Thorium-234 as a tracer for deep-sea mining sediment plume deposition

Domain: earth_science
Published: 2025-11-27
Editorial outcome: minor_revision

Abstract
Deep-sea mining for polymetallic nodules is currently exploratory, but commercial-scale operations require indicators of environmental change to support regulatory thresholds and inform adaptive management. In the Clarion-Clipperton Zone, where background sedimentation rates are low, seafloor imagery has validated mining plume deposition but cannot resolve repeated sedimentation as nodules become buried. Thorium-234 (234Th), a naturally occurring radionuclide with a 24.1-day half-life and strong particle reactivity, serves as a high-resolution geochemical tracer. Here we apply sedimentary 234Th to identify the spatial extent of plume deposition following the NORI-D mining test. Excess 234Th (234Thxs) activity was low at baseline but elevated after mining and declined to background within 1–2 km of the directly mined area. Results suggest that mining plumes scavenge and redistribute 234Thxs, establishing a geochemical benchmark for plume extent and an operational tool for tracing recent sedimentation under future commercial-scale mining scenarios.

Introduction
Commercial scale deep-sea mining has the potential to extract large quantities of polymetallic nodules from the abyssal Clarion-Clipperton Zone (CCZ). These nodules are rich in metals such as manganese, nickel, copper, and cobalt, which are sought after to support growing demand for critical minerals1,2. However, removal of seafloor substrate and deposition of sediment plumes will likely impact benthic habitats, and the spatial extent of such impacts must be considered. It is necessary to identify suitable indicators that can inform regulatory thresholds to determine the spatial extent and magnitude of plume deposition, which would aid adaptive management measures3–5.

The CCZ spans over 6 million km2 of seafloor in the eastern central Pacific, at abyssal plain depths of 4000-6000 m. It is located between the Clarion and Clipperton fracture zones from 5° to 20° N and 115° to 160° W6. Polymetallic nodules are abundant throughout the CCZ, having formed over millions of years by the hydrogenetic and diagenetic precipitation of metals from seawater and sedimentary porewater. The natural sediment accumulation rate is low, ranging from 0.2-1.15 cm per thousand years in the eastern CCZ, where sediments are predominantly fine-grained silts and clays7–10. The occurrence of fine-grained abyssal sediments raises concerns about the generation of sediment plumes from mining activities, as fine particulate matter may be susceptible to entrainment and advection over long distances by deep ocean currents or mesoscale eddies11,12.

Establishing environmental baselines and conducting small-scale test mining are fundamental to a precautionary and adaptive management approach, prioritizing the assessment of potential impacts before full-scale operations commence5. Robust pre-disturbance baselines are vital for quantifying impacts and evaluating the impacts to and recovery of benthic biota, ensuring effective mitigation strategies and adherence to environmental management standards3,13. Lessons from related deep-sea industries like oil and gas, such as gaps in baseline data that hindered impact assessment following the Deepwater Horizon blowout event, underscore this requirement14,15. Multi-year baselines that capture natural variability provide the most accurate point of comparison for evaluating disturbance impacts and recovery13.

From October to November 2022, Nauru Ocean Resources Inc. (NORI) - a wholly-owned subsidiary of The Metals Company - conducted the first fully integrated nodule collection system test in over 40 years in the NORI-D contract area16. This area is the southeastern-most contract area located between −116 to −118° W and 9.8 to 11.2° N. During the system test, a remotely-operated prototype collector vehicle (PCV), measuring 6 meters wide and 12 meters long, maneuvered 80 km generating mining tracks on the seafloor and collected ~3000 tonnes of polymetallic nodules16.

Collector vehicles generate benthic sediment plumes through seafloor disturbance from both vehicle movement and nodule collection processes. Observations from a pre-prototype collector component test in 2021, conducted by Global Sea Mineral Resources, revealed distinct plume phase characteristics. The initial phase formed a negatively buoyant gravity-driven turbidity current with 92-98 percent of the particulate matter remaining within two meters of the seafloor17. Following local settlement of the turbidity current, the residual fine-fraction plume was advected by ambient bottom currents, with AUV imagery detecting deposition up to 1.8 km from the source18,19.

While AUV imagery effectively captures initial deposition patterns in undisturbed environments, monitoring the cumulative impacts from a commercial-scale mining operation, where repeated sediment deposition may occur over the lifetime of the mine (years-decades), requires additional tools capable of distinguishing newly deposited sediment layers. Over time, the progressive burial of nodules will reduce visual contrast of nodules with the surrounding sediment, limiting the utility of imagery for detecting incremental sedimentation.

Thorium-234 (234Th) is a naturally occurring, short-lived radioisotope (half-life = 24.1 days) produced continuously in seawater by the alpha decay of uranium-238 (238U), which is conservative throughout the water column. 234Th is distributed throughout the marine water column, typically exhibiting depleted activity in the photic zone due to scavenging by settling particles and phytoplankton20. 234Th generally reaches equilibrium with its parent isotope, 238U, below the photic zone, and has been detected at depths exceeding 5000 m (Supplementary Fig. S1)20,21.

As a particle-reactive element, 234Th is readily scavenged by suspended particles like clay minerals and organic matter22,23. With a large ionic radius and relatively simple electronic configuration, 234Th complexes readily within organic (nitrate, phosphate, sulfate) and organic molecules (humics, polysaccharides, biomolecules)23–25. Laboratory experiments have demonstrated strong adsorption of 234Th onto various materials, including nanoporous silicates and silanol-rich clays at pH values typical of marine sediments25.

Owing to its particle reactivity and short half-life, 234Th has been widely used to estimate marine particle export fluxes and seafloor deposition26,27. 234Th has also been used to trace the high-resolution sedimentation of marine oil snow following the Deepwater Horizon spill and has also been applied to study resuspension dynamics in coastal, lacustrine, and deep-sea settings14,28–31. Depletions in bottom water 234Th inventories and temporal variability have been associated with scavenging by resuspended particles29. The short half-life and particle reactivity prove 234Th as a powerful tool for tracing recent episodic sediment resuspension driven by storms, currents, and tides, demonstrating its utility for investigating resuspension-deposition processes and its potential value for tracking deep-sea mining sedimentation.

The International Seabed Authority (ISA), the regulatory body in international waters, recommends in its guidance ISBA/25/LTC/6/Rev.1 the use of radionuclide measurements such as 210Pb and 234Th to assess bioturbation and sediment mixing32. This study repurposes short-lived 234Th as a high-resolution tracer for recent sediment deposition from mining plumes, aligning with ISA Recommendations on ‘ground-truthing’ plume models32.

The conceptual basis of this study is grounded in the naturally low sedimentation rates in the CCZ and the short half-life of 234Th. Together, these conditions are expected to yield low sedimentary surface activities of decay-corrected excess 234Th (234Thxs) (Fig. 1a). Sediments resuspended by deep-sea mining activities are expected to settle on a timescale of days to weeks33, making short-lived 234Th a suitable tracer for detecting the spatial influence of mining plumes. 234Thxs reflects the 234Th adsorbed to settling particulate matter in the water column, which are delivered to the seafloor during deposition events.

During plume generation, suspended particles scavenge 234Th present from surrounding bottom waters, resulting in a localized depletion of the water column inventory (Fig. 1b)21,31. As particles are advected from the source, additional scavenging occurs along the transport pathway. Consequently, as deposition occurs, sedimentary 234Th activity is expected to increase with distance from the point-source until a deposition threshold is reached, beyond which settling rates are insufficient to leave a detectable geochemical signature (Fig. 1c).

In this work, we demonstrate that sedimentary 234Th provides a sensitive tracer of recent deposition from a benthic deep-sea mining plume. We first establish a multi-year baseline of surface sediment 234Th activity to characterize natural temporal variability in the NORI-D area. Against this background, we detect elevated 234Th activities within 1–2 km of the prototype collector tracks, while sites beyond this range are indistinguishable from baseline and controls. These results highlight 234Th as a sensitive, short-timescale indicator of sedimentation, supporting its application for monitoring benthic plume extent and informing environmental management strategies.

Results
Characterizing natural variability in sedimentary 234Th is essential for distinguishing mining-related signals from background conditions in the NORI-D contract area (Fig. 2). The collector test area (CTA) spans a narrow bathymetric range (4248 − 4336 m), with <90 m difference between the shallowest and deepest points, minimizing the potential influence of depth-related factors on 234Th distribution (Fig. 2a). Within the test field itself, the depth range is even more consistent, varying by less than 20 m (Fig. 2b).

This analysis integrates samples collected during two baseline campaigns: May–June 2021 and August–October 2022. Together, these provide a multi-year perspective on natural temporal variability in surface (0–1 cm) sedimentary 234Th across the test field (Fig. 2a). Measured 234Thxs activities were consistently low, reflecting the region’s naturally low sedimentation rates. In 2021 (n = 34), values ranged from 0 to 2.74 dpm g-1, with a mean of 0.42 ± 0.13 dpm g-1 (Fig. 3a, b; Supplementary Table S1). The 2022 dataset (n = 26) showed a similar range (0 to 2.46 dpm g-1), but a higher mean of 1.07 ± 0.15 dpm g-1 (Fig. 3a, b; Supplementary Table S1). This increase was statistically significant (mean difference: –0.6500 dpm g-1, p = 0.0129), based on ANOVA followed by a Tukey’s HSD post-hoc test (Supplementary Fig. S2). The observed shift may reflect interannual variability driven by seasonal changes in primary production, ocean current regimes, or episodic benthic disturbances such as eddies or deep-sea storms.

A clear spatial distribution of surface 234Thxs activities was observed (Figs. 3 and 4) in samples collected in November-December 2022, days to weeks after the PCV mining test. All samples were analyzed within 90 days of collection and decay-corrected to account for radioactive decay occurring between sampling and measurement.

In the post-PCV test control reference zone (n = 4), located ~7 km northwest of the mined area, 234Thxs activities ranged from 0.36 to 1.05 dpm g-1, with a mean of 0.64 ± 0.15 dpm g-1 (Fig. 3a, b). These values were statistically indistinguishable from those recorded in the 2021 and 2022 baseline campaigns (Supplementary Fig. S2), consistent with the absence of mining-related deposition in this area. AUV imagery further confirmed this, showing no visible signs of sedimentation at the control sites (Fig. 3c).

Within the directly mined area (site TF_007), located at the point source of plume generation, 234Thxs activities were relatively low, ranging from 0.23 to 1.52 dpm g-1 (n = 5; mean = 0.69 ± 0.22 dpm g-1; Fig. 3a, b). Several factors potentially contribute to these low values. First, the PCV likely stripped away the natural surface sediment layer, where most 234Th accumulates. Second, the highest concentrations of resuspended sediment were present near the tracks, rapidly depleting the bottom water 234Th inventory through scavenging. 234Th adsorption is most effective with finer sediments due to their higher surface area-to-volume ratio, which allows for increased scavenging and adsorption through surface complexation22–24. Consequently, the resuspension of coarser sediments with lower surface area-to-volume ratios near the tracks may have resulted in less efficient 234Th scavenging and lower 234Thxs activities in the surface sediments redeposited at the site of the mining tracks (TF_007). AUV imagery corroborated this interpretation, revealing removal of sediment and nodules within the mined track (Fig. 3c).

234Thxs activities increased with distance from the mining tracks at sampling sites proximal to the mining activity. This trend is evident in significantly elevated activities at sites TF_032 (~200 m) and TF_025 (~500 m) northeast of the mined track. At TF_032 (n = 2), values were varied between 3.1 to 3.9 dpm g-1 (mean = 3.50 ± 0.4 dpm g-1), while TF_025 (n = 5) exhibited even higher values, ranging from 5.5 to 6.31 dpm g-1 (mean = 5.88 ± 0.14 dpm g-1) (Fig. 3a, b). Activities at both sites were significantly higher than those in all baseline (2021, 2022) and post-PCV control samples (p < 0.0001; Supplementary Fig. S2). Estimated mean differences in 234Thxs between TF_025 and the 2021 baseline, 2022 baseline, and post-PCV control were −5.46, −4.81, and −5.24 dpm g-1, respectively (all p < 0.01). For TF_032, differences were −3.08, −2.43, and −2.86 dpm g-1, respectively (all p < 0.01) (Supplementary Fig. S2). These values represent a 3–8-fold increase relative to background conditions, reflecting a sedimentation signal from the mining plume (Fig. 4).

The elevated 234Thxs at site TF_025 may be further explained by local bathymetry. A shallow seafloor depression of ~10 meters (Fig. 2b) likely enhanced sediment deposition. This is consistent with prior findings that mining plumes propagate as gravity-driven turbidity currents, shaped by seafloor topography17. AUV imagery supports this interpretation, revealing a surface layer of sediment visibly coating nodules at both TF_025 and TF_032 (Fig. 3c). These results further validate the utility of 234Th as a high-resolution tracer of recent fine-grained sediment deposition.

At site TF_033 (n = 1), located 1 km northeast of the directly mined area the 234Thxs activity was 1.86 dpm g-1, which was elevated compared to the baseline 2021 and post-PCV control reference zone but within the broader range of natural variability (Fig. 3a, b). This level is consistent with light sedimentation from the distal edge of the mining plume, but also falls within the envelope of background processes such as benthic storms, mesoscale eddies, or bottom currents that may cause episodic resuspension12,31.

The decline in 234Thxs from TF_025 to TF_033 is likely due to reduced particle flux and organic loading with increasing distance from the plume source. While scavenging efficiency is influenced by the surface-area-to-volume ratio of particles, the observed trend primarily reflects diminishing particle supply. AUV imagery at TF_033 reveals a thin dusting of sediment on the seafloor, but nodules remain clearly visible, indicating light but detectable deposition (Fig. 3c).

At sites STM_174 and STM_173, located 2 km and 3 km northeast of the mining tracks, 234Thxs activities ranged from 0.21 to 0.68 and 0.21 to 0.81 dpm g-1, respectively (n = 3 for each site), with both sites averaging 0.49 dpm g-1 (Fig. 3a, b). These values were not significantly different from 2021 or 2022 baseline levels (Supplementary Fig. S2), indicating that the influence of the mining plume was no longer detectable beyond 2 km (Fig. 4).

The return to background 234Thxs levels at STM_174 and STM_173 is supported by AUV imagery, which revealed no discernible sedimentation features relative to the control site (Fig. 3c). In contrast, TF_033, located 1 km from the mined area, exhibited elevated 234Thxs activity. This pattern suggests that the transition from detectable to undetectable plume deposition occurred between 1 and 2 km from the source. Collectively, these results constrain the spatial extent of mining-related sedimentation within ~2 km of the collector track.

This study provides geochemical validation of the spatial extent of sedimentation from a benthic sediment plume generated by a deep-sea nodule mining test. Previous modeling studies predicted sediment blanketing up to 2.5–4 km from a 4-day PCV operation under typical abyssal current regimes11. In contrast, the 234Th data from the NORI test indicate a more constrained depositional footprint of 1–2 km. This difference likely reflects the influence of local bathymetry, site-specific hydrodynamic conditions during the trial, and potential differences in collector vehicle design.

These results support the objectives of regulators, such as the ISA, which emphasize developing best practices for monitoring deep-sea mining impacts32. This geochemical approach is particularly valuable for monitoring cumulative impacts, as commercial-scale mining operations would involve multiple collector vehicles operating continuously over decades, potentially leading to repeated sediment deposition across extensive areas. Visual methods may become less effective over time as sediment accumulation reduces contrast between nodules and the surrounding seafloor. In contrast, 234Thxs provides a geochemical signal of sedimentation within a ~100-day timeframe, allowing recent deposition events to be identified even where visual evidence is no longer apparent. By providing a tool to quantify spatial sediment deposition and distinguish newly deposited layers, this study contributes to the development of methodologies that can inform operational thresholds, assess environmental impacts, validate plume dispersion models, and support adaptive management strategies.

This study contributes to the growing body of 234Th research27, and builds on previous radionuclide-based resuspension studies29–31, demonstrating that 234Th is a sensitive and robust geochemical indicator of benthic sedimentation associated with deep-sea mining activity. Sediment samples collected after the fully integrated nodule collection system test at NORI-D revealed a clear spatial gradient in 234Th activity, indicating detectable sedimentation between 1 and 2 km from the mined tracks. At TF_033, located 1 km from the mining site, 234Thxs activity was elevated but remained within the range of baseline variability, suggesting that sedimentation at that distance may be comparable to naturally occurring resuspension events such as benthic storms or eddies.

The results provide initial evidence validating the conceptual hypothesis that 234Th can serve as a geochemical indicator for recent sedimentation. The baseline dataset confirms naturally low sedimentation rates in the region, while post-PCV test results show that sediment transport is shaped by local bathymetry, bottom currents, and sediment characteristics. Unlike AUV imagery, which cannot resolve the timing of sediment deposition, 234Th provides a ~100-day detection window and can identify recent deposition events on a weekly timescale. This distinction is especially critical in a commercial mining context, where vehicles may revisit mined areas months or years later. As sediment accumulates and visual contrast diminishes, 234Thxs remains one of the few available indicators of new deposition, supporting monitoring of cumulative impacts.

234Th offers a valuable tool to inform operational thresholds, provide metrics for environmental monitoring and management plans (EMMPs), and validate plume dispersion models. Future work should integrate 234Th measurements with biological and ecological indicators to assess potential impacts on benthic communities. While 234Thxs reveals the spatial extent of deposition, it does not directly quantify ecological response, which remains a critical research priority. Although this study focuses on sediment-based excess 234Th to trace recent deposition, future efforts should include direct measurements of 234Th activity in suspended particles within the plume and incorporate mechanistic scavenging models to better constrain non-steady-state dynamics. These additions would improve estimates of total mass deposition and sedimentation rates. Complementary radionuclides, such as 210Pb, though limited in temporal resolution due to their longer half-life, could be used to examine historical deposition from legacy mining tests34. Together, these approaches offer a framework for assessing the spatial extent of plume-driven sedimentation and for developing science-based strategies to monitor environmental impacts from deep-sea mining.

Methods
Sampling campaigns
Sediment multicores were collected using an Oktopus GmbH 20-core multicore, equipped with 60 cm long core tubes with an inner diameter of 95 mm, during two baseline campaigns (2021, 2022) and one post-PCV test campaign (2022) in the NORI-D contract area of the CCZ. Baseline campaigns were conducted from May to June, 2021 (Campaign 5D) and from August to October, 2022 (Campaign 7A), the latter of which occurred immediately prior to NORI’s PCV test in October, 2022. In total, 34 and 26 multicores, respectively, were collected on baseline campaigns 2021 and 2022 for 234Th measurement. The post-PCV test campaign was conducted from November to December, 2022 (Campaign 7B2), directly following the completion of the nodule mining test (days-weeks), in order to sample for immediate impacts.

To assess the spatial extent and impact of sediment plume deposition from the PCV test, a strategic sampling plan was developed based on existing baseline data of current direction and speed. The survey design consisted of three main components: directly impacted tracks, a gradient of potential sedimentation sites, and a control area. A series of in situ sensors, such as ADCPs, camera landers, sediment traps, and sedimentation plates were deployed prior to the PCV test to capture the direction and magnitude of the prevailing current during the time of testing35.

Samples were collected from sites located within the mining tracks to evaluate the immediate effects of the PCV test on sediment 234Thxs activities. Sampling sites were positioned along a transect extending from the mining tracks in the direction of the prevailing current at the time of the PCV test. These sites were selected at increasing distances from the tracks (i.e., 200 m, 500 m, 1 km, 2 km, and 3 km) to investigate any gradual changes in sediment deposition and 234Th activities as the plume dispersed. A control area was established approximately 7 km northwest of the mining tracks, in a location expected to be unaffected by the sediment plume. Samples from the control area were used to compare 234Thxs activities with baseline levels and to verify that the area remained uninfluenced by the PCV test. The collection of multi-cores within the tracks of the PCV was simultaneously monitored by an ROV to ensure sampling accuracy (Supplementary Fig. S3). 24 multicores were collected for 234Th measurement during the post-PCV test campaign (7B2).

At-sea sampling protocols
Once the multicore was recovered on deck, cores were secured on a custom rack, measured, photographed, and assessed for quality with a QA/QC matrix that examines metrics such as “Nodule drag”, “Top water intact”, “Sediment-water interface”, and “Undisturbed stratigraphy”. All sediment cores from Baseline 2021, Baseline 2022, and the post-PCV test campaigns were extruded at sea and subsampled. The 0-1 cm interval was analyzed and reported in this study. The top water was carefully siphoned off to prevent sediment loss. Nodules present at the core surface were removed with forceps, rinsed with milli-q water into the sample bag, and stored separately. Subsampling was conducted using pre-cut and measured polycarbonate rings placed on top of the core tube (of equal diameter), with sediment sections obtained by slicing a thin plastic sheet between the ring and core tube. Sediment samples were stored in WhirlPak bags, frozen, then freeze-dried before analysis.

Short-lived radioisotope (234Th) analysis
Freeze-dried sediment samples were analyzed by gamma spectroscopy on Canberra High-Purity Germanium Coaxial Planar and Well Photon Detectors for total 234Th (63 KeV). Each sample was weighed, homogenized, and sealed in a plastic container before analysis. Total 234Th activities, in disintegrations per minute per gram (dpm g-1), were corrected for counting time, detector efficiency, and self-absorption using the IAEA-447 standard, following methods from Brooks et al.28 and described as follows. To account for self-absorption and matrix effects, standard calibration curves were generated by analyzing a series of IAEA-447 standards at varying masses (1, 3, 5, 7, 9, 12, 15, 17, 20, 30, 40, and 50 g). These were used to build a geometry-corrected efficiency template relating sample weight to detector response. Sample-specific corrections were applied to derive absolute 234Th activity in disintegrations per minute per gram dry sediment (dpm g-1). Activities were corrected for counting time, background radiation, and detector efficiency. Instrumental error associated with detector efficiency was estimated to be <3% of measured activity consistent with Brooks et al.28 and considered negligible relative to natural sample variability. All surface samples (0-1 cm) were analyzed within 90 days of initial collection. 234Th activity supported by the in-situ decay of its parent isotope 238U was determined by reanalyzing each 0–1 cm sample more than 120 days after collection, allowing all excess 234Th to decay (i.e., after ~5 half-lives). Supported activity was subtracted from total 234Th to calculate excess 234Th (234Thxs). Measured 234Th values were decay-corrected to account for time elapsed between collection and counting and are reported as mass-normalized (dpm g-1) activities.

All reported values and replicates are derived from independent sediment cores, with no technical replicates from the same core. Values are presented as mean ± standard error of the mean (SE), unless otherwise noted.

Statistical analysis
All statistical analyses and data visualizations were performed using R statistical software (version 4.1.1)36. The ggplot2 package was used to create plots, including bar plots and raincloud plots, to visualize the distribution and variability of 234Thxs activities across sampling sites and campaigns. To assess whether there were statistically significant differences in 234Thxs activities among the sampling sites, a one-way analysis of variance (ANOVA) was conducted using the ‘aov’ function in R. Following a significant ANOVA result, a Tukey’s Honest Significant Difference (HSD) post hoc test was performed to determine which specific sites differed significantly from one another. The Tukey’s HSD test was conducted using the ‘TukeyHSD’ function in R, which computes pairwise comparisons between group means and provides adjusted p-values to account for multiple comparisons. The results of the post hoc analysis were used to identify the statistical differences in 234Thxs activities between sites and sampling levels (Supplementary Fig. S2). Spatial maps depicting the sampling locations and 234Thxs activities were created using QGIS (version 3.32)37. Sampling sites were plotted on the maps using their geographic coordinates, and the 234Thxs activities were represented using a color gradient to visualize the spatial distribution of the radionuclide concentrations. Ocean Data View software38 (version ODV 5.8.2) was used to generate Supplementary Fig. S1.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This paper “Thorium-234 as a tracer for deep-sea mining sediment plume deposition” by O’Malley et al., investigates the\napplication of 234Th to trace the sediment plume in the abyssal Clarion-Clipperton Zone (CCZ). 234Th have been\nextensively used to study the sedimentation during episodic events. However, there are several factors to consider during\nthese episodic events such as steady state or non-steady state. At present, the paper only shows the thorium excess to\nsupport the resuspension/redistribution of particles during the mining process. However, it fails to measure any thorium\nactivity in the suspended particles, or the plume created during the mining process. The paper line 207 mentioned “The\nresults provide initial evidence validating the conceptual model of 234Th as an indicator of mining plume sedimentation.” yet\nthe paper has failed to incorporate any thorium-scavenging model. The paper presents a good datasheet to show the\ndifference before the mining and after mining. However, this datasheet is not relevant to be published on high-impact journal\nsuch as Nature Communications.", "reviewer_2": "This is a very interesting article that demonstrates the applicability of using the short-lived radionuclide, 234Th, as a tracer of\nfine-grained sedimentation following deep-sea mining. The article is very well written and easy to follow. I support\npublication. My only question is in the long term applicability of this work and have a few thoughts on expanding relevance.\nMy major comments are below:\n1) The authors argue that geochemical measurements are needed to support photographic and other evidence of mining\nplumes. Is this a requirement by a specific agency? If so, I would love to see this explicitly cited in the text. One could say\nthat the 234Th measurements clearly support the photographic evidence so why should one bother with 234Th, which is\nmuch more limited and costly. I would argue that what 234Th provides is a time component of when the deposition occurred\n– within 2-3 months of the actual event. This is alluded to in the article but not explicit – weakening the need and support for\nthe 234Th measurements. I encourage the authors to add this important point explicitly to the manuscript. As a side note –\none could argue that you could use 210Pb in a similar manner. Since these samples were gamma counted, did you note any\nchanges in 210Pbex activities?\n2) Another question that arises from deep-sea mining is not just the extent of the plume, but the sedimentation rates\nassociated with these activities. The authors mention sampling at 0-0.5 cm and 0.5-1 cm depth (but also note sampling to 5\nand 9 cm depth). There needs to be some clarity as the depths actually measured and presented in the text throughout – are\nyou averaging, considering each measurement within a core as a sep measurement? Average measurement per site, etc.?\nIs it possible to obtain a sedimentation rate or change in inventory to determine mass deposition and assumptions from\nscavenging? I recognize that this is has high errors – but so does the activity difference measurements. Again, I encourage\nthe authors to think through this possibility – at least for future work- as it could be another key\nmeasurement.\nSpecific comments:\n1) Line 105. “With a large ionic radius and relatively simple electronic configuration,”\n2) Line 126. This is the only time that inventories are really discussed. I would change this to surface activities or discuss\ninventories in general and potential applicability (see comment 2 above).\n3) Line 159. “Initial baseline 234Thxs activities from baseline campaign 2021 (n=34)…” Use baseline twice.\n4) Lines 162 and beyond – when providing average, would be useful to also provide standard deviations.\n5) Line 164. Add units to read “0.6500 dpm/g...)\n6) Line 170. Add dates of when post test sampling was conducted. Should add a note that the 234Th activities measured\nhere may have already decayed depending on time since scavenging and deposition.\n7) Line 179. I’m a little confused. Are these two measurements from separate surface samples or from two depths at the\nsame site: 0-0.5 and 0.5 to 1 cm? It would be good to clarify this throughout the manuscript. In Figure 3 you just say 0-1 cm?\nSo is this an average of two measurements?\n8) Line 195. Same question as #6. I would also rewrite as “Activities from the TF_032 site (n=2) were varied between 3.1\nand 3.9 dpm/g, respectively, and averaged… “ Also please average std devs.\n9) Line 200. Also please add units here too.\n10) Line 213. “…of the fine-grained sediment deposition\n11) Line 235. I would delete. Not sig is not sig. “(2021 baseline: estimate difference= 0.064, 236 p= 1; 2022 baseline: estimate difference= -0.586, p= 0.889)”\n12) Lines 245-246. This is time dependent. So within 2-3 months can make this measurement, but if you waited – this activity\nwould decay, correct? This needs to be discussed here and elsewhere.\n13) Lines 254 “…was more limited to between 1 and 2 km away…”\n14) Lines 264-265. “….benthic plume from the mining operations being detected at least 1 km away from the 265 PCV\ntracks, but less than from 1 to 2 km away.\n15) Sup Mat. Lines 344-345. Can you confirm how these depths impact activities? Are all measurements from these two\ndepth sections? All sections? Do you see trends with depth? You note sampling to 5 and 9 cm depth hence my question\nabove about sed rates. Did you measure 210Pbxs for comparison?\n16) Supp Mat Stat Analyses I think how you’ve done the stats should be noted somewhere in the main text, especially with\nsuch limited sampling in some cases.", "reviewer_3": "General comments\nThis manuscript reads very smoothly and polished as if it had already gone through substantial review, internally or\nexternally or both. In brief, I think the approach of using 234Th to trace the sediment plume is neat, and the data look sound.\nThe analytical method to count the same sample twice via gamma-spectrometry should produce reliable results (and it\nshould be interesting to see the changes in other radionuclides following the collector test). The visualization is excellent.\nThe statistics look appropriate as far as I can tell, admittedly no real expert on this aspect. The interpretation seems\nstraightforward. The topic is very timely. The literature, with one exception given below, looks adequate and complete.\nI feel that the focus on the sediment drape may actually be distracting, intentionally or not, from some way more severe\naspects of the mining activities. But this is not an issue of this study and does not affect its scientific validity. The authors are\ntransparent about the involvement of The Metals Company.\nIn summary, I recommend this nice manuscript with only very minor changes (see below) for publication in Nature\nCommunications.\nSpecific comments\nL50 I think it is misleading to state that the mining of polymetallic nodules, which grow at rates of millimeters per millions of\nyears, might be a step towards more sustainability in view of the Earth system.\nL85 if the term “organic matter” refers here to total organic carbon (TOC), it would already be covered by mentioning\nsediment, of which it is here only a (very small) fraction. If it refers to living organic matter, the term should better be replaced\nby “organisms” or “biota”.\nL278-279 it is monitoring just one aspect of environmental pressure, probably a minor one. For example, it does not touch\nupon the removal and disconnection of habitats, the presumably permanent alteration of benthic fluxes by removal of the\nstrongly absorbing nodules, or the spreading of fine particulate heavy metals like cobalt and radionuclides.\nFigure S1: The axis labelling is too small to read the values. For these GEOTRACES data, it is also good practice to cite the\nactually producing researchers of the data and not just the published data product. It might even be that the authors agreed\nto this when downloading the dataset, but I’m not exactly sure about the data policy at this time. In any case it should be\nconsidered to give more credit to the original studies."}, "author_response": "We appreciate the time and constructive comments from Reviewer #1. We agree that analyzing\n234Th in suspended particulate matter directly within the mining plume would provide valuable\ninsights into particle scavenging processes in the water column. However, as outlined in our\nmanuscript, our primary objective was to demonstrate a straightforward and operationally\nfeasible geochemical approach based on sedimentary 234Th to detect and map recent\nsedimentation deposition from nodule mining activity. While we appreciate that further\nmeasurements on suspended particulate matter would deepen our mechanistic understanding,\nthese were beyond the scope of our initial study. We regard this paper as a key foundational step\nthat can inform monitoring strategies for regulators and industry to establish and monitor\nsedimentation redistribution thresholds. We have included this recommendation for future work\nin the conclusions section (Lines 287-291).\n“Although this study focuses on sediment-based excess ²³⁴Th to trace recent deposition, future\nefforts should include direct measurements of ²³⁴Th activity in suspended particles within the\nplume and incorporate mechanistic scavenging models to better constrain non-steady-state\ndynamics. These additions would improve estimates of total mass deposition and sedimentation\nrates.”\nWe acknowledge the note regarding the “conceptual model” terminology and have changed this\nto “hypothesis” in the text. This clarifies that our work validates the conceptual\nhypothesis/framework rather than a fully mechanistic model. The proof-of-concept leverages\nwell-established GEOTRACES data (Supplementary Fig. S1) to confirm that 234Th is present in\nthe deep CCZ, justifying its use in tracing sediment re-distribution.\nRegarding the broader relevance and potential journal suitability, we respectfully maintain that\nour findings are both timely and highly relevant. Driven by the rapidly growing global demand\nfor critical minerals, deep-sea mining may become a commercial reality. Accordingly, we bear a\ncollective responsibility to ensure rigorous, science-based monitoring and adaptive management\npractices are established to conserve deep-sea ecosystems and prevent “serious harm”. The\nInternational Seabed Authority and the global community will ultimately decide whether deep-\nsea mining proceeds, but such decisions should be grounded in the best available science.\nOur study directly addresses this need by providing a scientifically robust and practical\nmonitoring tool. By demonstrating both the spatial extent of sediment deposition during the\ncollector test and the utility of sedimentary 234Th as a high-resolution tracer, we offer a readily\noperational method for establishing and enforcing sedimentation thresholds. This approach can\nbe integrated into emerging international regulations and industry-led environmental\nmanagement plans, ensuring that if deep-sea mining is approved, it proceeds with transparent\nand science-driven oversight.\nWe thank Reviewer #2 for the time, effort, and interest put into their extensive review, and we\nappreciate them ultimately improving the manuscript.\nFirst, addressing the question of whether a specific agency mandates geochemical verification\nalongside photographic evidence to verify a plume model -The International Seabed Authority\n(ISA) is the autonomous international organization established by the United Nations\nConvention on the Law of the Sea (UNCLOS) which governs seabed activities in international\nwaters. The ISA recommends the use of radionuclide measurements (210Pb and 234Th) in its\nRecommendations for the guidance of contractors for the assessment of the possible\nenvironmental impacts arising from exploration for marine minerals in the Area\n(ISBA/25/LTC/6/Rev.1). While these radionuclides are primarily emphasized for quantifying\nbioturbation and sediment mixing (Recommendation III.B.15.(e)), our study adapts and extends\nthis approach by applying short-lived 234Th as a high-resolution tracer for recent sediment\ndeposition from mining plumes. In doing so, we also address Recommendations V; VI.C.38.(e),\nwhich emphasizes “ground-truthing” data for plume models. By repurposing the ISA’s guidance\nfor bioturbation studies, we offer a monitoring tool that pinpoints where and when freshly\nresuspended sediment settles on the seafloor. This complements photographic evidence,\nproviding a toolset to inform best available practices and evolving environmental standards as\ndeep-sea mining progresses toward potential commercial reality.\nThis has been included from lines 107-112:\n“The International Seabed Authority (ISA), the regulatory body in international waters,\nrecommends in its guidance ISBA/25/LTC/6/Rev.1 the use of radionuclide measurements such as\n210Pb and 234Th to assess bioturbation and sediment mixing 32. This study repurposes short-lived\n234Th as a high-resolution tracer for recent sediment deposition from mining plumes, aligning\nwith ISA Recommendations on ‘ground-truthing’ plume models 32.”\nSecond, we address the question of “Why rely on 234Th in addition to AUV imagery for tracking\na sedimentation footprint?”. While AUV surveys offer valuable spatial coverage and visual\nconfirmation of seafloor conditions, there are several reasons why imagery alone may be\ninsufficient. In our experience, deploying and processing deep-sea AUV transects are actually\nmore resource-intensive than collecting sediment cores and conducting 234Th analyses.\nShort-lived 234Th provides a clear radiochemical “timestamp” that indicates whether observed\nsurface sediments were deposited during a specific period, such as during a mining operation, or\nif they resulted from other processes (occurring outside the time window) like bioturbation or\nbenthic storms. This helps to distinguish naturally formed sediment mounds or naturally\ndeposited sediment layers from those produced by a mining plume, a distinction that may be\ndifficult to ascertain from imagery alone.\nMost importantly, the key application of this tool is monitoring potential cumulative impacts as\ndeep-sea mining moves to a commercial scale. As collectors repeatedly traverse the same or\nadjacent areas years later, resuspended sediments can accumulate over previously deposited\nlayers. In such scenarios, AUV imagery cannot readily show whether newly arrived sediment has\nbeen added on top of older deposits, especially once nodules become buried. Repeated 234Th\nmeasurements, however, can pinpoint ongoing or subsequent sedimentation events within a\ndefined timeframe (~100 days), thus providing a means to track cumulative impacts and inform\nadaptive management strategies over the lifespan of a mine. This has been incorporated into the\nintroduction, discussion and conclusions section (Lines 8-10; 72-78; 252-254; 273-277).\nRegarding 210Pb measurements, gamma analysis does report 210Pb alongside 234Th, showing\nsimilar patterns in their distributions. However, 210Pb lacks the temporal resolution needed to\ndistinguish individual sedimentation events from mining activities occurring within months of\neach other. The 210Pb “hotspots” won’t fully decay for decades considering the 22-year half-\nlife. While this makes 210Pb less suitable as a tool for monitoring active mining operations, it\nproves valuable for investigating historic sedimentation events, such as the OMCO 1979 mining\ntest (Jones et al., 2025), providing insights into longer-term sediment accumulation patterns and\nenvironmental impacts. This has been included in future work (Lines 290-292).\n“Complementary radionuclides, such as ²¹⁰Pb, though limited in temporal resolution due to their\nlonger half-life, could be used to examine historical deposition from legacy mining tests ³⁶.”\nWe thank the reviewer for this important point regarding sediment depth intervals and the\npotential for deriving sedimentation rates. To clarify: although cores were initially sectioned at\nfiner intervals (0–0.5 cm, 0.5–1 cm, 1–2 cm, etc.), only the 0–1 cm surface interval was analyzed\nand reported in this study. This decision was made to maintain consistency and focus on the most\nrecent depositional layer, which is most relevant to the detection of recent sedimentation from\nthe mining plume using ²³⁴Thₓₛ. Polymetallic nodules rested on the upper surfaces of most cores.\nTheir removal causes minor disturbance to the topmost sediment, particularly the 0–0.5 cm layer.\nTo mitigate this, we merged the 0–0.5 cm and 0.5–1 cm intervals and treated the combined 0–1\ncm layer as a single “surface” sample for all analyses. Activities were normalized to dry mass\n(dpm g⁻¹), and only this surface layer was used for inter-site comparisons. To reduce confusion,\nwe have removed references to deeper intervals (e.g., 1–9 cm) from the methods text, as these\nlayers were archived but not included in the present analysis.\nLine 335; 358-360\n“The 0–1 cm interval was analyzed and reported in this study.”\n“Measured ²³⁴Thₓₛ values were decay-corrected to account for time elapsed between collection\nand counting, and are reported as mass-normalized (dpm g⁻¹) activities.”\nWe agree that there is scope to use 234Th (and Pb210) inventories in future studies to calculate\nsedimentation rates and mass deposition, particularly paired with water-column measurements\n(e.g., sediment traps, plume sampling) to account for scavenging processes and plume\ndispersion. These calculations require additional assumptions (e.g., uniform mixing, consistent\nplume characteristics) and as Reviewer 2 suggested would introduce greater uncertainties. For\nthis initial demonstration, our primary goal was to establish 234Th as a sensitive tracer for\ndetecting newly deposited sediments from mining plumes and mapping their spatial extent.\nNevertheless, we acknowledge that investigating mass deposition and net sedimentation rates\nrepresents a valuable next step, and highlight these exciting possibilities in our future work.\n(Lines 289-290)\n“These additions would improve estimates of total mass deposition and sedimentation rates.”\nSpecific comments:\n1) Line 105. “With a large ionic radius and relatively simple electronic configuration,”\nCorrected. (line 89)\n2) Line 126. This is the only time that inventories are really discussed. I would change this to\nsurface activities or discuss inventories in general and potential applicability (see comment 2\nabove).\nLine 116: We agree that the term “inventory” was incorrectly used as this would refer to the\nintegrated amount of 234Th in the entire sediment column. It has been changed to “surface\nactivities”.\n3) Line 159. “Initial baseline 234Thxs activities from baseline campaign 2021 (n=34)…” Use\nbaseline twice.\nCorrected.\n4) Lines 162 and beyond – when providing average, would be useful to also provide standard\ndeviations.\nStandard errors have been added to each average.\n5) Line 164. Add units to read “0.6500 dpm/g...)\nCorrected. (Line 156)\n6) Line 170. Add dates of when post test sampling was conducted. Should add a note that the\n234Th activities measured here may have already decayed depending on time since scavenging\nand deposition.\nAdded Line 165-166.\n“All samples were analyzed within 90 days of collection and decay-corrected to account for\nradioactive decay occurring between sampling and measurement.”\n7) Line 179. I’m a little confused. Are these two measurements from separate surface samples or\nfrom two depths at the same site: 0-0.5 and 0.5 to 1 cm? It would be good to clarify this\nthroughout the manuscript. In Figure 3 you just say 0-1 cm? So is this an average of two\nmeasurements?\nThis has been addressed with major revision #2 from Reviewer 2 and standardized in the\nmethods to clarify 0-1 cm is the only interval reported and analyzed.\nLine 335-336: “The 0–1 cm interval was analyzed and reported in this study.”\n8) Line 195. Same question as #6. I would also rewrite as “Activities from the TF_032 site (n=2)\nwere varied between 3.1 and 3.9 dpm/g, respectively, and averaged… “ Also please average std\ndevs.\nCorrected. Line 192.\n9) Line 200. Also please add units here too.\nCorrected.\n10) Line 213. “…of the fine-grained sediment deposition\nCorrected.\n11) Line 235. I would delete. Not sig is not sig. “(2021 baseline: estimate difference= 0.064, 236\np= 1; 2022 baseline: estimate difference= -0.586, p= 0.889)”\nNon-significant stats were removed.\n12) Lines 245-246. This is time dependent. So within 2-3 months can make this measurement,\nbut if you waited – this activity would decay, correct? This needs to be discussed here and\nelsewhere.\nAdded a sentence addressing this from Lines 165-166.\n“All samples were analyzed within 90 days of collection and decay-corrected, minimizing the\npotential for depletion before measurement.”\n13) Lines 254 “…was more limited to between 1 and 2 km away…”\nCorrected.\n14) Lines 264-265. “….benthic plume from the mining operations being detected at least 1 km\naway from the 265 PCV tracks, but less than from 1 to 2 km away.\nCorrected.\n15) Sup Mat. Lines 344-345. Can you confirm how these depths impact activities? Are all\nmeasurements from these two depth sections? All sections? Do you see trends with depth? You\nnote sampling to 5 and 9 cm depth hence my question above about sed rates. Did you measure\n210Pbxs for comparison?\nDecay corrected excess 234Th activity from 0-1 cm (integrated 0-.5 and 0.5-1) is reported and\nanalyzed in this study. This is detailed in the above response to major revision #2.\n16) Supp Mat Stat Analyses I think how you’ve done the stats should be noted somewhere in the\nmain text, especially with such limited sampling in some cases.\nA short reference to the statistical tests used to discern significance in this study has been added\nto line 156-157\n“This statistically significant shift (estimate difference: -0.6500 dpm/g, p=0.0129)\n(Supplementary Fig. S2), determined by ANOVA followed by a Tukey’s HSD post hoc test,\nunderscores the natural temporal variability of CCZ deposition rates, potentially driven by\nseasonal changes, ocean currents, or episodic benthic disturbances (eddies, benthic storms).”\nWe are appreciative of the time, effort, and thorough review provided by Reviewer 3. We believe\nthat their feedback has improved the manuscript.\nSpecific comments\nL50 I think it is misleading to state that the mining of polymetallic nodules, which grow at rates\nof millimeters per millions of years, might be a step towards more sustainability in view of the\nEarth system.\nWe agree that the term \"sustainable\" may be misleading, as polymetallic nodules are not\nrenewable on human timescales. The intention was to highlight their potential role in supplying\nminerals for renewable energy storage. We have revised the wording to clarify this distinction.\n(Line 25-27).\n“These nodules are rich in metals such as manganese, nickel, copper, and cobalt, which are\nsought after to support growing demand for critical minerals 1,2.”\nL85 if the term “organic matter” refers here to total organic carbon (TOC), it would already be\ncovered by mentioning sediment, of which it is here only a (very small) fraction. If it refers to\nliving organic matter, the term should better be replaced by “organisms” or “biota”.\nWe appreciate this clarification. Given that the average TOC content in the southeastern CCZ is\nless than 1%, the term \"organic matter\" could be misleading. We have adjusted the wording to\nbetter reflect the intended meaning.\nL278-279 it is monitoring just one aspect of environmental pressure, probably a minor one. For\nexample, it does not touch upon the removal and disconnection of habitats, the presumably\npermanent alteration of benthic fluxes by removal of the strongly absorbing nodules, or the\nspreading of fine particulate heavy metals like cobalt and radionuclides.\nWe acknowledge that 234Th monitoring captures only one environmental pressure\n(sedimentation) and agree that direct nodule removal, habitat disconnection, and alterations to\nbenthic fluxes represent significant long-term impacts. However, the spatial footprint of nodule\nremoval is well-defined and predictable, as the exact locations of PCV operations are\ndocumented. In contrast, sediment plumes are more difficult to track and model, necessitating the\ndevelopment of a tracers like 234Th to quantify their extent. We have reworded this section to\nclarify that 234Th serves as an important indicator for pressures associated with plume\nsedimentation (Line 284-287).\n“Future work should integrate ²³⁴Thₓₛ measurements with biological and ecological indicators to\nassess potential impacts on benthic communities. While ²³⁴Thₓₛ reveals the spatial extent of\ndeposition, it does not directly quantify ecological response, which remains a critical research\npriority.”\nFigure S1: The axis labelling is too small to read the values. For these GEOTRACES data, it is also\ngood practice to cite the actually producing researchers of the data and not just the published\ndata product. It might even be that the authors agreed to this when downloading the dataset,\nbut I’m not exactly sure about the data policy at this time. In any case it should be considered to\ngive more credit to the original studies.\nWe have updated the axis labels in Figure S1 to improve readability. Additionally, we have\nrevised the citation for the GEOTRACES IDP to include the original producing researchers,\ngiving proper attribution.\n“GEOTRACES Intermediate Data Product Group, Buessler, K.; Kenyon S; Pike, 2023. The\nGEOTRACES Intermediate Data Product 2021v2 (IDP2021v2). NERC EDS British\nOceanographic Data Centre NOC. doi:10.5285/ff46f034-f47c-05f9-e053-6c86abc0dc7e.”"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have thoroughly incorporated all previous feedback from all three reviewers including my comments, leading to\nsignificant improvements to the manuscript's quality. I recommend for publication in its current form.", "reviewer_2": "I've taken a look at the revision by the authors. They've done a very nice job of responding to my comments. I have no\nadditional concerns."}, "author_response": ""}]

Decision letter