Hg sinks, and enhancing air-sea exchange20,21. Warming also remobilizes glacial and permafrost OC and Hg to the ocean, where they are prone to remineralization and for net surface water ionic Hg reduction in warmer seawater, potentially elevating CO2 and elemental Hg (Hg0) evasion22,23. This raises the critical question of whether future anthropogenic warming will increase or decrease carbon and Hg sequestration in the Southern Ocean.

We therefore aimed to determine whether climate warming can alter the temporal patterns of OC and Hg sequestration in the Southern Ocean in order to improve our understanding of their interrelated behavior in the ocean. This work may also inform whether different mitigation strategies for OC, Hg, and other heavy metals, are required under future warming. However, the period of anthropogenic warming is too short to study this mechanism, as much of the anthropogenic carbon and Hg likely remain in the water column24. Given the Holocene’s climate similarity to the present, we focused on the period from 12,000 years BP to the present, particularly during the glacier retreat since 8000 years BP25, to avoid any hysteresis between warming and sequestration.

We focused on the Ross Sea (Fig. 1) because it represents a typical ice-shelf sea in west Antarctica. The dense shelf water formed on the Ross Sea continental shelf is one of the main sources of AABW, playing a significant role in the deep-ocean carbon cycle26. Characterized by high salinity, oxygen-rich conditions, and elevated nutrient levels27,28, the Ross Sea exhibits greater phytoplankton productivity than other Antarctic regions27, largely dominated by diatoms and haptophytes29. The sediments, mainly comprised of organic-rich mud and diatomaceous oozes28, are also potential hotspots for pollutant accumulation30. Importantly, warming events during the mid-late Holocene caused rapid retreat of the Ross Ice Shelf (Supplementary Fig. 1), making much of the sea ice-free31. While previous work has examined OC burial rates in the Ross Sea32 and Hg contamination in surrounding land masses33,34, a comprehensive palaeoclimatic record of OC and Hg burial is still lacking.

Utilizing 20 sediment cores from the Ross Embayment and the associated extended open ocean, along with chemical (OC, Hg, total nitrogen, grain size, biomarkers (n-alkanes, and sterols), major and trace elements) and isotopic (δ13C, Δ14C, Hg isotopes) analyses, we: (1) reconstruct the Holocene paleoceanic record of OC and Hg burials; (2) identify the sources of OC and Hg to clarify their fate; and (3) examine the impacts of mid-late Holocene warming on the spatiotemporal patterns of OC and Hg sequestration. Focusing on the Ross Sea during the Holocene provides significant insights into how the Southern Ocean’s carbon and Hg cycles may respond to future anthropogenic warming.

Results
Spatial patterns in carbon and mercury sequestration
Sediment cores from the Ross Sea show a depth-integrated average OC% ranging from 0.23 ± 0.14% to 1.6 ± 0.26%, with an overall average of 0.59 ± 0.48% (Fig. 2). The OC% in the Ross Embayment (eight cores) is significantly higher than in the open ocean (twelve cores, P < 0.01). Our OC% measurements contribute to the limited existing data for Ross Sea sediments35, aligning with several previous observations (0.14%–2.0%)32. Depth-integrated average Hg concentrations in these cores range from 3.5 ± 4.3 to 79 ± 9.2 ng g−1, with significantly higher levels in the Ross Embayment (51 ± 26 ng g−1) compared to the open ocean (12 ± 15 ng g−1, P < 0.01, Fig. 2). Spatial analysis confirms a positive correlation between Hg concentration and OC% in Ross Sea sediments (R2 = 0.89, P < 0.01, Fig. 3b).

For Hg, there have been no determinations of Hg contamination in Ross Sea sediments to date36. Our results are comparable to Hg levels reported in the diatom-rich sediments of the Pacific/Indian Ocean sector of the Southern Ocean (42 ± 16 ng g−1)19 and sediments from the deep Arctic Ocean basins (10–120 ng g−1)37. However, they are slightly lower than sediments from the Greenland coast (4.0–280 ng g−1)38, probably because Arctic shelves have received more anthropogenic Hg since industrialization. Our results are also lower than ornithogenic (soil) deposit-dominated sediments in the Ross Sea region (6.5–190 ng g−1), where Hg accumulates in bird guano, which is rich in pollutants from their marine-based diets34,39.

Our measurements show that OC and Hg burial rates are highest near the ice shelf in the Ross Sea, diminishing significantly toward the open ocean (P < 0.01). In the Ross Embayment, the average sedimentation rate is 21 ± 37 cm kyr−1, a factor of 6 higher than the open ocean (Fig. 2). Aligning with the spatial distribution of OC% and Hg concentrations, we find that the Embayment records depth-integrated burial rates of 1.2 ± 1.8 g C m−2 yr−1 for OC and 5.9 ± 7.7 μg m−2 yr−1 for Hg, greater than the open ocean (0.05 ± 0.04 g C m−2 yr−1 and 0.25 ± 0.32 μg m−2 yr−1, respectively) by a factor of 24 (Fig. 2). This suggests that similar processes drive Hg and OC removal in the Ross Sea water column.

Our OC burial rates in the Ross Sea align with prior work32, with Embayment rates between 0.07 and 5.6 g C m−2 yr−1 and open ocean rates between 0.01 and 0.17 g C m−2 yr−1. Our observed Hg burial rates are higher than the Arctic Ocean Basin (0.1–0.2 μg m−2 yr−1)37 but lower than many continental shelves38,40 like the Greenland coast (4.3–400 μg m−2 yr−1), mainly attributed to differences in sedimentation rates. The Ross Sea, a diatom-rich Antarctic marginal sea, has sedimentation rates similar to that found in the Southern Ocean diatom ooze (75 cm kyr−1)41. Interestingly, our sedimentation and Hg burial rates are substantially lower than a single core from the Adélie Basin in Antarctica (2000 cm kyr−1 and 580–1200 μg m−2 yr−1, respectively)19,42, which are exceptional and exceed rates found in many estuarine sediments19. Our samples likely represent broader Southern Ocean conditions. Extrapolating our OC and Hg rates in surface sediments across the Ross Sea, we estimate burials of 0.72 Tg C of OC yr−1 and 4.7 Mg yr−1 of Hg since industrialization. This likely underestimates Hg burial due to the more than 3-fold increase in atmospheric Hg concentrations and the integrated surface sediments being slightly older than industrialization. Notably, however, the area-normalized Hg burial rate in Ross Sea’s open ocean (0.25 ± 0.32 μg m−2 yr−1) is much lower than the known atmospheric deposition flux (~2 μg m−2 yr−1) into the Southern Ocean43, suggesting substantial net Hg0 evasion from seawater in this region. Sea ice acts as a barrier, preventing the escape of seawater Hg0 to the atmosphere (the sea ice capsuling effect). Therefore, the loss of sea ice could result in significant increases in Hg evasion44,45.

Source of sedimentary carbon and mercury
Our study reveals that marine-derived OC dominates in the Ross Embayment and open ocean, while glacial and terrestrial inputs (vegetation, soils, and penguin and seal feces and hair)46–48 contribute additional OC to the Ross Embayment sediments. These inputs also enhance the scavenging and burial of seawater and terrestrial Hg in ocean sediments. Analyzing particle sizes across sediment cores, we found negative correlations between median particle size d(50) and OC% and Hg concentration (P < 0.05, Fig. 3). Similar to previous studies49, positive correlations were observed between OC and Hg and particle sizes in the 0–30 μm range (P < 0.05), while the converse is true in the 30–125 μm range (P < 0.05, Fig. 4a). This indicates that higher OC and Hg concentrations are predominantly linked to finer particles (0–30 μm), whereas larger particles (30–125 μm) tend to dilute these concentrations. Particles larger than 125 μm, mainly ice-rafted debris with low OC content, have limited surface area and binding sites50, making them ineffective carriers of Hg, which explains the lack of correlation between Hg and OC (Fig. 4a).

We find that fine particles (0–30 μm) have higher Corg/Ntot ratios and more negative δ13Corg values compared to particles sized 30–125 μm (Figs. 3a, 4a). Glacial-derived and terrestrial OC typically exhibit higher Corg/Ntot ratios and more negative δ13Corg values than marine sources51,52. Our finding indicates that fine particle OM predominantly originates from glacial and terrestrial inputs, while coarser particle OM derives from large phytoplankton and fecal pellets (Fig. 3). Previous studies have shown that fine-grained particles (e.g., clays and silts) in coastal systems are often associated with terrestrially derived OC53, which is more resistant to microbial decay, enhancing burial efficiency54. Additionally, Hg concentration increases with the fraction of glacial particles and decreases as the phytoplankton fraction increases (Fig. 3). The higher contribution of terrestrial materials in the Ross Embayment, relative to the open ocean, is further supported by the higher abundance of land-derived major and trace elements (e.g., Aluminium, Iron, Rubidium, Potassium, Zirconium, Titanium) and higher Potassium/Calcium and Rubidium/Strontium ratios (Supplementary Fig. 2). Thus, we hypothesize that glacial and terrestrial inputs influence the spatial distribution, levels, and burial of OC and Hg in Ross Embayment sediments.

To substantiate our hypothesis that glacial and terrestrial inputs influence OC and Hg accumulations in Ross Embayment sediments, we utilize lipid biomarkers, Corg/Ntot ratios, δ13Corg, and Hg isotopes. All sites show a bimodal distribution of long- and short-chain n-alkanes, suggesting the mixing of marine OC (short-chain) with terrestrially exported OC (long-chain) from Antarctic meltwater48 (Supplementary Fig. 3a). The decrease in sterol indicators of terrestrial plants (campesterol) and soil fungi (ergosterol) across the land-sea transect reflects the expected seaward reduction in contributions of terrestrial OC (Fig. 4b). This is further confirmed by a greater proportion of long-chain n-alkanes in the Ross Embayment (E1 to E8) compared to the open ocean (O1 to O7; Fig. 4b, Supplementary Text 3), consistent with changes in the terrigenous/aquatic n-alkane ratio (TAR), the long-chain to short-chain n-alkane ratio (L/H ratio), and the carbon preference index for long-chain n-alkanes (CPIlong) across the transect55 (Methods, Supplementary Fig. 3). These seaward decrease in total sterols and the increase in n-alkane concentration suggests the selective loss of labile OC (e.g., sterols) and preservation of recalcitrant OC (e.g., n-alkanes) during land-sea transport55. This progressive degradation is further evidenced by the increasing stigmastanol/stigmasterol ratio from nearshore to offshore stations (Supplementary Fig. 3).

Lipid biomarkers and the Corg/Ntot ratio-δ13Corg mixing model indicate that in situ marine production is the primary source of sediment OC in the Ross Sea, while the Ross Embayment locations exhibit relatively higher contributions from glacial and terrestrial OC compared to the open ocean (Fig. 4b, Supplementary Figs. 3, 4). Additionally, a higher proportion of terrestrial OC strongly correlates with increased glacial OC (R2 = 0.86, P < 0.01, Fig. 3a, Supplementary Table 5), likely due to meltwater runoff physically introducing glacial OC from coastal bedrock47 and transporting substantial terrestrial biospheric OC into coastal seawater56. However, the contribution of ocean production, as indicated by lipid biomarkers (brassicasterol, dinosterol, and short-chain n-alkane relative abundance), exceeds that predicted by the mixing model (Fig. 4b, Supplementary Figs. 3, 4). This is likely due to the early diagenetic transformation of sedimentary Corg/Ntot and δ13Corg, especially in regions where OC appears highly degraded (e.g., O1 to O7)57. This could introduce uncertainties in the mixing model computations that result in larger estimates of %terrestrial and %glacial OC, compared to those indicated by the lipid biomarkers.

Furthermore, Hg isotopes illustrate that atmospheric deposition is the primary source of Hg in Ross Sea sediments. Surface sediments in the Ross Sea exhibit near-zero Δ200Hg values (Ross Embayment: −0.02 ± 0.01‰, open ocean: 0.01 ± 0.02‰, Fig. 4c, Supplementary Text 4). Based on mass balance estimates of Δ200Hg isotopes in the Southern Hemisphere atmosphere, we estimated that 60 ± 14% of the Hg in the Ross Sea surface sediments originates from atmospheric Hg0 deposition58–60. The mass balance for mixed atmospheric HgII and Hg0 sources show a positive shift in Δ199Hg in Ross Sea sediments (Fig. 4d), with a Δ199Hg/Δ201Hg regression slope of 1.04 ± 0.14‰ (Fig. 4e), indicating a mixed atmospheric origin and photoreduction of HgII in the surface ocean58.

Compared to Southern Ocean surface waters, Antarctic ice, snow, and terrestrial particles contain much higher unit Hg concentration39,61. Deposited Hg in polar ice, which increased significantly during the Last Glacial Period62, may be transported to the marginal sea via meltwater during the Holocene warming. The Corg/Ntot ratio and δ13Corg correlate positively and negatively with Hg concentration (P < 0.01, Fig. 3), respectively, reflecting contributions of glacial and terrestrial Hg (Supplementary Table 9). Observations suggest this Hg mainly originated from atmospheric deposition33,62, consistent with our results of Hg isotopes. Overall, glacial and terrestrial inputs shape the spatial pattern of OC burial in the Ross Sea. This OC captures significant amounts of atmospherically sourced Hg deposited into seawater and terrestrial ecosystems, greatly enhancing Hg burial.

Temporal decoupling of carbon and mercury
A significant finding of this study is that Holocene warming has accelerated Hg enrichment in Ross Sea sediments, yet it did not correspondingly enhance carbon sequestration in these sediments. From the last glacial period to the Early Holocene (14,000–10,000 years BP), global climate change initiated a rise in Antarctic temperatures and CO2 levels63,64. During the mid- to late-Holocene period (8000–2000 years BP), Antarctic temperatures further increased and remained high up until industrialization (Supplementary Fig. 1). Concurrently, the Ross Sea’s ice grounding line retreat aligned with the rapid warming phase during this period25. This retreat, coupled with ice melt and terrestrial ice debris inputs, significantly affected west Antarctica’s marine environment.

Our analysis reveals that OC burial rates in both the Ross Embayment and the adjacent open ocean have remained relatively stable over the past 12,000 years. Some sediment cores (O1, O2, O4, O6-O8, O12, E4-E6, Fig. 1) display higher OC% in the uppermost layers (Supplementary Figs. 5, 6, Supplementary Text 1, 2), corresponding to the depth of oxygen penetration65, likely reflecting fresh inputs that will exponentially decay over time during burial. Neglecting the degradation of this OC would overestimate its burial flux. Thus, we employed a Power decomposition model to quantify the impact of OC post-deposition remineralization66. Our reconstruction of Holocene OC burial in the Ross Sea shows no significant temporal trends in either the Ross Embayment or the open ocean (Fig. 5a, b). While cores O4, O7, and O8 show increased OC burial, others exhibit fluctuating or declining trends (Supplementary Fig. 7, Supplementary Text 1, 2).

During the Holocene, climate warming led to a loss of almost half of the ice shelf area in the Ross Sea67 and a retreat of the ice grounding line by over 50%68, introducing a substantial amount of glacial and terrestrial OC into the seawater69 (Fig. 6b). This glacier retreat also created extensive open water areas, boosting phytoplankton growth—primarily diatoms—and enhancing marine primary productivity and initial atmospheric CO2 uptake20,47. Moreover, meltwater provided large amounts of nutrients like silicates70 and iron71, which further promoted primary production and CO2 uptake over time72. However, most sediment cores showed no significant increase in fluxes or fractions in glacial and terrestrial OC (Supplementary Fig. 8, Supplementary Text 1, 2). Therefore, although glacier retreat increased external OC input and enhanced the biological carbon pump, it concurrently decreased the density and volume of newly formed bottom waters. This weakened carbon transport to the seabed and enhanced OC degradation in water column26,73. As a result, OC burial remained largely unchanged. Atmospheric CO2 increased by ~70 ppm during the Holocene64, likely turning the Ross Sea into an OC “reactor” that intensified climate fluctuation through positive carbon cycling loops.

Surprisingly, throughout the Holocene, sediment Hg burial in both the Ross Embayment and the open ocean increased rapidly. We observed that Hg burial in the Ross Sea rose by a factor of 5 (Fig. 5b), with almost all sediment cores exhibiting a consistent upward trend (Supplementary Fig. 7). In contrast, ice core Hg concentrations remained relatively constant during this period74, suggesting these trends cannot be attributed to changes in atmospheric Hg concentration.

We find that Hg burial increased by factors of three in the Embayment and eight in the open ocean, although the amount of Hg buried in the Embayment was much greater. The difference in glacial and terrestrial versus marine OC fluxes are likely the major factors driving the larger magnitude of increase in Hg burial in the open ocean compared to the Embayment. Marine production, the primary source of OC in the open ocean (Fig. 4b, Supplementary Figs. 3, 4), likely facilitated Hg transport to the seafloor as productivity increased during the Holocene. Meanwhile, longer residence time and higher mineralization of sinking OC in the open ocean (as evidenced by selective loss of sterols, selective enrichment of n-alkanes, increasing stigmastanol/stigmasterol ratios, and decreasing CPIlong, Fig. 4b, Supplementary Fig. 3) led to smaller particle sizes with higher specific surface areas, enhancing Hg adsorption45,75. The loss of sea ice in the open ocean also increased atmospheric Hg deposition into seawater, further contributing to this effect. Differences in Hg background concentrations between the open ocean and Embayment sediments are another major factor. Early Holocene open ocean sediments had extremely low Hg concentrations (mostly <5 ng g−1), while the Embayment’s glacial and terrestrial sources contributed Hg-rich sediments (mostly 30–60 ng g−1, Fig. 2; Supplementary Figs. 6, 7), resulting in higher early Holocene Hg concentrations. Consequently, the temporal changes in the Hg burial in the open ocean were more sensitive to warming.

The temporal decoupling of carbon and Hg results from differences in removal during particulate sinking (Fig. 6)75. Glacier retreats have greatly increased open water areas, boosting the seawater Hg0 evasion44,45. However, these open areas also enhance atmospheric Hg deposition onto the ocean. Warming-enhanced marine productivity incorporates dissolved Hg into particulate matter21, and fine particulate debris from glaciers and snow introduces high concentrations of Hg into seawater (Fig. 4a)61; these mechanisms therefore enhance Hg inputs into the Ross Sea. As particulate OC sinks and is remineralized into CO2, Hg concentration in settling particles gradually increases, enabling deeper transport into the water column45,75. These processes have caused the temporal decoupling of OC and Hg during the mid-Holocene warming (Fig. 5b). Therefore, our results demonstrate that increased Hg input caused by Holocene glacier retreat completely offsets the enhanced Hg evasion, leading to greater Hg sequestration in Ross Sea sediments (Fig. 6).

Potential feedback of Antarctic carbon and mercury cycles to warming
The sequestration of carbon and Hg in marine sediments is crucial, as it affects their atmospheric and oceanic residence time, impacting both climate change35 and ecological health10,36. Our study utilized chemical analyses and isotopic techniques to explore how carbon and Hg cycles in the Ross Sea might respond to climate warming. Our analyses suggest that warmer seawater temperatures and increased microbial activities likely promote OC remineralization into CO226,76. This implies that warming weakens the physical transport of OC by AABW73, the critical process for transferring CO2 absorbed from the Southern Ocean surface to the deep water. Despite the Southern Ocean absorbing 40% of anthropogenic atmospheric CO215,16, the weakening of AABW could reduce its OC sequestration capacity73, creating a positive feedback loop that exacerbates climate warming.

Moreover, the temporal decoupling of Hg and OC highlights how climate warming differently affects their burial. Our results imply that even without anthropogenic Hg emissions during the Holocene, warming-induced Antarctic glacier retreat significantly boosted atmospheric Hg sequestration in the ocean and enhanced the influx of glacial and terrestrial Hg into seawater (Fig. 6). Since warming can accelerate OC remineralization, it should also exacerbate Hg methylation in seawater12,13. Anthropogenic Hg emissions since industrialization have tripled Hg levels in global surface seawater77. Thus, warming-driven glacier retreat is likely to amplify the impact of these emissions on the Southern Ocean ecosystem78.

Unlike process-based models, relying solely on observational efforts is insufficient for providing a complete understanding of large-scale carbon and Hg cycles. Meanwhile, theoretical and technical gaps remain in understanding the precise sources and fate of carbon and Hg in the Southern Ocean. The limited variation in carbon and Hg isotope signals in this study presents challenges for more precise source assessments. Despite inevitable variability from sediment spatial heterogeneity, our measured temporal trends can contribute to carbon and Hg cycles simulations, offering key insights into their fate under warming in the Southern Ocean (Fig. 6). Due to low sedimentation rates in Ross Sea sediments32, we cannot provide high-resolution chronological records of recent anthropogenic Hg pollution. Additionally, the scarcity of calcareous foraminifera in Southern Ocean sediments introduces stratigraphic age uncertainties79–81. Aligned with our finding (3–8 fold), Zaferani et al.’s sediment core analysis in the Adélie Basin shows a mid-Holocene (~6000 years BP) increase in Hg burial by 2–3 fold, similar to the rise driven by anthropogenic Hg emissions since industrialization19. This raises the important question of whether the last 200 years of anthropogenic emissions have impacted Southern Ocean Hg levels as significantly as the past 10,000 years of glacial retreat.

In summary, our research emphasizes the need to moderate expectations regarding the Southern Ocean’s capacity to buffer against climate warming, and the enrichment of toxic Hg in the region due to warming should not be overlooked. Reducing greenhouse gas and Hg emissions remains the best strategy to mitigate these feedbacks.

Methods
Sample collection
Seafloor sediment cores measured in this study were recovered by Columbia University and Oregon State University (Fig. 1, Supplementary Table 1). Among these cores, NBP1502A-44KC (E3), NBP9401-16TC (E6), 17TC (E7), 20TC (E8), 31TC (E5), 33PC (E4); NBP9501-11TC (E2), 26PC (E1), NBP9802-3MC1 (O2), 3TC2 (O3), 4PC1 (O5), 5MC2 (O6), 6PC1 (O8), 7TC1 (O10), 7TC2 (O9), 8TC1 (O11), and 9TC1 (O12) were collected based upon four field seasons aboard the research vessel (RV) Nathaniel B. Palmer, the first National Science Foundation-supported vessel dedicated entirely to scientific operations in the Southern Ocean82. The other cores (KN7812-5BC (O7), 7BC (O1), 10BC (O4)) were collected on board the RV Knorr. Most of the samples were collected using trigger core and box core. These two core types have been widely applied for recovering surface sediments80. We retrieve the topmost 30 cm of sediment to encompass the Holocene period. The sampling interval of each sediment core was 1–2 cm. Prior to analyses, all samples were freeze-dried at –50 °C to constant weight. Then all dried samples were milled and homogenized. Dry bulk density is calculated by water content (wet) and porosity.

Sediment lithological description and chronology
The physical characteristics of the sediment cores, including lithology, magnetic susceptibility (10−5 SI units), smear slide analyses, structure, and disturbance record, are derived from previous expedition research83,84. Marine sediment lithology can indicate the historical origin of sediments and serve as a proxy for changes in the upper ocean environment and terrestrial inputs. The main lithologies of global marine sediments include siliciclastic, volcaniclastic, transitional, and biogenic85. Due to the influence of ice shelf erosion, the lithology of marine sediments in Antarctic marginal seas (such as the Bellingshausen Sea and Ross Embayment) is mainly diatomaceous mud (transitional)85. In the open ocean area of the Southern Ocean, the composition of sediments is predominantly characterized by diatom ooze85. Further sediment lithological descriptions are obtained from field survey data and can be found at Pangaea.de.

Due to the lack of calcareous foraminifera in Antarctica, sedimentation rates and core chronology used in this study were mostly calculated using the 14C age of acid-insoluble OM79–81 (Supplementary Fig. 9), a method previously applied to sediment cores in the Antarctic shelf79, with reservoir ages and standard errors for the sites obtained from the literature (Supplementary Table 11). The 14C data used in this study were obtained from laboratory measurements and published literature67,80,86–89. Radiocarbon (14C) determination of some surface sediments (Supplementary Table 11), direct acidification using a weak 1M HCl solution was chosen in accordance with Kennedy’s recommendations90 and steps during method intercomparisons. All acidified samples were prepped individually in tin capsules (15–60 mg based on samples’ OC%) and combusted in Elemental Analyzer (EA). The CO2 was then captured in Gas Interface System (GIS) and transported for further 14C analysis. Analyses of 14C were conducted using an accelerator mass spectrometer (AMS) MICADAS (Mini Carbon Dating System) at Yale Analytical and Stable Isotope Center (YASIC) at Yale University. All samples are corrected using 14C-free blanks, primary NIST standard oxalic acid II. 14C results were presented as 14C ages, Δ14C values, and fraction modern (Fm) values (Supplementary Table 11). Fm values were calculated from Δ14C values based on the below equation:

14C age = −8033 × ln(Fm) (1)

Fm = e14C/8033 (2)

Δ14C = (Fm × e(1950−sample year)/8267 − 1) × 1000 (3)

More comprehensive discussions of age model development have been provided in the published literature67,80,86–89. These studies use detailed sedimentological, physical properties, and geochemical data to develop strict measurement strategies and provide reliable 14C ages. The depletion of 14C in Antarctic waters is known as the “Antarctic reservoir effect”. All 14C dates from the Antarctic must be corrected by a factor that accounts for the pool of “old” carbon in the Antarctic waters. Fixed reservoir corrections are unlikely to yield reliable absolute ages for single-dated stratigraphy within sediment cores. Andrews et al. suggested that dates from a single core should be corrected by its core top age81. The 14C age correction for these cores was, therefore, based on the age of surface OM at each core site, rather than a standard correction for all ages at all sites (Supplementary Table 11). O11 and O12 lack evidence of surface radioactive carbon isotopes; thus, sedimentation rates (SRs) of these cores were estimated based on Thorium-230 (230Th)86,87. Some sediment cores (O7, O12, E1, E3, E5, and E7) do not have ages that cover the entire Holocene, and these cores were temporarily excluded from subsequent time series analysis.

Organic carbon and mercury
Sediment samples were analyzed for Hg concentrations using a solid-phase Milestone DMA-80 analyzer at the Ministry of Education Laboratory of Earth Surface Processes, Peking University. All samples were measured in duplicate, with all relative standard deviations (RSDs) for analytical precision being <15% (Supplementary Table 2). Approximately 0.04–0.10 g of each sample was used. We analyzed one replicated standard for every ten samples. Analytical accuracy, as indicated by the average recoveries of certified reference material (CRM) TORT-3 (NRC), TORT-2 (NRC), and SRM-2976 (NIST) are 103.6 ± 6.5% (n = 62), 101.3 ± 3.9% (n = 5), and 100.7 ± 2.9% (n = 4), respectively. The method blank is 0.041 ± 0.015 ng g−1 (n = 30) for Hg in all samples.

The OC and TN analysis processes were completed at the Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences. For the OC measurement, a total of ~300–400 mg of each sample was treated with 1N HCl for 12 h at room temperature to remove inorganic carbons. Then, all acidified samples were rinsed to neutral pH and freeze-dried. The sample was wrapped in tin foil and put into an elemental analyzer (vario MACRO CUBE, ELEMENTAR). For calibration, the CRM D-Phenylalanine (C9H11NO2, Alfa Aesar) was analyzed every 15 samples. Based on the amounts of CO2 generated from the OM, the OC contents were measured with an accuracy of ±5%. While measuring OC, TN% was obtained by combusting nitrogen-containing compounds and reducing them to N2, with an accuracy of ±5%.

Carbon isotope
To remove inorganic carbons, the bulk sediments were acidified using a 1M HCl solution. Before analysis, about 10–30 mg decarbonated sediment was weighed and placed in a tin capsule. Stable isotopes (δ13C and δ15N) were analyzed using the Thermo Scientific MAT 253 Stable isotope ratio mass spectrometer system (Thermo Fisher Scientific) at the Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences. The oxidation and reduction reactors were heated to a temperature of 960 °C. The oven was set to maintain a temperature of 65 °C. Helium was used as the carrier gas with a flow rate of approximately 100 mL min−1. O2 was used for flash combustion at a flow rate of 180 mL min−1. Stable isotope ratios were expressed in δ notation as parts per thousand (‰) deviations from the international calibration standards according to the following equation:

δ = (R sample / R standard − 1) × 103 (4)

where R is the 15N/14N or 13C/12C ratio, and air and Vienna Pee Dee Belemnite (PDB) were used as international standards for δ15N and δ13C measurements, respectively. The isotopic values were determined using official CRM IVA 33802174 (IVA analysentechnik), consisting of certified urea, which provided values of δ13C = −39.73 ± 0.16‰, PDB δ15N = −0.73 ± 0.16‰. The relative standard deviations of δ13C and δ15N are 7.0% and 10.5% (n = 38), respectively. To ensure accurate calibration, the CRM was analyzed every ten samples. The precision of the measurements was ±0.20‰ for δ13C and ±0.14‰ for δ15N. During the 14C measurement, the 13C value was also obtained. Comparison of measurement results between AMS-MICADAS and the MAT 253 Stable isotope ratio mass spectrometer system reveals that the RSD was less than 3.0%.

Mercury isotope
According to the total Hg concentrations in sediment samples, ~0.2–1.0 g of each surface sediment (0–1 cm) sample was pre-concentrated for Hg using the combustion-trapping method91, and 2 mL of 40% double-distilled acid (2 HNO3 / 1 HCl, v/v) was used to trap the volatilized elemental Hg (Hg0). Procedural blanks and CRMs were processed with the samples in the same manner. The procedural blanks account for <1% (n = 37) of Hg mass in the samples, and the Hg digestion and preconcentration recoveries are all in the range of 80–115% (n = 12) for both the samples and procedural CRMs. The processed sample solutions were diluted with Milli-Q water to Hg concentrations of ~0.5–0.8 ng g−1, and were measured for Hg isotope ratios by coupling a customized cold vapor generation system to multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS, Nu Plasma 3D at Tianjin University, China). Specific details on instrument mass deviation and program settings were provided in Sun et al.91.

Here, the mass-dependent fractionation (MDF) is expressed as small delta (δxxxHg) per mil (‰) derived from the NIST 3133 Hg bracketing standard:

δxxxHg(‰) = [(xxxHg/198Hg)sample / (xxxHg/198Hg)NIST3133 − 1] × 1000 (5)

where “xxx” represents the mass number of the measured Hg isotopes: 199, 200, 201, 202, and 204. Mass independent fractionation (MIF) is expressed as capital delta (ΔxxxHg). ΔxxxHg is the deviation of the measured isotope ratio from the isotope ratio predicted using the kinetic MDF law:

ΔxxxHg(‰) = δxxxHg − β × δ202Hg (6)

The mass-dependent scaling factor β is 0.2520 for 199Hg, 0.5024 for 200Hg, 0.7520 for 201Hg, and 1.4930 for 204Hg.

Hg isotope ratios of secondary standard UM-Almad’en standard NIST 8610 and procedural CRM (GSD-11 GBW07311) analyzed during the analytic session agree with those reported in previous studies91. The typical analytic uncertainties of samples, estimated as the larger 2 times standard deviation (SD) uncertainties of Hg isotope ratios in NIST 8610 and GBW07311, are provided along with the analytical precision of two Hg standards measured during the Hg isotope analysis in Supplementary Table 10. The Hg isotope of various materials in the polar environment compartment used for comparison is also given in Supplementary Table 8.

Grain size
About 0.5 g of each dried bulk sample was passed through a 2 mm sieve. The fraction was then treated with 20% H2O2, 10% HCl, and 1 mol L−1 Na2CO3 to remove OC, CaCO3, and opal, respectively. Finally, the sample was dispersed in 2 mL of Sodium Hexametaphosphate ((NaPO3)6)92. The grain-sized distribution was determined using a Mastersizer 2000 diffraction grain sizer (MALVERN). Fine sand (63–250 μm), silt (2–63 μm), clay (<2 μm), and average particle size d(50) were obtained, respectively. We also categorized particle sizes into ranges of 1–2 μm, 2–10 μm, 10–20 μm, 20–30 μm, 30–40 μm, 40–50 μm, 50–63 μm, 63–100 μm, 100–125 μm, 125–200 μm, 200–1000 μm, 1000–1500 μm, and 1500–2,000 μm to investigate the correlation between debris size and the concentrations of OC, Hg, Corg/Ntot ratio, and δ13Corg.

Burial of mercury and organic carbon
We calculated the Hg burial rate based on the sediment Hg concentration, sedimentation rate, and dry bulk density as follows36:

Hg burial rate,ij = Hg conc,ij × D ij × SR ij × 10 (7)

where Hg burial rate,ij is the Hg burial rate in μg m−2 yr−1 in depth j of core i; Hg conc,ij is the Hg concentration in ng g−1 in layer j of core i; D ij is the dry bulk density in g cm−3 in layer j of core i; and SR ij is the sedimentation rate in cm yr−1 in layer j of core i.

When using marine sediment cores to determine OC burial rates, post-deposition mineralization (γ) needs to be considered to avoid overestimating OC burial. That is, OC could decompose to a certain extent over time after deposition, and the ultimately buried OC only accounts for a part of the pre-deposition, also known as burial efficiency. Here, we utilized the model proposed by Middelburg, the Power decomposition model, which introduced a degradation rate constant that changes with time66:

dOC t,ij / dt = −k(t) × OC t,ij (8)

where OC t,ij is the OC concentration of depth j of core i after degradation in time t (yr) since deposition, and k(t) is the degradation rate and is a function of time. If k is assumed to be constant within a short time interval, k can be calculated by the following formula:

k = 1/Δt ln(OC t,ij / OC(t+Δt),ij) (9)

By substituting the OC observation data in the sediment core into the above equation and integrating it, the functional expression of k with respect to t (yr) can be obtained:

log k = a log t + b (10)

Therefore, the concentration of OC at the initial deposition of depth j of core i (OC 0,ij) and the proportion of post-deposition mineralization (γ ij) after t (yr) can be obtained:

OC 0,ij = OC t,ij × e(t 0 (Δt × k t)) (11)

γ ij = (OC 0,ij − OC t,ij) / OC 0,ij (12)

As time increases, the degradation rate gradually decreases, and γ ij will converge to the maximum value. Thus, the burial efficiency calibration coefficient (φ i) of each sediment core is obtained by:

φ i = 1 / (1 − lim t→∞ γ ij) (13)

Finally, we calculated the burial rates of OC based on the following equation:

OC burial rate,ij = OC % × φ i × D ij × SR ij × 104 (14)

where OC burial rate,ij is the OC burial rate in g m−2 yr−1 in depth j or core i.

Major and trace element concentrations
Total concentrations of major and trace elements (Sulfur (S), Titanium (Ti), Iron (Fe), Rubidium (Rb), Strontium (Sr), Zirconium (Zr), Aluminium (Al), Silicon (Si), Potassium (K), Manganese (Mn), Calcium (Ca), Niobium (Nb), and Yttrium (Y)) in bulk sediments were determined by X-ray Fluorescence (XRF), Geotek XYZ system at Oregon State University. The Olympus XRF is mounted on the Geotek XYZ system, including a Konica Minolta spectrophotometer and point magnetic susceptibility sensor. An energy calibration check was performed on a 316 stainless steel alloy coupon to standardize the instrument, using a voltage of 40 kV for 15 s. Core samples are measured in Geochem mode with two beams, one at 10 kV and one at 40 kV, each for 10 s. The analysis interval was 1 cm and results for each element are given in ppm or % (Supplementary Table 12)93. In the Delta device, light elements (abbreviated as “LE”), contain everything lighter than magnesium, plus all other detected elements not identified otherwise. CRM (NIST 2710a (Montana Soil I) and NIST 2711a (Montana Soil II)) and replicates were measured in each set of samples for accuracy and precision control. Nine replicate measurements were performed within a 120-s counting period for NIST 2710a and NIST 2711a for calibration verification and precision testing. The measurements for lighter elements deviated by 20–30% from the certified reference values, while the trace and heavier elements deviated by ~10–15%, with RSDs ranging from 0.1% to 10%. Repeated analysis of samples gave RSDs less than 5% for Fe, Sr, Si, Ca, and K; less than 10% for Rb, Zr, Ti, and Al; less than 15% for S and Mn; less than 20% for Y and Nb.

Extraction of lipid biomarkers
Residual sediments sampled from 5 to 10 cm intervals were combined into mixed samples (Supplementary Tables 13, 14) and used for sterol and n-alkane extraction. Briefly, total lipids were extracted by Dionex Accelerated Solvent Extractor 200 (ASE 200) at 100 °C, using 9:1 Dichloromethane (DCM):Methanol (MeOH) as a solvent. Total lipid extracts were saponified with 0.5N potassium hydroxide (KOH) in MeOH at 70 °C for 2 h. Neutral fraction was recovered in hexane (Hex), followed by separation of apolar and polar fraction by column chromatography, using 9:1 Hex:DCM and 1:1 DCM:MeOH as solvents, respectively. Both fractions were analyzed with a Thermo Trace 1310 gas chromatograph (GC) coupled to a Thermo TSQ 8000 Mass Spectrometer (MS) in split-less mode, using a 30-m DB-5MS column. Instrumental conditions for the analyses of both fractions were provided in Supplementary Text 3.

For apolar fraction, the concentration of C13 to C35 n-alkanes was calculated from calibration curves created by external standard and separated into chain-length groups: long-even (C26, C28, C30, C32, C34), long-odd (C27, C29, C31, C33, C35), mid-even (C22, C24), mid-odd (C23, C25), short-even (C16, C18, C20), and short-odd (C15, C17, C19, C21). The terrigenous/aquatic n-alkane ratio (TAR)94, long-chain to short-chain n-alkane ratio (L/H ratio)94, the carbon preference index of long-chain n-alkanes (CPIlong)95 and short-chain n-alkanes (CPIshort)96 were calculated according to the Eqs. 15 to 18, when Ci represents the concentration of n-alkanes with i carbon atoms. Sterols were recovered in polar fraction, after derivatized with N,O-bis (trimethylsilyl) trifluoroacetamide (BSTFA), and quantified by external standards.

TAR = (C27 + C29 + C31 + C33) / (C15 + C17 + C19) (15)

L/H = Σ(C23 to C35) / Σ(C13 to C21) (16)

CPIlong = 0.5 × [(C25 + C27 + C29 + C31) / (C26 + C28 + C30 + C32) + (C27 + C29 + C31 + C33) / (C26 + C28 + C30 + C32)] (17)

CPIshort = (C15 + C17 + C19) / (C16 + C18 + C20) (18)

The short-chain n-alkanes, brassicasterol, and dinosterol represent compounds primarily derived from marine sources. The long-chain n-alkanes, stigmasterol, sitosterol, campesterol, and ergosterol represent compounds mainly derived from terrestrial sources55. In the Antarctic region, sea-ice algae and penguin guano are important sources of even-dominated short-chain n-alkanes97–102. Ross Sea phytoplankton could be an important source of stigmasterol103–105, while penguin guano is enriched in cholesterol101 (see details in Supplementary Text 3).

Organic carbon source apportionment
The Corg/Ntot ratio and δ13Corg values have been considered effective indicators for determining OC sources in sediments51,52,106. A three-end-member mixing model using Corg/Ntot ratio and δ13Corg source markers was used to determine the relative proportions of marine primary production-derived OC (OCmar), terrestrial OC (OCterr), and glacier-derived OC (OCglacial) which still keep their original δ13Corg and Corg/Ntot ratio characteristic. The OCmar here represents particulate OC generated by primary producers in surface seawater, algae hosted in sea ice, and any autotrophic or heterotrophic byproducts from the marine environment47. The OCterr here represents a portion of the biospheric pool within the active carbon cycle, primarily consisting of vegetation components (fresh vegetation litter and degradation products or debris), soil components, as well as biological feces and hair46,47. OCmar and OCterr collectively represent the biospheric pool within the active carbon cycle (Supplementary Text 3). In contrast, OCglacial is integral to the geological carbon cycle, including surface runoff and subglacial discharges from glacial meltwater, as well as freshwater discharges caused by ice sheet loss69:

fmar × δ13Cmar + fterr × δ13Cterr + fglacial × δ13Cglacial = δ13Csample (19)

fmar × (C/N)mar + fterr × (C/N)terr + fglacial × (C/N)glacial = (C/N)sample (20)

fmar + fterr + fglacial = 1 (21)

where δ13Cmar, δ13Cterr, and δ13Cglacial are the respective δ13Corg end-members of marine primary production, terrestrial inputs, and glacial OC. The same equations were applied to Corg/Ntot ratio. The fmar, fterr, and fglacial represent the OC fraction from the three apportionments, respectively. Referring to previous studies on δ13Corg and Corg/Ntot ratio of various end-members of Antarctic land and ocean (Supplementary Table 9), here we used end-member values of −25.2 ± 5.1‰ and 5.6 ± 2.0 for δ13Cmar and C/Nmar107,108, −27.8 ± 1.5‰ and 14.6 ± 4.0 for δ13Cterr and C/Nterr109–111, −26.8 ± 1.1‰ and 9.1 ± 3.2 for δ13Cglacial and C/Nglacial112–114, respectively. The end-member values used are from the average of all data reported from the three sources (Supplementary Text 3). Although the most common strategy for source proportion calculation is by setting up a system of linear equations using fixed end-member value, but this methodology introduces a bias that the calculated fractions and corresponding uncertainties are dependent of the variability of the source distribution, and the calculated results of different source proportions are fixed value which usually are unreliable. Thus, in this study, the Monte Carlo simulation strategy is implemented to assess source variability to avoid errors from arbitrary assignments of end-member values.

Statistical analysis
All statistical analyses were performed using R software (version 4.2.3). A vertical profile point-line graph with B-spline connections was generated for Hg, Hg burial, and OC burial in sediment cores. Due to the positive skewness in the mean values of Hg, OC content, and other environmental physicochemical parameters in sediment cores, a non-parametric test (Mann–Whitney U test) was employed to compare the differences between the two groups for each variable, with a significance level set at P < 0.05. We used a linear regression model to identify any potential auxiliary parameters that may influence variations in Hg and OC content in each sediment core. The Monte Carlo simulations were conducted, assuming that end-member values follow a normal distribution. The computations involved random sampling from these normal distributions, satisfying Eqs. (19)–(21) while constraining the solutions between 1 and 0 (representing fraction contributions). Distributions of the fractions were generated by iteratively performing random sampling 100,000 times and organizing the results into histograms. The resulting coefficient of variation for the average relative contribution is 0.03%, indicating the statistical robustness of our model. To analyze the trends of OC and Hg burial in the Ross Sea from the Holocene to the present, after excluding sediment cores O7, O12, E1, E3, E5, and E7 that are not old enough to cover the Holocene, the ages of the Ross Embayment and open ocean sediment cores at each depth were sorted. The sorted Hg burial rate and OC burial rate were averaged at a time interval of 500–1000 years, and the min-max normalization method was used to process the Hg burial rate and OC burial rate. Polynomial curve fitting (no more than 3rd order) was performed on the normalized data points, and the significance level was P < 0.05.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript provides organic carbon and Hg data from analyses of 20 sediment cores taken from the Ross Sea embayment and the adjacent open sea (pelagic sediments). In principle, the authors found that Hg and carbon concentrations as well as accumulation rates of both elements are higher in embayment sediments as compared to sediments of the open sea. Based on C/N ratios they concluded that carbon in embayment sediments is of terrestrial origin, while carbon in pelagic sediments comes from primary productivity (mainly diatoms). Similar, they concluded that the higher Hg burial in the embayment is attributed to glacier melting and other terrestrial sources. The main hypothesis is that global warming will increase retreat of Antarctic glaciers in this area thus Hg but not carbon fluxes to the sea. Furthermore, they observed in their sediment cores, that carbon accumulation in their core remains more or less constant while Hg burial increases by a factor of 3 in embayment sediments and a factor of 8 in the open sea sediments.\n\nThe data is interesting because data on Hg and carbon in Antarctic sediments is still restricted to view studies only. However, the relationship between carbon and Hg burial in marine sediments as well as the observation that coastal sediments show higher carbon and Hg burial than those of the open sea is not entirely new and not surprising. Moreover, the embayment cores were taken in relatively shallow water while the open sea cores are taken from depths below ~ 3000 m. This makes a large difference regarding mineralisation of organic matter and recycling of Hg in the water phase from sinking particles. Although carbon and Hg concentrations and burial appears to be lower in open sea sediments, the overall increase of Hg accumulation in open sea sediments is much higher (8-fold), which somehow contradicts that assumption that Hg from terrestrial sources and glacier retreat may play an important role in Hg fluxes to the sea. This might be an issue for Ross Bay but not for entire Antarctica. In this context, a discussion on total carbon fluxes from the terrestrial environment compared to OM production in the open sea might be helpful. What are these terrestrial carbon sources (mainly excrements from marine mammals and penguins)? I guess that such terrestrial C and Hg fluxes might be locally of importance, but not on a larger scale, if compared to the high productivity in the Antarctic Ocean (statement of the authors on Hg accumulation on open sea diatom-ooze sediments).\n\nThe manuscript is not very well written and structured and sometimes difficult to follow. It seems that a lot information in the data is somehow hidden in the different models and figures (e.g. SI Fig.3 and Fig.4) which average and mix up the information. This is rather confusing than really supporting the core message. A closer look at the data of individual cores shows that there is a lot of variation (e.g in sedimentation rates and often “trends” are not always seen) and that the message is probably not as clear as it is stated by the authors. This variability needs more explanation and discussion.\n\nMoreover, there is a lot of speculation on the origin and fate of carbon and Hg which is poorly supported by data or is taken from speculations in other studies. I did not understand what the difference between glacier and terrestrial organic matter is. This was somehow taken from other studies, but it remains unclear if this differentiation is of importance here and if the data show supports this differentiation. (The contribution of terrestrial fluxes e.g. could be easily determined by analysing lithogenic elements.)\n\nThe interpretation of the Hg isotope data is also highly speculative and only relies on Hg isotopes as a source tracer (although it is quite likely that all Hg is actually coming from the atmosphere (there is no other source), I think that we also have to expect process-controlled fractionation which is not discussed in the ms.\n\nIn summary, my main concern is the lack of a clearer explanation of the processes affecting the different accumulation of C and Hg in the individual sediment cores which allows to derive general controlling meachnism and effects which are coupled to global warming etc.\n\nI won’t go into more detail here as I think that the manuscript in its present form (confusing structure and line of reasoning) is not suitable for publication in Nature Communications.", "reviewer_2": "Zhou et al., investigated the relationship between the marine carbon pump(s) and the export and sequestration of Hg, as well as the impact of climate warming on these processes. Particularly, they examined how did organic carbon (OC) and Hg sequestrations responded to glacial retreat during the Holocene warming using multiple sediments from the Ross Sea, Southern Ocean. Using isotopic data with a mixing model, they showed that glacial and terrestrial-sourced OC and Hg largely influence the spatial distribution and accumulation of these components in Ross Sea sediments. Interestingly, they found that the burial rates of OC remained constant during the Holocene warming, while the burial of Hg significantly increased from 8 kyr BP to present. They suggest that this observed decoupling resulted from higher inputs of Hg to the Ross Sea, triggered by the retreat of the ice-sheet during the Holocene warming. These enhanced Hg inputs would have increased the concentrations of Hg in the exported organic particles, ultimately increasing Hg burial rates. They infer that, considering the modern anthropogenic inputs of Hg, current global warming could promote Hg bioaccumulation in marine food webs.\n\nThe authors presented a comprehensive dataset of OC and Hg from several sediment cores which provide an extended spatial resolution with samples from near the ice shelf, and from the open ocean. I believe that their results represent an important contribution to the field, and that their conclusions are important and generally supported by their data. I appreciate the amount work done regarding the geochemical and data analyses but I think that the introduction of some of the background concepts could be improved (for the general readers), and that some data should be interpreted more in-depth. In addition, more work could be done regarding the referencing in some parts of the manuscript.\n\nDetailed comments and suggestions:\nIntroduction\nL 35-37: Provide a reference, i.e., IPCC report.\nL37-38: Replace “The biological pump” by “The marine biological carbon pump” and explain better for general readers. The 3 main processes involved in this biological carbon pump are (1) photosynthesis, (2) sinking and (3) remineralization. The physical carbon pump should also be mentioned here (not only the biological one), especially because the authors refer to the process of water mass formation in the subsequent discussion.\nL42: Few words about the origin/sources of this anthropogenic Hg should be added here.\nL49: “transported” rather than “carried”.\nL54: It is true that observations are limited in polar ecosystems due to the remoteness of these areas, but I would also expect less anthropogenic inputs of Hg there compared to other locations. What is the main source of Hg to the Southern Ocean?\nL56-59: Upwelling would actually represent a source of CO2 to the atmosphere, not a sink. Also, I am not sure that Fogwill et al. 2020 is the best reference to support this statement.\nL63: “sinks” instead of “deposition”.\nL63-65: References 26-28 focused on the Arctic Ocean. Aren’t there similar studies undertaken in the Southern Ocean?\nL72-73: Rephrase “short” by “too short to study this mechanism”.\nL79-80: There are several locations of AABW formation around Antarctica, not only the Ross Sea.\nL82: Typo “Antarctic”\nL93: Complete “identify the source of OC and Hg to …\"\n\nSpatial patterns in carbon and mercury sequestration\nL98 and onward: This section generally presents the data and compares it with the existing literature but is lacking of more in-depth interpretation of the results.\nL105-106, Figure 2: Can the authors comment on why the core O12 presents such higher concentrations compare to the other cores from the open-ocean area?\nL108-109: Which sector of the Southern Ocean?\nL116: It would be clearer if the authors referred to the same SR units as the ones used in the corresponding figure.\nL124-126, L128-130: Why do we observe such a difference for the Ross Sea compare to these other locations?\nL132: Typo “Notably”\nL134: Please explain what is the ‘sea ice capsuling effect”.\n\nSource of sedimentary carbon and mercury\nL141: How do the authors justify that it originates mainly from the atmosphere? It looks likes it is explained later on, but in that case, this statement comes up too early in the discussion.\nL143-148: What about the lack of correlation observed for the largest particles?\nL148-149: Figure 3a shows correlation coefficients, not Corg/Ntot and δ13C values.\nL149-152: It is not clear how do the authors get to this conclusion.\nL173-175: There is no data of Δ200Hg in Figure 2.\nL178-181: Could the authors add the plot of Hg in the atmosphere to this graph to further support this statement?\nL182-184: This sentence sounds odd. Please rephrase. My understanding is that terrestrial particles have higher Hg concentration than ice and snow?\nL186-188: “reflecting the contribution of glacial and terrestrial Hg”: based on what? The values of Supp. Table 9?\nL205-206 and L212-214: The authors should precise which sediment cores are they referring to, and explain the differences observed in the OC% and burial.\nL221: The references 80 and 81 are studies focusing on the Arctic. Could the authors provide similar references for the Ross Sea or at least the Southern Ocean?\nL222-224: Complete this sentence with ‘over time’.\nL224-228: Why would this process exacerbate the degradation of OC in seawater? If I understand well, the authors are suggesting that the warming basically decreased the efficiency of the physical carbon pump. And what about the biological pump? It was mentioned before that glacier retreat should have enhance primary productivity, so this should induce a higher OC export via the biological pump. Please clarify.\nL235-236: Hg burial is shown in Supp. Fig 7, not 5 & 6.\n\nPotential feedback of Antarctic carbon and mercury cycle to warming\nL253: ‘residence time’ instead of ‘lifetime’.\nL259-260: The Southern Ocean absorbs 40% of the *anthropogenic* atmospheric CO2.\nL277: I would use ‘contribute to’ rather than ‘constrain’.\nL284-286: This sentence is unclear. What do the authors mean by Hg pollution being the same magnitude than the anthropogenic emissions?\n\nMethods\nL345: Most of the sediments were collected by trigger core. According to Supp. Table 1 only one sediment core was collected by multi-core.\nL348: Remove the ‘-‘ 50°C.\nL351: It would make more sense to have the section on the core’s chronologies and lithological descriptions before the elemental concentrations and isotopic analyses.\nL354-355: Please refer to a figure or table with these results. That would be Supp. Table 2?\nCould the authors comment on the higher standard deviation obtain for some samples, e.g., E4?\nL356-358: Add a sentence on the precision and accuracy of these analyses.\nL366: Can the authors expand on the TN% analyses? Shouldn’t it be organic N as well, rather than total?\nL384-385: Please report the deviation to the reported value.\nL388-404: Consider moving this paragraph on 14C in the section ‘Chronology’.\nL388: ‘some surface sediment’? Which ones? And why not all of them?\nL396-397: Again, please refer to the figure or table where these values are reported so the reader does not need to look for it.\nL421-422: What about 202Hg (shown in Fig. 2)?\nL428-432, Supp Table 10: Please also report the certified values for these standards.\nL438: Change ‘to’ by ‘with’.\nL460-462, Supp Fig 10: It would be nice to have a zoom in the first 30 cm since it is the sediment interval used in this study. It would also be good to have another figure with the corrected 14C ages, along with the uncorrected ones.\nL463: Precise which reservoir ages were used.\nL464-465: Provide references.\nL471-472: Please report these corrected 14C ages in Supp. Table 11.\nL473-475: Precise which sediment cores are involved.\nL509-510: Provide references.\nL528-529: It is unclear how these end-member values were obtained. Are they averaged values from the data shown in Supp. Table 9?\nL539: Provide references of these reports.\nL559: Which sediment cores were not old enough to cover the Holocene? Maybe this information should be added to the section ‘Chronology’.\n\nFigures\nFigure 1: Dorschel et al. is reference n°90, not 92.\nFigure 3: Please add a title and units to the y-axis of Fig. 3. Also precise the unit of ‘OC’ and ‘Hg’ at least in the figure’s caption.\nFigure 5: The authors do not refer to Figure 5 in the main text. This Figure is actually very nice and informative and should be discussed accordingly.\nSupp. Figure 2: What does OC_fpp stands for?", "reviewer_3": "The complexity of Hg cycling in marine polar environments challenges predictions of future mercury cycling in a warming climate. This manuscript utilizes palaeoceanographic data during the Holocene warming period to examine how Hg sequestration in marine sediments was impacted by warming seawater temperatures and glacial melt. The authors found no significant variation in carbon sequestration during Holocene warming, but a 3-8 fold increase in Hg sequestration. The conclusion is that enhanced deposition of Hg to ice-free surface waters and glacial input drove higher rates of Hg sequestration. This is a significant finding as other hypotheses (i.e., Hg(0) evasion w/out ice cap, biomass dilution) suggest that Hg levels in polar marine environments will decreases with climate warming.\n\nDiscussion of historic atmospheric Hg concentrations would strengthen the conclusions. Is there ice core data during this period that shows fluctuation in atmospheric Hg concentrations? Perhaps the 3-8 fold increase in Hg sequestration is influenced by changes in natural atmospheric Hg concentrations (e.g., from volcanic emissions).\n\nThe significance of this work extends to climate change and carbon sequestration research. The study design and data analysis are sound, figures and supplementation information provide a clear representation of the data.\n\nOther comments:\nLine 42: Pre-industrial Hg data are presented, but the introduction focuses on anthropogenic Hg. This is important for the context and implications of the work, but there should also be discussion of natural Hg sources and variations and Hg(0) dynamics at the surface ocean.\nLine 130: This extrapolation likely underestimates Hg burial since industrialization, which I think is worth mentioning. The authors use pre-industrial Hg burial rates for this estimation, though concentrations of atmospheric Hg have increased more than 3-fold since industrialization"}, "author_response": "We sincerely thank the reviewers for their valuable comments and suggestions on our\nmanuscript, which enabled us to disseminate our work to the highest possible quality. We\nhave thoroughly reviewed and carefully considered all the comments and suggestions and\nhave incorporated them into a revised manuscript. For example, in response to Reviewer\n1’s feedback on “individual core variability,” we have expanded the manuscript and\nSupplementary Information with detailed explanations of the geochemical parameters for\neach sediment core. We have also clarified our use of Hg and carbon isotopes to enhance\nour understanding of the tracer methods. For Reviewer 2, we have strengthened the\nbackground and improved data interpretation for better clarity, making the study more\naccessible to a broader audience. Additionally, following Reviewer 3’s suggestion, we have\nincluded a discussion on historical variations in atmospheric Hg concentrations to provide\na more comprehensive context for changes in Hg burial in marine sediments. Additional\nminor edits have been made to improve clarity.\nIn response to the Editor’s suggestions, we have thoroughly addressed concerns regarding\nthe “variability of individual records,” “varied remineralization at the core sites,” and the\n“interpretation of Hg data,” which align with issues raised by Reviewers 1 and 2. We\nunderstand the importance of addressing all reviewers’ comments thoroughly before\nresubmission. Their feedback is essential to improving the quality of our manuscript, and\nwe are committed to ensuring that the revised manuscript meets the highest standards.\nFinally, we have ensured the manuscript now adheres to the format required by Nature\nCommunications. Please refer to the revised manuscript to see these changes. Our detailed\nresponses to each point are provided below. If additional clarifications or revisions are\nneeded, please don’t hesitate to inform us."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors made changes according to the reviewers comments and did improve the clarity of the manuscript. However, I still have concerns if the data shown and interpretations given really explain what we see and thus are robust enough to support the conclusions.\n\nIn principal, I follow the authors conclusion that warming after the last glaciation has increased Hg burial in sediments specifically in the Ross Embayment attributed to Hg release from melting glaciers.\n\nHowever, I think that a major weakness of this manuscript is, that it solely relies on Hg and carbon concentrations as well as related C/N ratios and C-isotopes to reconstruct sources and decomposition of carbon found in the sediment cores.\nHowever, C/N ratios or C-isotopes have the disadvantage of indicating both source and decomposition. The used mixing model suggest that 63 % of OC in the Ross Embayment is derived from terrestrial sources. For me this is hard to believe regarding the low precipitation rates and very low OC in ice in Antarctica on one hand and the high productivity at the Ross Embayment at the other. Where should this large amount of OC come from? The area is entirely covered by ice. Even if C/N ratios are higher than in the open ocean sediments, they are still low and more in the range of phyto- and zooplankton. I further wonder if terrestrial inputs (whatever this might be) could be high enough to contribute significantly to the OC in the sediments. To support this statement, I think that the authors need stronger proxies such as DNA analyses or specific organic proxies (especially if the data and the conclusion should meet the high quality standard of NatComm). I also wonder, why the authors did not conducted more geochemical analyses of their sediment cores (including major and trace elements) to get a better understanding of material sources and sedimentation processes instead of relying and speculating on grain size.\n\nThe authors stated that Ross shelf has lost ~ 50 % of its ice cover during the Holocene. I miss a calculation what happens with all the atmospheric derived Hg stored in this huge amount of ice is released tot he ocean. I speculate that it is more likely that it is scavenged by algae than released back to the atmosphere. Regarding burial and concentrations of carbon and Hg it seems that the authors assume a linear relationship between both. However, release of large amount of Hg from melting ice (which is not organically bound) could explain the high increase in Hg burial in sediments without increasing burial of OC (high capacity for Hg binding). Other studies have shown that this could be oberserved in Antarctic lakes (Pérez et al., 2019).\n\nDue to the comparatively small number of and relatively weak proxies used, the interpretation of the data relies to a significant extent of findings of other studies (Hg isotopes, Hg evasion etc., water phase Hg biogeochemistry which is actuall poorly understood). Therefore, the authors should avoid to much speculation on the implication of their findings to processes such atmospheric Hg fluxes to the Southern Ocean and Hg evasion, Hg methylation, global warming, Hg uptake in food chain etc.\n\nM Pérez-Rodríguez, H Biester, JR Aboal, M Toro, AM Cortizas (2019) Thawing of snow and ice caused extraordinary high and fast mercury fluxes to lake sediments in Antarctica. Geochimica et Cosmochimica Acta 248, 109-122", "reviewer_2": "I believe that the authors addressed numerous comments which significantly improved the initial manuscript. However, several issues (mostly minors) remain and should be addressed for this work to be suitable for publication.\n\nOne general remark: I feel like the authors refer almost more often to the supplementary figures than to the ones in the main text. It would be great if this could be improved, although, I acknowledge that it may not be possible due to journal format and requirements.\n\nAbstract\n- L25: This sentence is incomplete: ‘respectively’ of what? I guess embayment vs open ocean.\n\nIntroduction:\n- L49: There is a word missing: ‘and be laterally transported ‘\n- L59: ‘CO2’ instead of ‘its’ would clarify the sentence.\n- L62-64: This is an important information but this sentence is out of place, it does not follow the beginning of the paragraph.\n- L65-66: ‘where Hg gets scavenged’ instead of ‘where Hg scavenged’\n\nSpatial patterns in carbon and mercury sequestration:\n- Overall, this section reads much better now!\n- L111-112: This sentence could be moved L106, to follow the OC results.\n\nSource of sedimentary carbon and mercury:\n- This section is much clearer now.\n\nTemporal decoupling of carbon and mercury:\n- L251-253: Please remove the second ‘increased’ L252 or reformulate the sentence.\n\nMethods\n- L376: ‘collected onboard’ instead of ‘based on’\n- L392: I do not think there is a need to refer to Supp. Fig 9 here.\n- L430: Please add ‘concentrations’\n- L436-439: Could the authors add the number of samples analysed (n=?) for the standards and blanks.\n- L446: Why refer to reference 84 here when reporting accuracy?\n- L447-449: This sentence should be moved at the beginning of the paragraph.\n- L478-480: Same comment as above, please add n=xx.\n- L499-502: These two sentences can be merged.\n- L508: ‘the sample was’ instead of ‘it is’.\n\nSupplementary:\n-I appreciate that the authors added the supplementary texts 3 and 4, which significantly clarify the origin of the end-members presented in the results/discussion, and give important background required for general readers.\n- Supp. Figure 2: This figure presents some important results, and is cited multiple times in the text, so I suggest that the authors move it into the main manuscript.\n- Supp. Figure 10: This figure is not essential, especially since there is not a picture of every core, and now that the authors added the supplementary text 1, which gives a comprehensive description of every core.\n- Supp. Table 8: Please add some lines or more space to separate the data between each sample, right now the column ‘Materials’ is confusing. Same comment for Supp. Table 9."}, "author_response": "We sincerely thank the reviewer for their valuable comments and suggestions, which\nhave been instrumental in improving our manuscript. The constructive feedback\nprovided by the reviewer has been crucial in strengthening our study.\nIn response to the specific issues raised, particularly regarding the capability of the\nproxies to support our conclusions, we have made significant revisions to enhance the\nrobustness of our analysis. Detailed discussions have been added to the revised\nmanuscript to explain the selection and validation of the proxies (i.e., biomarkers and\nelement abundance) used in our study. We have also provided a more thorough\njustification for how these proxies are linked to carbon sources and Hg enrichment\nprocesses. Additionally, we have expanded our literature review to incorporate recent\nstudies that support using these proxies, reinforcing their validity in the context of our\nresearch objectives.\nAlong with the manuscript revisions, we have provided a point-by-point response to all\nreviewers’ comments, clearly outlining how we have addressed each concern.\nWe believe these revisions significantly strengthen our manuscript and adequately\naddress the reviewers’ concerns. We appreciate their guidance throughout this process\nand look forward to resubmitting our revised manuscript for further consideration."}, {"round": 3, "reviewer_comments": {"reviewer_1": "The authors addressed all my questions and concerns regarding the interpretation of the data, specifically the sources of Hg in the different sedimentary environments. The authors added additional proxy data to support their conclusions regarding the sources of Hg which has really improved the study. I think that the manuscript is now suitable for publication."}, "author_response": "We are pleased to hear that you are satisfied with the revisions made to the interpretation of the data and the proxy data we provided to support our conclusions.\nWe sincerely appreciate your continued support for the manuscript’s publication."}]

Decision letter