The thriving interest in harvesting deep-sea mineral resources, such as polymetallic nodules, calls for environmental impact studies and, ultimately, for regulations for environmental protection. Industrial-scale deep-sea mining of polymetallic nodules most likely has severe consequences for the natural environment. However, the effects of mining activities on deep-sea ecosystems, sediment geochemistry and element fluxes are still poorly understood. Predicting the environmental impact is challenging due to the scarcity of environmental baseline studies as well as the lack of mining trials with industrial mining equipment in the deep sea. Thus, currently we have to rely on small-scale disturbances simulating deep-sea mining activities as a first-order approximation to study the expected impacts on the abyssal environment.
Here, we investigate surface sediments in disturbance tracks of seven small-scale benthic impact experiments, which have been performed in four European contract areas for the exploration of polymetallic nodules in the Clarion–Clipperton Zone (CCZ) in the NE Pacific. These small-scale disturbance experiments were performed 1 d to 37 years prior to our sampling program in the German, Polish, Belgian and French contract areas using different disturbance devices. We show that the depth distribution of solid-phase Mn in the upper 20 cm of the sediments in the CCZ provides a reliable tool for the determination of the disturbance depth, which has been proposed in a previous study from the SE Pacific (Paul et al., 2018). We found that the upper 5–15 cm of the sediments was removed during various small-scale disturbance experiments in the different exploration contract areas. Transient transport-reaction modeling for the Polish and German contract areas reveals that the removal of the surface sediments is associated with the loss of the reactive labile total organic carbon (TOC) fraction. As a result, oxygen consumption rates decrease significantly after the removal of the surface sediments, and, consequently, oxygen penetrates up to 10-fold deeper into the sediments, inhibiting denitrification and Mn(IV) reduction. Our model results show that the return to steady-state geochemical conditions after the disturbance is controlled by diffusion until the reactive labile TOC fraction in the surface sediments is partly re-established and the biogeochemical processes commence. While the re-establishment of bioturbation is essential, steady-state geochemical conditions are ultimately controlled by the delivery rate of organic matter to the seafloor. Hence, under current depositional conditions, new steady-state geochemical conditions in the sediments of the CCZ are reached only on a millennium scale even for these small-scale disturbances simulating deep-sea mining activities.
The accelerating global demand for metals and rare-earth elements drives the economic interest in deep-sea mining (e.g., Glasby, 2000; Hoagland et al., 2010; Wedding et al., 2015). Seafloor minerals of interest include (1) polymetallic nodules (e.g., Mero, 1965), (2) massive sulfide deposits (e.g., Scott, 1987) and (3) cobalt-rich crusts (e.g., Halkyard, 1985). As the seafloor within the Clarion–Clipperton Zone (CCZ) in the NE Pacific holds one of the most extensive deposits of polymetallic nodules with considerable base metal quantities, commercial exploitation of seafloor mineral deposits may focus on the CCZ (e.g., Mero, 1965; Halbach et al., 1988; Rühlemann et al., 2011; Hein et al., 2013; Kuhn et al., 2017a). The exploration and, ultimately, industrial exploitation of polymetallic nodules demand international regulations for the protection of the environment (e.g., Halfar and Fujita, 2002; Glover and Smith, 2003; Davies et al., 2007; van Dover, 2011; Ramirez-Llodra et al., 2011; Boetius and Haeckel, 2018). The International Seabed Authority (ISA) is responsible for regulating the exploration and exploitation of marine mineral resources as well as for protecting and conserving the marine environment beyond the exclusive economic zones of littoral states from harmful effects (ISA, 2010). The ISA has granted temporal contracts for the exploration of polymetallic nodules in the CCZ, engaging all contract holders to explore resources, test mining equipment and assess the environmental impacts of deep-sea mining activities (ISA, 2010; Lodge et al., 2014; Madureira et al., 2016).
Although a considerable number of environmental impact studies have been conducted in different nodule fields, the prediction of environmental consequences of potential future deep-sea mining is still difficult (e.g., Ramirez-Llodra et al., 2011; Jones et al., 2017; Gollner et al., 2017; Cuvelier et al., 2018). In case of the CCZ, the evaluation of the environmental impact of deep-sea mining activities is challenging due to the fact that baseline data on the natural spatial heterogeneity and temporal variability in depositional conditions, benthic communities and the biogeochemical processes in the sediments are scarce (e.g., Mewes et al., 2014, 2016; Vanreusel et al., 2016; Mogollón et al., 2016; Juan et al., 2018; Volz et al., 2018, 2020; Menendez et al., 2018; Hauquier et al., 2019). In addition, there is no clear consensus on the most appropriate mining techniques for the commercial exploitation of nodules, and technical challenges due to the inaccessibility of nodules at great water depths between 4000 and 5000 m have limited the deployment of deep-sea mining systems until today (e.g., Chung, 2010; Jones et al., 2017).
The physical removal of nodules as hard-substrate habitats has severe consequences for the nodule-associated sessile fauna as well as the mobile fauna (Bluhm, 2001; Smith et al., 2008; Purser et al., 2016; Vanreusel et al., 2016). With slow nodule growth rates of a few millimeters per million years (e.g., Halbach et al., 1988; Kuhn et al., 2017a), the deep-sea fauna may not recover for millions of years (Vanreusel et al., 2016; Jones et al., 2017; Gollner et al., 2017; Stratmann et al., 2018). In addition to the removal of deep-sea fauna as well as seafloor habitats, the exploitation of nodules is associated with (1) the removal, mixing and re-suspension of the upper 4 cm to more than several tens of centimeters of the sediments; (2) the re-deposition of material from the suspended sediment plume; and (3) potentially also the compaction of the surface sediments due to weight of the nodule collector (Thiel and Forschungsverband Tiefsee-Umweltschutz, 2001; Oebius et al., 2001; König et al., 2001; Grupe et al., 2001; Radziejewska, 2002; Khripounoff et al., 2006; Cronan et al., 2010; Paul et al., 2018; Gillard et al., 2019). The wide range of estimates for the disturbance depth may be associated with (1) various devices used for the deep-sea disturbance experiments (Brockett and Richards, 1994; Oebius et al., 2001; Jones et al., 2017), (2) distinct sediment properties in different nodule fields of the Pacific Ocean (e.g., Cronan et al., 2010; Hauquier et al., 2019) and (3) different approaches for the determination of the disturbance depth (e.g., Oebius et al., 2001; Grupe et al., 2001; Khripounoff et al., 2006). Based on the observation that bulk solid-phase Mn contents decrease over depth in the surface sediments of the Disturbance and Recolonization Experiment (DISCOL) area in the Peru Basin, SE Pacific, Paul et al. (2018) have suggested that the depth distribution of solid-phase Mn and associated metals (e.g., Mo, Ni, Co, Cu) could be used to trace the sediment removal by disturbances. In addition, other solid-phase properties such as total organic carbon (TOC) contents, porosity and radioisotopes may be suitable for the determination of the disturbance depth.
The most reactive TOC compounds, found in the bioturbated uppermost sediment layer, are the main drivers for early diagenetic processes (e.g., Froelich et al., 1979; Berner, 1981) and are expected to be removed during mining activities (König et al., 2001). Thus, strong biogeochemical implications can be expected in the sediments after deep-sea mining activities. König et al. (2001) have applied numerical modeling to study the consequences of the removal of the upper 10 cm of the sediments in the DISCOL area. They showed that the degradation of TOC during aerobic respiration, denitrification and Mn(IV) reduction may be decreased for centuries, increasing the oxygen penetration depth (OPD).
Here, we investigate the impact of various small-scale disturbances on geochemical conditions, biogeochemical processes and element fluxes in surface sediments of the CCZ. These small-scale disturbance tracks were created up to 37 years ago in four different European contract areas for the exploration of polymetallic nodules, including the German BGR (Bundesanstalt für Geowissenschaften und Rohstoffe) area, the Belgian GSR (Global Sea Mineral Resources NV) area, the French IFREMER (Institut Français de Recherche pour l'Exploitation de la Mer) area and the Polish IOM (Interoceanmetal) area. In order to determine the disturbance depths of the different small-scale disturbances in the different European contract areas, we correlate the depth distributions of solid-phase Mn and TOC between disturbed sites and undisturbed reference sites using the Pearson product–moment correlation coefficient. On this basis, we (1) assess the short- and long-term consequences of small-scale disturbances on redox zonation and element fluxes and (2) determine how much time is needed for the re-establishment of a new steady-state geochemical system in the sediments after the disturbances. Our work includes pore-water and solid-phase analyses as well as the application of a transient one-dimensional transport-reaction model.
As part of the European JPI Oceans pilot action
“Ecological aspects of deep-sea mining (MiningImpact)”, multiple-corer (MUC) and gravity-corer
(GC) sediment cores were taken during the RV
Sampling sites (black circles; black star) in various European contract areas for the exploration of manganese nodules within the Clarion–Clipperton Zone (CCZ). Investigated stations are located in the German BGR area (blue), eastern European IOM area (yellow), Belgian GSR area (green) and French IFREMER area (red). The two stations within the German BGR area are located in the “prospective area” (BGR-PA; black star) and in the “reference area” (BGR-RA; black circle). The contract areas granted and governed by the International Seabed Authority (ISA; white areas) are surrounded by nine areas of particular environmental interest (APEIs), which are excluded from any mining activities (green shaded squares). Geographical data provided by the ISA.
MUC and PC cores investigated in this study, including information on geographic position, water depth, type and age of the disturbances (years: yr; months: mth; days: d).
The different investigated European contract areas within the CCZ include
the BGR, IOM, GSR and IFREMER areas. Comprehensive pore-water and
solid-phase analyses on the MUC and GC sediment cores from undisturbed sites
have been conducted in previous baseline studies and are presented elsewhere
(Volz et al., 2018, 2020). These analyses include the
determination of pore-water oxygen;
The CCZ is defined by two transform faults, the Clarion Fracture Zone in the
north and the Clipperton Fracture Zone in the south, and covers an area of
about 6
Information of sedimentation rate (Sed. rate), flux of particulate
organic carbon (POC) to the seafloor, bioturbation depth (Bioturb. depth),
and oxygen penetration depth (OPD) based on GC cores from the investigated sites
and determined in the study by Volz et al. (2018). Information for the
BGR-PA area is taken from an adjacent site (A5-2-SN; 11
Since the 1970s, several comprehensive environmental impact studies of
deep-sea mining simulations have been carried out in the CCZ, including the
Benthic impact experiment (BIE; e.g., Trueblood and Ozturgut, 1997;
Radziejewska, 2002) and the Japan deep-sea impact experiment (JET;
Fukushima, 1995). In addition, numerous small-scale seafloor disturbances
have been carried out in the CCZ in the past 40 years using various tools such
as epibenthic sledge (EBS) and dredges (e.g., Vanreusel et al., 2016; Jones
et al., 2017). The EBS is towed along the seabed for the collection of
benthic organisms (and nodules), thereby also removing the upper few
centimeters of the sediments (e.g., Brenke, 2005). In 2015, some of these disturbances that were up
to 37 years old were revisited as part of the BMBF–EU JPI Oceans
pilot action “Ecological aspects of deep-sea mining (MiningImpact)”
project in order to evaluate the long-term consequences of such small-scale
disturbances on the abyssal benthic ecosystem (Table 1; Fig. 2;
Martínez Arbizu and Haeckel, 2015). For comparison, DISCOL, which was conducted in a nodule field in
the Peru Basin (PB) in 1989, was revisited as part of MiningImpact (Boetius,
2015; Greinert, 2015). In the framework of DISCOL, a seafloor area of
Examples of undisturbed reference sediments in the German BGR-PA
area and the French IFREMER area and pictures of small-scale disturbances
for the simulation of deep-sea mining within the CCZ, which are investigated
in the framework of this study (years: yr; months: mth; days: d). Copyright: ROV
ROV-operated push cores were sampled at intervals of 1 cm for solid-phase
analyses. Bulk sediment data and TOC contents have been corrected after Kuhn (2013) for the interference of the pore-water salt matrix with the sediment
composition (Volz et al., 2018). The mass percentage of the pore water was
determined gravimetrically before and after freeze-drying of the wet
sediment samples. The salt-corrected sediment composition
Total acid digestions were performed in the microwave system MARS Xpress
(CEM) after the protocols by Kretschmer et al. (2010) and Nöthen and
Kasten (2011). Approximately 50 mg of freeze-dried, homogenized bulk
sediment was digested in an acid mixture of 65 % sub-boiling distilled
Total organic carbon (TOC) contents were determined using an Eltra CS-2000
element analyzer. Approximately 100 mg of freeze-dried, homogenized sediment
was transferred into a ceramic cup and decalcified with 0.5 mL of
10 % HCl at 250
In order to determine the disturbance depths, solid-phase bulk Mn contents
were correlated between disturbed sediments and undisturbed reference
sediments using the Pearson product–moment correlation coefficient
While the solid-phase bulk Mn contents of the disturbed sediments were
determined in the framework of this study, solid-phase bulk Mn contents from
undisturbed reference sediments were taken from Volz et al. (2020). The
highest positive linear correlations of solid-phase Mn contents
(
A transient one-dimensional transport-reaction model (Eq. 3; e.g.,
Boudreau, 1997; Haeckel et al., 2001) was used (1) to assess the impact of
small-scale disturbances on biogeochemical processes, geochemical conditions
and element fluxes in sediments of the CCZ and (2) to estimate the time
required to establish a new steady-state geochemical system after a
small-scale disturbance. We applied a transient transport-reaction
model for the sites in the BGR-RA and IOM areas (Table 1). These sites were
chosen due to distinctively different sedimentation rates and OPD (Table 2).
We adapted the code of the steady-state transport-reaction model, which
was originally presented by Volz et al. (2018), and used pore-water oxygen,
The bioturbation and bioirrigation profiles, i.e., biologically induced
mixing of sediment and pore water, respectively, are represented by a
modified logistic function:
Assuming steady-state compaction, the model applies an exponential function
that is parameterized according to the available porosity data at each
station (e.g., Berner, 1980; Supplement Fig. S1):
Organic matter was treated in three reactive fractions (3G-model) with first-order kinetics. The rate expressions for the reactions (R1–R6) include inhibition terms, which are listed together with the rate constants (Table S3).
Calculated Pearson correlation coefficients
Based on the Pearson correlation coefficient
Most of the small-scale disturbances investigated in the framework of this study were created with an EBS (Table 1; Fig. 2). Based on the visual impact inspection of the EBS disturbance tracks in the CCZ, the sediments were mostly pushed aside by the EBS and piled up next to the left and right side of the tracks (Fig. 2). In particular, the freshly created 1 d old EBS tracks in the BGR-RA and GSR areas indicate that the sediments were mostly scraped off and accumulated next to the freshly exposed sediment surfaces (Fig. 2). Small sediment lumps occur on top of the exposed sediment surfaces on the EBS tracks, which indicates that some sediment slid off from the adjacent flanks of the sediment accumulation after the disturbances (Fig. 2). However, the mostly smooth sediment surfaces of the EBS tracks suggest that sediment mixing during the EBS disturbance experiments may be mostly negligible (Fig. 2; Table 1). In the 8-month-old dredge track in the GSR area, small furrows occur at the disturbed sediment surface, most likely caused by the shape of the dredge (Fig. 2).
The sediment porosity shows little lateral variability and ranges between 0.65 and 0.8 throughout the upper 25 cm of the sediments at all investigated disturbed sites (Fig. 3). At the disturbed IOM-BIE site, sediment porosity is about 5 % higher in the upper 4 cm of the sediments than below. Bulk Mn contents in the upper 25 cm of the sediments at the disturbed sites are between 0.1 and 0.9 wt % (Fig. 3). Solid-phase Mn contents decrease with depth at all investigated sites. Total organic carbon (TOC) contents in the upper 25 cm of the sediments at the disturbed sites are within 0.2 wt % and 0.5 wt % (Fig. 3). The TOC contents slightly decrease with depth at all investigated sites.
Solid-phase Mn and TOC contents for all disturbed sites investigated in the framework of this study (years: yr; months: mth; days: d).
The Pearson correlation coefficient
Correlation of solid-phase Mn and TOC contents between the disturbed sites and the respective undisturbed reference sediments (grey shaded profiles) using the disturbance depths determined with the Pearson correlation coefficient (compare Table 3). For the undisturbed reference sediments, solid-phase Mn contents are taken from Volz et al. (2020) and TOC contents are taken from Volz et al. (2018; years: yr; months: mth; days: d).
The removal of the surface sediments in the transient transport-reaction
model for the BGR-RA and IOM-BIE sites is associated with the loss of the
reactive labile organic matter (Figs. 5 and 6). About 10 kyr after the
removal of the upper 10 cm of the sediments in the model for the BGR-RA
site, oxygen penetrates about 10-fold deeper into the disturbed sediments
than in undisturbed sediments (Table 2; Fig. 5; Volz et al., 2018). At the
IOM-BIE site, oxygen reaches the maximum OPD at about 100 years after the
removal of the upper 7 cm of the sediments. At this site, the oxygen front
migrates only
Model results of the transient transport-reaction model showing depth profiles of TOC,
Detailed model results of the transient transport-reaction model
(Fig. 5) showing depth profiles of TOC,
Naturally, the solute fluxes across the SWI are
strongly affected after the surface sediment removal (Fig. 7). The transient
transport-reaction model suggests that the oxygen fluxes into the sediments
are lowered by a factor of 3–6 after 10–100 years at the IOM-BIE and
BGR-RA sites, respectively. This trend is mirrored by the decreased release
of
Pore-water fluxes of oxygen (
Our work demonstrates that the depth distribution of solid-phase Mn provides a reliable tool for the determination of the disturbance depths in the sediments of the CCZ (Fig. 4; Table 3). The success of the correlation of solid-phase Mn contents between disturbed and undisturbed reference sediments benefits from several factors.
Sediment mixing during the small-scale disturbance experiments is
negligible: the visual impact assessment of the investigated disturbance
tracks in the CCZ suggests that sediment mixing during the small-scale
disturbance experiments was insignificant (Fig. 2). This observation is in
agreement with a recent EBS disturbance experiment, which was conducted
in the DISCOL area in the Peru Basin in 2015 (Greinert, 2015). The freshly created EBS track
in the DISCOL area was revisited 5 weeks after the disturbance experiment,
where the surface sediment was mostly removed and deeper sediment layers
were exposed without visible sediment mixing (Boetius, 2015; Paul et al.,
2018). In a study on the geochemical regeneration in disturbed sediments of
the DISCOL area, Paul et al. (2018) have shown that the
bulk Mn-rich top sediment layer, which has been observed in undisturbed
sediments, is removed in the 5-week old EBS disturbance track. Thus, an
important prerequisite for this method is met, and the authors have proposed
that the depth distribution of solid-phase Mn may be suitable for the
evaluation of the impact as well as for the monitoring of the recovery of
small-scale disturbance experiments. The solid-phase Mn maxima in the surface sediments appear
to be a regional phenomenon across the CCZ area as has been observed
throughout the different exploration areas studied in the framework of this
study (Volz et al., 2020): the investigated disturbed sediments as well as
the undisturbed reference sediments in the CCZ show decreasing
solid-phase Mn contents with depth in the upper 20–30 cm of the sediments
(Figs. 3; 4; Volz et al., 2020). In the undisturbed reference sediments,
solid-phase Mn contents show maxima of up to 1 wt % in the upper 10 cm of
the sediments, with distinctly decreasing contents below this level (Fig. 4; Volz et
al., 2020). Similar bulk solid-phase Mn distribution patterns have been
reported for other sites within the CCZ (e.g., Khripounoff et al., 2006;
Mewes et al., 2014; Widmann et al., 2015). Volz et al. (2020) have suggested
that the widely observed solid-phase Mn enrichments in CCZ surface sediments
formed in association with a more compressed oxic zone, which may have
prevailed as a result of lower bottom-water oxygen concentrations during the
last glacial period than today. Strong indication for lower glacial
bottom-water oxygen concentrations throughout the eastern Pacific Ocean have
been provided by a number of independent proxies (e.g., Anderson et al.,
2019, and references therein). As a consequence of the condensed oxic zone,
upwardly diffusing pore-water Lastly, the OPD at all sites is located at sediment depths greater than
0.5 m, and, thus, diagenetic precipitation of Mn(IV) in the surface sediments
(e.g., Gingele and Kasten, 1994) since the last glacial period can be ruled
out (Table 2; Mewes et al., 2014; Volz et al., 2020).
Based on the depth distribution of solid-phase Mn, our work suggests that
between 5 and 15 cm of the surface sediments was removed and pushed aside
by the different small-scale disturbance experiments in the CCZ (Table 3;
Fig. 4). This range of disturbance depths is in good agreement with other
estimates for small-scale disturbances by similar gear in the CCZ and in the
DISCOL area, which suggest that the upper 4–20 cm of the sediments was
removed (e.g., Thiel and Forschungsverband Tiefsee-Umweltschutz, 2001; Oebius et al., 2001; König et al., 2001;
Grupe et al., 2001; Radziejewska, 2002; Khripounoff et al., 2006; Paul et
al., 2018). However, as the disturbed sites investigated in this study and
the respective undisturbed reference sites are located up to 5 km apart from
each other, the correlation of solid-phase Mn may be influenced by some
spatial heterogeneities in solid-phase Mn contents (Table 1; Mewes et al.,
2014). Furthermore, it should be noted that for the correlation of
solid-phase Mn contents between the disturbed and undisturbed reference
sites, we did not consider that (1) particles may have re-settled on the
freshly exposed sediment surfaces from re-suspended particle plumes (e.g.,
Jankowski and Zielke, 2001; Thiel and Forschungsverband Tiefsee-Umweltschutz, 2001; Radziejewska, 2002; Gillard et al.,
2019), (2) sediment slid off from adjacent flanks of the sediment
accumulation after the disturbances (Fig. 2), and (3) sediments were
deposited after the small-scale disturbances at sedimentation rates between
0.2 and 1.2 cm kyr
The geochemical conditions found at the study sites in the CCZ are the
result of a balanced interplay of key factors, such as the input of fresh,
labile TOC, sedimentation rate and bioturbation intensity (e.g., Froelich et
al., 1979; Berner, 1981; Zonneveld et al., 2010; Mogollón et al., 2016;
Volz et al., 2018). Together they characterize the upper reactive layer,
which in turn plays a crucial role in the location of the OPD in the
sediments of the CCZ (e.g., Mewes et al., 2014; Mogollón et al., 2016;
Volz et al., 2018). Oxygen is consumed via aerobic respiration during the
degradation of organic matter, while bioturbation transports fresh, labile
TOC into deeper sediments (e.g., Haeckel et al., 2001; König et al.,
2001). The presence of labile TOC throughout the bioturbated zone
significantly enhances the consumption of oxygen with depth, where oxygen is
not as easily replenished by seawater oxygen. Thus, the availability of
labile TOC in the bioturbated layer controls the amount of oxygen that
passes through the reactive layer into deeper sediments (e.g., König et
al., 2001). Below the highly reactive layer, refractory organic matter
degradation and secondary redox reactions – such as oxidation of
The removal of the upper 5–15 cm of the sediment results, on one hand, in an almost complete loss of the labile TOC fraction (Fig. 4), as this fraction is restricted to the upper 20 cm of the sediment in the CCZ (e.g., Müller and Mangini, 1980; Emerson, 1985; Müller et al., 1988; Mewes et al., 2014; Mogollón et al., 2016; Volz et al., 2018). On the other hand, studies on faunal diversity and density in small-scale disturbances in the sediments of the CCZ and in the DISCOL area show that most of the biota is lost immediately after the disturbance experiment (Borowski and Thiel, 1998; 2001; Bluhm et al., 2001; Thiel et al., 2001; Vanreusel et al., 2016; Jones et al., 2017; Gollner et al., 2017). Thus, a drastic decline or standstill of bioturbation can be expected in the surface sediments.
Conceptual model for time-dependent pore-water fluxes of oxygen
(
Based on the results of the transient transport-reaction model, geochemical
recovery after small-scale sediment disturbances can be divided into two
main phases (Fig. 8).
Since the labile TOC fraction and bioturbating fauna are mostly removed,
downward diffusion of oxygen is the main driver shaping solute profiles
towards a new geochemical steady-state system in the absence of the reactive
layer (Figs. 5 and 6). This entails the downward migration of the OPD, as
oxygen is no longer effectively consumed in the upper sediment layer. The
presence of oxygen outcompetes denitrification and Mn(IV) reduction and
induces The second phase is characterized by the increasing influence of
reactive fluxes across the seafloor. It takes approximately 1000 years before
any significant build-up of an upper labile TOC layer is re-established
(Fig. 6), at which point solute profiles slowly shift towards their
pre-disturbance shape (Fig. 7). Interestingly, during the transition time,
when oxygen is still present at depth but aerobic respiration in the upper
sediments has already began to pick up,
With the importance of bioturbation and the mining-related removal of
associated fauna in mind, solute and in particular nutrient fluxes across
the seafloor should also be considered. The release of nutrients complements
the close link between sediment geochemistry and the food web structure
(e.g., Smith et al., 1979; Dunlop et al., 2016; Stratmann et al., 2018) and
further emphasizes their interdependencies. Figure 7 depicts fluxes of
oxygen,
It should be noted that while bioturbation has a pivotal influence on the undisturbed steady-state profile, it only plays a secondary role in re-establishing the steady-state geochemical system at the disturbed sites in the CCZ. Studies suggest that faunal abundances fully recover within centuries after the disturbance even though the benthic community may be different than prior to the disturbance (e.g., Miljutin et al., 2011; Vanreusel et al., 2016). Due to the extremely slow build-up of the reactive layer with labile TOC, the bioturbation “pump” is active again before any significant amount of labile TOC is present about 1–100 kyr after the disturbance. Thus, full recovery is mainly controlled by the re-establishment of the upper reactive layer, i.e., the delivery rate of labile TOC to the seafloor.
The transport-reaction model reveals that under current depositional
conditions, the new steady-state geochemical system is established after
1–10 kyr at the IOM-BIE site, while the re-establishment of steady-state
geochemical conditions at the BGR-RA site takes 10–100 kyr (Figs. 5 and 6).
Shorter recovery times at the IOM site compared to the BGR-RA site are
related to higher sedimentation rates (1.15 instead of 0.65 cm kyr
We studied surface sediments from seven small-scale disturbance experiments for the simulation of deep-sea mining, which were performed between 1 d and 37 years prior to our sampling in the NE Pacific Ocean. These small-scale disturbance tracks were created using various disturbance devices in different European contract areas for the exploration of polymetallic nodules within the eastern part of the Clarion–Clipperton Zone (CCZ). Through correlation of solid-phase Mn contents of disturbed and undisturbed reference sediments, we (1) propose that the depth distribution of solid-phase Mn in the sediments of the CCZ provides a reliable tool for the estimation of the disturbance depth and (2) show that 5–15 cm of the sediments was removed during the small-scale disturbance experiments investigated in this study. As the small-scale disturbances are associated with the removal of the surface sediments characterized by reactive labile organic matter, the disturbance depth ultimately determines the impact on the geochemical system in the sediments. The application of a transient transport-reaction model reveals that the removal of the upper 7–10 cm of the surface sediments is associated with a meter-scale downward extension of the oxic zone and the shutdown of denitrification and Mn(IV) reduction. As a consequence of lower respiration rates after the disturbance experiments, the geochemical system in the sediments is controlled by downward oxygen diffusion. While the re-establishment of bioturbation within centuries after the disturbance is important for the development of steady-state geochemical conditions in the disturbed sediments, the rate at which geochemical steady-state conditions are reached ultimately depends on the delivery rate of organic matter to the seafloor. Assuming the accumulation of labile organic matter to proceed at current Holocene sedimentation rates in the disturbed sediments, biogeochemical reactions resume in the reactive surface sediment layer, and, thus, the new steady-state geochemical system in the disturbed sediments in the CCZ is reached on a millennial timescale after the disturbance of the surface sediments.
Our study represents the first study on the impact of small-scale disturbance experiments on the sedimentary geochemical system in the prospective areas for polymetallic nodule mining in the CCZ. Our findings on the evaluation of the disturbance depths using solid-phase Mn contents as well as the quantification of the development of a new geochemical steady-state system in the sediments advance our knowledge about the potential long-term consequences of deep-sea mining activities. We propose that mining techniques potentially used for the potential commercial exploitation of nodules in the CCZ may remove less than 10 cm of the surface sediments in order to minimize the impact on the geochemical system in the sediments. The depth distribution of solid-phase Mn may be used for environmental monitoring purposes during future mining activities in the CCZ. However, based on our current knowledge and in combination with ongoing natural environmental changes (e.g., bottom-water warming, acidification, changes in the POC flux to the seafloor), it is difficult to assess whether the surface sediment removal may trigger a tipping point for deep-sea ecosystems. This study also provides valuable data for further investigations on the environmental impact of deep-sea mining, such as during the launched JPI Oceans follow-up project MiningImpact 2.
The data are available via the data management portal OSIS-Kiel and the WDC
database PANGAEA, including the solid-phase bulk sediment Mn and TOC
contents (
The supplement related to this article is available online at:
The study was conceived by all co-authors. JBV carried out the sampling and
analyses onboard during the RV
The authors declare that they have no conflict of interest.
This article is part of the special issue “Assessing environmental impacts of deep-sea mining – revisiting decade-old benthic disturbances in Pacific nodule areas”. It is not associated with a conference.
We thank captain Lutz Mallon, the crew and the scientific party of RV
This research was funded by the Bundesministerium für Bildung und Forschung (grant no. 03F0707A+G) as part of the JPI Oceans pilot action “Ecological aspects of deep-sea mining (MininigImpact). Further financial support was provided by the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research. The article processing charges for this open-access publication were covered by a Research Centre of the Helmholtz Association.
This paper was edited by Jack Middelburg and reviewed by two anonymous referees.