What Causes Excess Deepening of the Sediment Mixed Layer in the Deep Ocean?

The sediment mixed layer (SML) in the deep ocean is an important interface with a rich diversity of benthic organisms. With increasing ocean mineral exploration, and eventual mining, the effect of sediment mixing on deep ocean ecosystems has raised considerable concern. We evaluate the distribution patterns and driving factors of SML depth in deep ocean nodule fields using naturally occurring 210Pb–226Ra isotopes. Results show that average SML depth has increased in Mn‐nodule fields since the end of the last century. SML processes are associated with significant desorption of 226Ra from sediments, resulting in a departure from radioactive equilibrium. By estimating possible driving factors, we conclude that anthropogenic exploration activities, rather than natural physical and/or biological drivers, are the most likely mechanism for intensified sediment mixing. 210Pb–226Ra disequilibria may be a potential tracer for quantifying the impact of human exploration on deep‐ocean sediment mixing and associated biological and geochemical effects.


Introduction
Deep-ocean sediments, the largest ecosystem on Earth, have unique attributes that are essential to the biosphere (Ramirez-Llodra et al., 2010).Sediment mixing can significantly impact the biogeochemical cycling of many elements as well as the benthic community structure in the ocean (R. C. Aller, 1990Aller, , 1994;;Howard et al., 2020;Jones et al., 2017).If previously buried organic carbon stocks are disturbed and exposed, enhanced organic carbon remineralization and associated carbon dioxide release could potentially increase ocean acidification and the accumulation of carbon dioxide in the atmosphere (Bianchi et al., 2021;Clare et al., 2023).Quantifying the depths, distribution patterns, and driving factors of the sediment mixed layer (SML) has drawn increased attention over the past few decades (Hayes et al., 2021).Global estimates of SML depths report mean values of 9.8 ± 4.5 cm (Boudreau, 1994) and 5.75 ± 5.67 cm (Teal et al., 2008).However, most studies have focused on continental margins, especially near large river delta-front estuaries (Song et al., 2022), while mixing dynamics in deep-sea sediments are still relatively unexplored (Harris, 2014).
Sediment mixing in the ocean, is typically controlled by natural biological and/or physical forces, but it can be modified by anthropogenic drivers.Biological activity of benthic organisms (e.g., bioturbation) has long been recognized as a driver of sediment reworking in the deep-ocean (DeMaster & Cochran, 1982b;Guinasso & Schink, 1975;Hollister & McCave, 1984;Howell et al., 2021;Z. Yang et al., 2020).Softsediment benthic communities commonly exhibit high species diversity, with ca.21-250 macrofaunal species observed in only 0.25 m 2 of deep-ocean mud (Dunn et al., 2018).Physical forces such as internal solitary waves (Jia et al., 2019) and benthic storms (Gardner et al., 2017) may also be responsible for sediment mixing and resuspension.Physical forcing by such phenomena in the deep sea are generally concentrated in areas with large topographic relief, such as the High Energy Benthic Boundary Layer Experiment (HEBBLE) sites in the Northwest Atlantic (Gardner et al., 2017).As human population growth continues to increase in the Anthropocene, potential disruption of deep-sea sediment by anthropogenic activities (e.g., fishing, mining, oil and gas exploration, telecommunication networks) are also expected to increase (Clare et al., 2023;Harris, 2014;Puig et al., 2012).Specifically, with the increasing demand for a wide array of rare metals and rapid economic development and globalization, polymetallic ore deposits from deep-ocean sediments are now recognized as important natural resources (Hein et al., 2020;Levin et al., 2020).Consequently, increasing exploration activities of the deep seabed, particularly for manganese (Mn) nodules, has created new concerns regarding the potential impact of these activities on deep-ocean ecosystems (Ramirez-Llodra et al., 2011). Sincethe1970s,numerousbaselinesurveysandsmall-scaleoceanfloordisturbanceexperimentshavebeenlaunchedin theEasternPacific(Jonesetal.,2017).TheseprojectsincludedtheMnnoduleproject(MANOP),theKoreaDeepOcean Study and other studies in the Clarion-Clipperton Zone (CCZ), where there is a high abundance of Mn nodules (Jones et al., 2017).Other experiments were carried out to evaluate the environmental impacts of experimental deep-ocean mining.These projects included the Disturbance and Recolonization Experiment (DISCOL) in the Peru Basin, the Benthic Impact Experiment, the Japan Deep-Sea Impact Experiment (JET), and other studies in the CCZ (Jones et al., 2017;Volz et al., 2020).These investigations confirmed that some deep-ocean ecosystems are particularly vulnerabletomechanicaldisruptionoftheoceanfloorsurfaceMiningtoolssuchasepibenthicsledges(EBS)havebeen showntoremovetheupper fewcentimetersofthedeep-ocean sediments,theeffectsofwhichtakehundredsofyearsto recover (Volz et al., 2020).Consequently, the United Nations Convention on the Law of the Sea and the International Seabed Authority (ISA) have been tasked with protecting and managing seabed mining in regions beyond national jurisdictions (Levin et al.,2016).Assessments of biological andgeochemical impacts of Mn nodules extractionon the ocean floor and overlying water column are needed to help manage the extensive and frequent baseline surveys and resource exploration activities, which remains largely undocumented (Levin et al., 2016;Tyler, 2003).
Radioisotopes (e.g., 210 Pb-226 Ra) have been widely used to trace sediment mixing, especially the intensity of bioturbation (Boudreau, 1994;DeMaster & Cochran, 1982b;Guinasso & Schink, 1975).Due to its natural decay properties (t 1/2 = 22.3 years), 210 Pb is well suited for tracing decadal scale variation patterns of deep ocean sediment mixing (Z.Yang et al., 2020).Studies from different times and regions using this isotope system, lay a foundation for assessing the extent and driving forces of sediment mixing in the deep ocean (Boudreau, 1994;Teal et al., 2008).
In this paper, we use the 210 Pb-226 Ra isotope pair as a tool to: (a) understand the characteristics of sediment mixing in Mn-nodule fields compared to non-nodule field regions in the ocean; and (b) determine the main drivers of SML depths spatiotemporal distribution patterns.Both previously published papers (Boudreau, 1994;Teal et al., 2008), along with new 210 Pb-226 Ra isotope data (including 309 sampling sites), were used to characterize SMLs-using the parameters of mixing coefficient (D B ) and depth (L).Bottom shear stress and surface chlorophyll were used as proxies to examine decadal patterns of naturally driven physical and biological mixing, and the possible role of anthropogenic-driven sediment mixing in nodule fields of the deep Pacific Ocean.

Data Sources
Global data for sediment 210 Pb and 226 Ra profiles throughout the deep oceans were collected from peer-reviewed literature and our own investigations.The Web of Science (Thomson Reuters, New York, NY), Google Scholar, and Bai Du Scholar were utilized to search the literature using the following key words or phrases: 210 Pb-226 Ra in deep-ocean sediments; bioturbation and/or particle mixing.This data mining has provided an excellent base to show relationships between sediment characteristics and radionuclide profiles, along with additional 210 Pb-226 Ra profiles (Boudreau, 1994;Solan et al., 2019;Teal et al., 2008).Radionuclide profiles are based on measurement of nuclide activity in a sediment layer, measured either by gamma spectrometry or alpha spectrometry.Results obtained by these two spectrometric methods are comparable based on Cochran et al. (1990).We also present new data from three cores collected in 2021 and 2022 in the northwest Pacific Ocean (see Data Set S1 in Supporting Information S1).Sediment samples were collected using box cores from which subcores were taken.Sediment samples from the subcores were dried at 60°C for 24 hr, ground to a fine powder, and sealed for 20 days until 222 Rn-226 Ra equilibrium was reached.The radioactivities of 226 Ra and 210 Pb were then measured by gamma spectrometry using a high-purity germanium detector (ORTEC GWL-150-15-AWT, USA).We compiled data from 58 studies reporting 210 Pb ex profiles from 309 sites distributed globally (Figure S1 in Supporting Information S1).All sites were located at water depths greater than 1,000 m.
Mn nodules occur in all oceans but have only been well documented in four regions where the density of nodules is high enough to justify industrial exploration (Bücker et al., 2014).We classified the sites of collected data into three categories based on the sampling period and area.These included pre-exploration (PreE) and postexploration (PostE) at nodule exploration sites (NF), as well as samples from non-nodule and unexplored sites (Non-NF).We refer to these categories as PreE-NF sites, PostE-NF sites, and Non-NF sites respectively.Our categorization of pre-or post-exploration time is based on a reference year of 1994, when the ISA was founded, and exploration activities occurred more frequently thereafter.PreE-NF cores (n = 15) and PostE-NF cores (n = 35) were collected in the CCZ (eastern Pacific), and the DISCOL project area in the Peru Basin.Non-NF cores were collected in the Atlantic (n = 165), Pacific (n = 60), Indian (n = 5), Arctic (n = 21), and Southern Ocean basins (n = 8).

Bioturbation Model Based on 210 Pb ex Profiles
To quantitatively analyze bioturbation effects, the bioturbation coefficient (D B ) and mixed depth (L) were used to estimate bioturbation intensity (Z.Yang et al., 2020).The D B value represents the mixing rate of sediments, which is analogous to particle diffusion.The L value denotes the depth of mixing based on the time scale indicated by the 210 Pb ex penetration, allowing both D B and L to be estimated based on tracer profiles in the sediment cores.The distribution of 210 Pb ex can be used to derive mixing by bio diffusion based on the advection-diffusion equation (Officer, 1982): where, A is the 210 Pb ex activity, D B is the bioturbation coefficient, S represents the sediment accumulation rate, and λ is the decay constant of 210 Pb (0.0312 years 1 ).Detailed calculation methods are described in Text S1 of Supporting Information S1.Sediment accumulation rates in the abyssal ocean are commonly on the order of millimeters per thousands of years.Consequently, any 210 Pb ex occurrence in sediment profiles below the sediment-seawater interface must be due to sediment reworking.The mixing could be caused by physical, biological, and/or anthropogenic drivers and the depth of mixing (L) by the intensity of these drivers (Jumars & Wheatcroft, 1989).

Bottom Shear Stress
Bottom shear stress generated by deep-ocean circulation was used as an indicator of a bottom current driver of sediment mixing.To assess the historical impact and variability of such a driving force, we analyzed the long-term

Geophysical Research Letters
10.1029/2024GL108928 changes of bottom shear stress in different ocean basins from 1980 to 2020.Shear stress (τ) may be calculated using the approximate formula: where, ρ is the seawater density (kg/m 3 ), U is the oceanic circulation velocity (m/s), and C d represents the seabed friction coefficient which is approxiamatly 3 × 10 3 (Shi et al., 2012).
Ocean bottom circulation velocity estimates were derived from the database of Simple Ocean Data Association version 3 (Simple Ocean Ocean Data Association version 3 (SODA3)), which is maintained by the National Oceanic and Atmospheric Administration (NOAA).

Chlorophyll
We used the total amount of chlorophyll in the euphotic layer of the ocean as an indicator of the food availability for the benthic biomass.Here, we use a comprehensive suite of earth system models from the Coupled Model Intercomparison Project Phase 6 (CMIP6, Eyring et al., 2016), to analyze historical long-term variations (from 1950 to 2015) of the total chlorophyll content in the oceanic euphotic layer.Compared with satellite observation data, the RMSE and bias of the multi-model mean chlorophyll simulated by CMIP6 is 0.24 and 0.03 mg m 3 (Fu et al., 2022).We choose to use the multi-model mean of CMIP6 for analyzing these long-term changes in ocean chlorophyll.

Spatial-Temporal Variations of the SML
The extent of sediment mixing was characterized by the mixing coefficients (D B ) and the mixed-layer depths (L) (DeMaster & Cochran, 1982b;Guinasso & Schink, 1975), which were estimated based on excess 210 Pb ( 210 Pb ex = 210 Pb total -226 Ra) profiles in sediment cores.Spatial and temporal variations of sediment mixed layers exist in nodule exploration fields (NF) and non-nodule exploration fields (Non-NF), which are shown in Figures 1a-1d.
Both D B and L values in NF were significantly higher than in Non-NF (Figures 1a and 1b, and Figure S2 in Supporting Information S1).Profiles of 210 Pb ex in Non-NF cores showed maximum activities in surface sediments with an exponential decrease with depth (Figure 1e).The L values were commonly <15 cm at almost all stations (193 out of 200) with an average value of 7.3 ± 3.7 cm.D B values were less than 1 cm 2 year 1 at most stations (187 out of 203) with an average of 0.3 ± 0.9 cm 2 year 1 .Both D B and L were higher on continental shelf margins and seamount areas (Kuehl et al., 1993;Legeleux et al., 1994).The highest D B occurred at the HEBBLE sites, a region in the northwest Atlantic, well-known for its benthic storms with D B reaching greater than 10 cm 2 year 1 at several stations (DeMaster et al., 1985).In the Arctic and Southern Ocean basins, the D B was generally <5 cm; this agrees with previous work in the Arctic Ocean which reported depths <1 cm (Not et al., 2008).
Profiles of 210 Pb ex in nodule exploration fields show clear differences in SMLs before (Pre-) and after (Post-) active exploration.If we use 1994 as the year distinguishing between Pre-or Post-exploration (when the ISA was founded) there appears to be clear evidence of deepening of the mixed layer which we attribute to exploration activities.SML parameters increased significantly in NF after the mid-1990s, a feature not observed in Non-NF (Figures 1c and 1d).The pre-exploration nodule fields (PreE-NF) 210 Pb ex profiles were similar to those from Non-NF regions (Figures 1e and 1f).Both the D B and L values, in PreE-NF sites, were consistent with those at Non-NF sites; average values by site type were 0.2 ± 0.2 cm 2 year 1 and 7.8 ± 3.3 cm, respectively.However, the postexploration nodule fields (PostE-NF) profiles showed much deeper SML depths compared to the PreE-NF profiles in the same region (Figure 1f).Thirty-seven percent of the SML depths in the PostE-NF stations (13/ 35) were deeper than 30 cm; Moreover, D B values (1-300 cm 2 year 1 ) from these cores, 1-3 orders of magnitude greater than PreE-NF and Non-NF sites.

210 Pb ex Inventories Within the SML
210 Pb ex in sediments originates from minor atmospheric inputs to the surface ocean which is scavenged by particles in the oceanic water column which then accumulate in marine sediments.Indeed, previous studies have found that the integrated deficiency of 210 Pb in the water column is quantitatively consistent with inventories of 210 Pb ex observed in deep-ocean sediments (Bacon et al., 1976;Cochran et al., 1990; J. N. Smith et al., 2003).The average 210 Pb ex inventories in sediments at PreE-NF sites were 35 ± 11 dpm cm 2 (n = 8), which is consistent with the water column deficiencies.However, the 210 Pb ex inventories in the PostE-NF cores (average = 148 ± 91 dpm cm 2 ; n = 34) were about five times the inventories prior to exploration, indicating significantly higher 210 Pb ex than expected in PostE-NF.This elevated 210 Pb ex inventory may be caused either by an increase of unsupported 210 Pb or a decrease of 226 Ra in the sediment.While there was no evidence of increased total 210 Pb activity in sediments over the past few decades (Figure 2a), the 226 Ra profiles did change more dramatically in recent sediments (Figure 2b).The 226 Ra activities were lower on average by 20 dpm g 1 in the top 40 cm compared to deeper sections.This deficiency is roughly equivalent to the increased inventory of 210 Pb ex mentioned above.This suggests that the increase in 210 Pb ex inventory was likely due to desorption and loss of 226 Ra from sediments.Some processes can interfere with the secular equilibrium of the 210 Pb-226 Ra isotope pair, resulting in the reduction of 226 Ra (K d < 500) and the preservation of 210 Pb (K d = 10 4 -10 6 ) (Cochran & Krishnaswami, 1980;Yuan et al., 2023).Intense sediment reworking could induce such 226 Ra loss.This process would have had to occur within a last couple of decades to be observed, as 210 Pb would grow toward a new radioactive equilibrium with 226 Ra with a mean life (1/λ) of 210 Pb (τ = 32 years).Interestingly, the surface 210 Pb ex concentrations are systematically lower in PostE-NF profiles than in PreE-NF profiles, and the trend is opposite in the deeper layers (Figure 1f).This provides valuable evidence for mixing between surface sediments (high 210 Pb ex ) and deeper sediments (lower 210 Pb ex ).
An important question is whether sediments release 226 Ra during disturbances, and if so, where does the 226 Ra go.Previous studies (Cochran & Krishnaswami, 1980;Kadko, 1980;Yuan et al., 2023) have shown that almost all the 226 Ra in deep-ocean sediment is generated from decay of its parent 230 Th.The isotope 230 Th is scavenged out of the water column and is adsorbed onto the surfaces of sediment particulate matter.Thus, the 226 Ra produced by the subsequent decay of 230 Th would also be primarily attached to particle surfaces.However, a fraction of the adsorbed 226 Ra can be released into pore waters, via alpha recoil, diffusing into the overlying water column and/or it could be adsorbed by manganese oxides (Cochran & Krishnaswami, 1980).An estimated 70% of the total 226 Ra produced from decay of adsorbed 230 Th, is potentially exchangeable (Cochran & Krishnaswami, 1980;Yuan et al., 2023).Since sediment accumulation rates in the deep sea are on the order of mm/kyr (millimeters per millennium), sediment at depths of a few centimeters would represent at least 10,000 years-if burial was the main process.This is long enough for 226 Ra to reach secular equilibrium with 230 Th.Therefore, if there is excess 210 Pb within sediments deeper than about 15 cm, it must be due to mixing and 226 Ra desorption.
Dissolved 226 Ra profiles in the Pacific water column have not varied considerably over the past few decades (Chung & Craig, 1980;Kemnitz et al., 2023;Xu et al., 2022), so it is unlikely that a large fraction of the sedimentary 226 Ra has recently diffused into the overlying oceanic water column.Mn nodules are known to have 226 Ra activities 2-10 times higher than its parent 230 Th, and higher 226 Ra accumulation appears preferentially at the base and not at the top of the nodules, indicating that they have adsorbed large amounts of 226 Ra from pore waters (Chih-An & Teh-Lung, 1984;Krishnaswami & Cochran, 1978;Moore, 1984;Volz et al., 2023).It thus seems possible that Mn nodules and crusts on the ocean floor adsorbed much of the 226 Ra that may have been released from the sediments and can explain where the 226 Ra released from sediments is stored.Removal of these Mn nodules and crusts might cause an apparent increase in the depth of 210 Pb ex penetration and sediment mixing layer.

Causes for Intensified Sediment Disturbance
Results presented here indicate that sediment mixing in Mn nodule exploration areas (e.g., the CCZ and Peru Basin) has significantly increased depth of the SML over the past few decades.Possible explanations for deepening the SML include Mn nodule exploration activities, an increase in benthic biomass activity, and/or an enhancement of bottom currents strong enough to erode sediments.To explore which factor is dominant, two nonnodule field regions (the northeast Atlantic plain and the northwest Pacific plain) and one nodule field region (the CCZ) were selected for comparison of the temporal variations of physical and biological driving forces over the past few decades.The probability of Mn nodule occurrence in these three regions is <20% (NE Atlantic), 20%-50% (NW Pacific) and >50% (CCZ) (Dutkiewicz et al., 2020).
Because water depths in all three zones exceed 2,000 m, the main natural ocean dynamic processes likely to disturb bottom sediments are deep ocean circulation and benthic storms (J.Y. Aller, 1989;Jia et al., 2019).Therefore, in this work we focus on bottom shear stress generated by deep ocean dynamics.The decadal variability of the bottom shear stress in these three zones is shown in Figures 3a-3c.During the past four decades (from 1980 to 2020), the average bottom shear stresses in the northwest Pacific Ocean, northeast Atlantic Ocean, and CCZ were 0.99 ± 0.09 10, 0.17 ± 0.06 10 and 0.38 ± 0.02 10 4 N m 2 , respectively.The shear stress in the northwest Pacific Ocean is about five times stronger than the northeast Atlantic Ocean.However, the sediment mixing parameters (both L and D B values) in these two non-NF regions are similar.The shear stress in the CCZ was less than 40% of that in the northwest Pacific Ocean with slightly decreasing decadal pattern suggesting that erosive deep ocean currents in the CCZ are not the cause for the deeper SML there.
Benthic storms can cause resuspension of fine sediment particles and increase the sediment mixing intensity in some areas, such as seen at the HEBBLE sites in the North Atlantic (Gardner et al., 2017).However, there is no evidence showing that benthic storms mix sediment to depths greater than 20 cm (DeMaster et al., 1985;Volz et al., 2020).Observational records indicate that benthic storms and sediment resuspension events, due to abyssal benthic flows, are relatively weak in the manganese nodule zone compared with other places around the world ocean (Chen et al., 2023).Bottom current speeds measured over nodule fields are too low (<5 cm s 1 ) to erode sediments (Dutkiewicz et al., 2020), indicating that the enhanced sediment mixing, observed in the CCZ during the post exploration period, was not caused by natural ocean dynamic processes, further supporting evidence for anthropogenic disturb.
Bioturbation driven by increased benthic organisms' activity could also intensify sediment reworking.Usually, about 99% of benthic organisms living within the abyssal sediments are found within the upper 12 cm depth (Shirayama, 1984).Previous research (Jumars & Wheatcroft, 1989) has proposed that the depth of bioturbation and irrigation is spatially invariant, implying that the burrowing capacity may be universally constrained within the top sediment layers.Therefore, bioturbation is most likely to be the dominant factor driving shallow sediment mixing (Müller & Mangini, 1980), similar to that found in the Non-NF and PreE-NF regions (<15 cm).Wei et al. (2010) using ocean floor biomass and abundance databases found that Mn nodules rich areas generally are located below areas of low surface productivity (<300 mgC m 2 day 1 in summer) and low biomass concentrations (<1 log mgC m 2 ).
If benthos abundance or activities increased over the past few decades, bioturbation would also have to be considered as a possible driver of change in SML depths.Based on some early studies deep-sea mining has a negative effect on the abundance of deep-sea benthos (Simon-Lledó et al., 2019;Stratmann et al., 2018;Washburn et al., 2021).Because temporal variation data on deep ocean floor biomass is limited (Moreau et al., 2020), we used the global data of surface ocean chlorophyll and assume a link between food resources, and deep-sea benthic organisms that induce bioturbation.While a very small percentage of surface ocean primary production typically makes it to deep-sea benthic communities, these organisms are food limited and have been shown to respond to surface ocean inputs (Tecchio et al., 2013), which could then impact bioturbation (C.R. Smith et al., 2008;Vanreusel et al., 2016).The average annual chlorophyll concentration in the surface waters above the northeast Atlantic abyssal plain, the northwest Pacific abyssal plain, and the CCZ, over the past 70 years, are 0.45 ± 0.03, 0.45 ± 0.03 and 0.16 ± 0.01 mg m 3 , respectively (Figures 3d-3f).Chlorophyll content above the northeast Atlantic abyssal plain and northwest Pacific abyssal plain is similar and 3 times higher than that above in the CCZ.Biological mixing caused by macrobenthic organisms is unlikely to disturb deeper sediment in PostE-NF sites, because the SML depths are beyond the typical bioturbation depths (Shirayama, 1984).In addition, the population density of the larger late successional stage macrofauna is typically lower after disturbance.It thus appears that if we exclude the influence of benthic ocean dynamics and biological drivers, we reach the conclusion that human exploration activities are the most likely cause of the observed increase in mixing depth as seen in the PostE-NF 210 Pb ex profiles.

Outlook and Future Perspectives
Sediments in the Mn nodule exploration areas were increasingly disturbed over the past few decades (Figure 1).Neither physical nor biological driving forces that we considered seem likely to deepen the SML.We therefore conclude that manganese nodule exploration over the past few decades has impacted sediment mixing in Mn nodule fields.The spatial scale of anthropogenic exploration activities thus far covers about 1.2 million km 2 with 18 exploration contracts, each covering 75,000 km 2 , authorized by ISA (Hein et al., 2020;Levin et al., 2020).As economic interest in deep ocean Mn nodules continues, more extensive exploration and possibly mining are expected, and the study of associated environmental impacts will be necessary.
Commonly employed sampling methods associated with exploration activities, such as core collection (box cores, multiple or piston samplers) and EBS, could deepen the SML, with some flocculent surficial sediment being completely removed and/or laterally transported and redeposited nearby (Amon et al., 2016;Jung et al., 1998).
Comparisons of solid-phase Mn and total organic carbon (TOC) profiles in disturbed and reference (undisturbed) sites, revealed that sediments from the upper 5-15 cm were removed during various small-scale mining experiments (Volz et al., 2020).These processes could result in a 210 Pb ex "apparent deposition" profiles but with much deeper SMLs than what is common for open ocean conditions.
Intense sediment reworking would dramatically alter the biogeochemistry of different elements and compounds at the oceanic benthic boundary layer.In most cases, the reactive labile fraction of the TOC would be remineralized relatively quickly (R. C. Aller, 1994;Glud et al., 2021).Oxygen consumption rates are known to decrease significantly after the removal of surface sediments, and denitrification and Mn (IV) reduction are inhibited (Haeckel et al., 2001;Volz et al., 2018).Exchangeable 226 Ra and similar elements (e.g., Ba) would be more likely of escaping from sediment grains into pore waters after significant disturbance, especially at the sediment-water interface.Nodule removal, sediment disturbance, resuspended sediment plume perturbations, and the potential toxicity of released metals could cause serious harm to the marine environment, that could last for millennia (Paul et al., 2018).
In the future, as high-tech, green energy industries develop, deep-ocean contributions to mining of critical metals seem inevitable.Exploration techniques and apparatus with low or negligible disturbance to deep-sea sediment should be encouraged.Such approaches could include ROVs with underwater cameras, deep-towed video or photo sledges, and multibeam echo-sounding systems (Levin et al., 2016).The probability of nodule occurrence can even be predicted via machine learning (Dutkiewicz et al., 2020).Recently, the natural radioactive radon isotope ( 222 Rn) was proposed as a non-invasive tracer to evaluate nodule occurrences, as significant linear relationships were observed between benthic excess 222 Rn fluxes and concentrations of Fe-Mn nodules on the ocean floor (Guo et al., 2022).Development of new environmental-friendly technologies to access marine mineral resources would help to move the ocean toward a more sustainable future.

Figure 1 .
Figure 1.Spatial-temporal distributions of the sediment bioturbation coefficient (D B ) and mixed layer depth (L) are shown in panels (a)-(d).The 210 Pb ex profiles in non-nodule field (Non-NF, n = 259) regions and nodule fields (NF, n = 50) regions are shown in panels (e) and (f), respectively.The square frames indicate nodule fields in panels (a) and (b).In panels (c) and (d), column height represents the annual mean of D B and L. In panels (e) and (f), blue circles and red circles represent 210 Pb ex profiles collected during Pre-and Post-exploration period, respectively.The colored dotted lines are the average values of

Figure 2 .
Figure 2. Total 210 Pb (a) and 226 Ra (b) profiles collected from sediments in the Clarion-Clipperton Zone.The blue and red solid circles are average values for pre-exploration and post-exploration periods, respectively.The colored dotted lines are the average values of total 210 Pb and 226 Ra at different depths.

Figure 3 .
Figure 3. (a-c) Decadal variations of bottom stress and (d-f) chlorophyll in the NW Pacific abyssal plain, the NE Atlantic abyssal plain, and the Clarion-Clipperton Zone.