Parts of dive SO239_019_AUV2 run across the entire working area providing data from different terrains that can be linked to the ship-based bathymetric information. The correlation between photo analysis and this less resolving bathymetry indicates a trend of decreasing nodule coverage at elevations and steeper sloping areas (Fig. ). Video data acquired during previous cruises provide similar observations . The distribution pattern seen in the imagery also points towards small-scale Mn-nodule coverage variability which is possibly related to minor topographic changes in meter to sub-meter scale. As only one track covers the central region of the working area, clear correlations between Mn-nodule occurrence and large-scale ship-based bathymetry are difficult to assess. Finding clear correlations is further complicated by the uncertainty of the AUV navigation (up to 30 m), which prevents a precise georeferencing of the photos between sub-areas A1 and A2. More robust visual reference data could be provided by conducting a sparse mesh survey across the entire area or by a contiguous photo mosaic across different terrains.

The assessment of small-scale Mn-nodule coverage heterogeneity was based on the western A1 and eastern A2 sub-areas; here, overlapping photo mosaics and AUV-based bathymetric data in meter resolution exist (Fig. a). Sub-area A2 (700m×500m, 0.35 km2) is bound to the east by a 5–7 m high “ridge” with a relatively steep slope (slope A2E, 3–7∘). The western part of this area (A2W) shows only minor morphological variation and a total relief of approximately 2 m (Fig. ). Despite the rather small relief changes, variations in Mn-nodule coverage can be observed (Figs.  and ).

Figure a illustrates the detailed bathymetry of the studied area with red dots indicating lower Mn-nodule coverage (≤12.5 %) as indicted by image inspection. Those areas with a BPI50 >0, slopes ≤1.8∘ and plan curvature values > -0.02 radiansm-1 were found to show the best correlation with the lower Mn-nodule coverage in sub-area A2 (Fig. d, f, h). A NW–SE-oriented, elongated patch in the central part of A2 that corresponds to a flat-topped (slope ≤1.5∘), slightly convex-shaped elevated structure (<1 m above the surrounding terrain) shows a low Mn-nodule coverage. A higher Mn-nodule coverage instead occurs at steeper slopes (>1.5∘) and in morphological depressions (negative BPI values, negative plan and total curvature), indicating a sediment depositional environment. Two distinct depression structures (Pit 1 and Pit 2 structures in Fig. ), both approximately 60–80 m in diameter and 1–2 m deep, show a different pattern; here, the visible Mn-nodule coverage is significantly lower (0–8 %). The almost-spherical pit structures are bound by slopes of >2∘ and thus produce slightly increased vector ruggedness measurement values >1.0×10-4 (VRMs; Fig. a) and the lowest observed BPI-values (Figs. c and b). Similar structures are observed throughout the entire study area (blue shaded areas in Fig.  and circular features seen in the slope map of Fig. b). Based on additional ROV and benthic camera surveys, it is assumed that these pit structures exhibit very few to no Mn nodules .

No further correlation between Mn-nodule coverage and bathymetric derivatives was found, and no relation to absolute water depth could be observed (Fig. b). However, Fig. b shows a significantly lower variability of Mn-nodule coverage for water depths shallower than 4019 m (only approximately 4 % variability, compared to 6–7 % variability in deeper areas); these areas correspond to steeper slopes (approximately 2 to 3.5∘) associated with the eastern bounding elevation (slope A2E, Fig. a) of sub-area A2. Along this west-facing slope the Mn-nodule coverage clearly increases with increasing depth. Areas featuring low slope values show higher variability in Mn-nodule coverage (Fig. b).

A lower Mn-nodule coverage (<12.5 %) is predicted for the green areas marked in Fig. a when using the BPI50, slope and plan curvature classification of the A2 sub-area (Fig. b). Although the resulting area does not match completely with the areas of low coverage derived from the photo analyses (red dots/shades in Fig. ), it represents the best correlation that could be achieved. Based on this result, a Mn-nodule coverage of <12.5 % can be expected in 39 % of the study area (green shades in Fig. ) and is likely to be very low or zero in at least 1 % of the area (blue shaded parts).

In sub-area A1 (230×600 m, 0.138 km2), no correlation is observed between the photo-analyzed Mn-nodule coverage (red) and the seafloor classification of A2 (green; Fig. c). In addition, scatterplots (Fig. c–h) show different dependencies between Mn-nodule coverage to BPI, slope and plan curvature between A1 and A2. In both areas, though, the coverage attains more uniform values towards steeper slopes (Fig. e, f). A stronger correlation is shown in A2 and an inverse correlation in A1. In both areas, the highest variability but also the lowest values of Mn-nodule coverage occur in generally flat areas (curvature values around 0, low slope values; Fig. e, g).

Comparing the terrain statistics of areas A1 and A2 (Fig. ) reveals differences in their bathymetric settings, which might cause this discrepancy. Sub-area A2 mainly slopes towards west to southwest, as indicated by the aspect distribution and to a lesser degree in “opposite” northeast to east directions caused by the general pattern of N–S-striking graben and horst structures. In comparison, the main slope direction in A1 is towards a southerly direction. The slope distribution in A1 indicates a dominance of slopes up to 1∘. This is reflected by the large area of flat seafloor as determined by the AUV-BPI440 value distribution (Fig. ). In A2, slopes are steeper, the terrain is more variable and a larger number of depressions are observed compared to A1. The VRM shows similar values for both areas (Table A6). These differences in bathymetric derivative values point at a lower terrain variability in A1, confirmed by the more consistent depth values in A1 relative to A2. Considering the generally lower Mn-nodule coverage within A1 (Fig. a) it is concluded that lower Mn-nodule coverage correlates with lower terrain variability and lower slope values. This generalized observation is consistent with findings for A2. Although a direct one-to-one relationship valid in both sub-areas could not be derived, the general trend indicates higher Mn-nodule coverage with more variable terrain, along smooth slopes and in concave-shaped terrain (depressions).

In general, properties such as sedimentation rate , type and thickness of the sediment are believed to determine Mn-nodule growth ; for sediment deposition environments the interplay between bottom currents and bathymetry plays an important role . The depositional properties vary on a regional scale, considering large geomorphological terrain types, but are also impacted on a local scale of only a few kilometers and even less . Varying considerations of scale and regional differences in nodule exposure between different oceans across different studies have thus led to partly contradicting statements of the relationship between the Mn-nodule coverage/size and bathymetric settings.

Several investigations report small Mn nodules and low coverage in depressions and plains which are considered as sediment accumulation sites, in contrast to seamounts, slopes and crests . Other studies discussed comparatively larger diagenetic Mn nodules in plains which are also considered as sediment accumulation areas. More abundant but smaller hydrogenetic Mn nodules have been observed in more rugged terrain . Such terrains are interpreted to increase current velocities and turbulence caused by channel effects reducing sediment accumulation.

present a correlation between Mn-nodule size and sedimentation rate, where large nodules correlate with a smaller amount of clay fraction in the sediments that they interpreted to be caused by stronger bottom currents/lower sedimentation rate. A similar observation is presented by , who state that diagenetic/large Mn-nodule formation is linked to a periodical redistribution of the surface sediment layer.

With respect to the large scale of the ship-based bathymetry in Fig. , the working area of this study is located in a sediment-accumulating flat terrain with smooth bathymetry, characterized by the occurrence of medium to large (>4 cm) Mn nodules . However, a more detailed view allows the identification of terrain variability on a scale of several tens to hundreds of meters that enables a more detailed assessment of the associated Mn-nodule coverage variability (Fig. ). Larger nodules/higher coverage values occur in depressions and at sloping seafloor when compared to broad-scale bathymetry. Larger nodule sizes could be the result of stronger bottom currents preventing/reducing the deposition of sediment on nodules and/or favoring nodule growth. For another area in the German claim, box core (BC) samples taken by the Federal Institute of Geosciences and Natural Resources (BGR Hannover, Germany), revealed larger (diagenetic) nodules in a very broad-scale flat terrain. This area has been classified by as an area of sediment accumulation and is compared to a rougher, supposedly sediment “winnowing” area, with many smaller nodules formed hydrogenetically. The interpretation of sediment accumulating and winnowing areas is based on broad-scale ship-based bathymetry of much coarser resolution compared to this study.

According to the study by , the data presented here indicate lower sedimentation rates associated with stronger bottom currents in the depressions supporting the growth of larger Mn nodules. Increased bottom currents within the depressions could possibly be induced by convergent channeling or turbulence of bottom currents, which contradicts the assumption of lower current strength and therefore higher sedimentation within depressions.

Variability in Mn-nodule coverage within several tens of meters or less can be correlated with AUV-based bathymetry. In sub-area A2, patches of low Mn-nodule coverage correlate with low bathymetric elevations even when the relief differs by less than 1 m. The strongest correlation between low Mn-nodule coverage was determined with slightly convex-shaped elevated structures (surfaces <1∘ slope, positive plan curvature and positive BPI values). These parameters most likely define local-scale sedimentation environment affecting the local balance between sediment accumulation and erosion. The presented data show that favorable nodule growth/occurrence conditions coincide with gentle sloping sites and low relief depressions, where sediment is assumed to accumulate slowly.

Within sub-area A2, a smaller variability of Mn-nodule coverage can be observed in correlation with slope A2E towards the east. This is in agreement with observations by , who also observed more uniform nodule coverage in sloping areas. The authors point out that this could be a result of a larger exposure of the Mn nodules rather than absolute difference, since they discovered discrepancies between direct sampling and results of photo analyses.

Rather special for the presented data set are the pronounced pit structures, observed throughout the AUV-mapped area with very little to no Mn nodules observed at the sediment surface. This is in contradiction to the wider depressions, where a higher Mn-nodule coverage was observed. The existence of such pronounced depressions most likely leads to a reduction of bottom current velocities resulting in a higher sediment deposition of suspended sediment and potentially even sediment slumping from the sides. This could result in sedimentation rates too high for Mn-nodule formation or a simple cover/burial of previously formed and still-existing Mn nodules below the sediment surface. The formation process of the pits is unclear, but could be karst structures , which are younger than the Mn-nodule formation which would point towards Mn-nodule burial within the pits. At the same time, a higher sedimentation rate in a low current regime would also mean a higher accumulation of clay size particles, which are proposed to not be favorable for nodule growth . Another possibility could be that these pit structures are pockmarks, formed by pore water release with a significant change in local pore water geochemical properties and eventually warmer temperature that prevented Mn-nodule formation in the past. Unfortunately, the pit structures could not be sampled in more detail and it is unknown whether Mn nodules exist at all or if different geochemical conditions are present within the pits. Similar, but larger, structures exist in the disturbance and recolonization experiment (DISCOL) area , showing very similar geochemical conditions as other Mn-covered areas in both highly detailed sediment surface analyses as well as deeper sediment cores.

When comparing the relationships between the bathymetric derivatives and the Mn-nodule coverage, it becomes evident that correlations visible in A2 cannot be seen in A1 (Fig. a, c), where areas of lower nodule coverage could not be matched with distinct terrain types. This result points towards additional parameters that influence Mn-nodule occurrence. Geochemical processes could be involved that drive the Mn-nodule formation; these in turn depend on the sediment properties (composition, sedimentation rate, porosity, etc.). Bottom currents could additionally influence the sedimentation rate and affect the geochemical processes in the benthic boundary layer and Mn-nodule surface. Local differences in the hydrodynamic regime near the bottom seem likely, as the bathymetric derivatives vary between the two sub-areas. Sub-area A2 is bound towards east and north by elevated terrain (7 to 10 m higher) which could have a focusing effect on bottom currents eventually causing a more erosional environment. In contrast, sub-area A1 is unbound by elevated terrain within 2 km distance. This might cause a stronger influence of seasonally changing bottom currents, preventing a clearer correlation of Mn-nodule coverage with the seafloor morphology.

The observations made on a broad scale (several hundreds of meters; grid cell size of 55 m) show that high Mn-nodule coverage correlates with depressions (Fig. ), which is consistent with observations on a smaller scale (scale of tens of meters; grid cell size of 5.5 m) for sub-area A2 (Fig. ). Outside of A2 decreasing Mn-nodule coverage correlates with steeper sloping areas, which is contradicting to observations on a small scale, where the lowest Mn-nodule coverage correlates with extremely low slope angles of less than 1.8∘. This contradicting finding highlights that simple and generalized correlations between Mn-nodule occurrence and bathymetric but also geochemical properties in the sediment might not be possible on the regional scale (10 to 1000 km) but on the local scale (100 m–10 km). This is because the formation parameters also change on such local scales which are not possible to accurately predict using ship-based multibeam data, “sparse” box coring (distances of few kilometers) and limited information about current regimes.

The extent of the visible sediment blanketing, that varies over several tens of meters, can be related to a focusing of the sediment plume settling or the prevention of it through small-scaled morphological changes in form of barriers (steeper slopes facing against the current) or the opening of plume transportation pathways (sloping terrain with the current). Varying terrain in general will modify the current regime near the bottom and thus the settling properties of the sediment plume; it might also enhance the interactions between the particles due to increased turbulence that might stimulate increased flocculation and thus scavenging of very small particles that otherwise would be much further distributed. The shorter transport of sediment in north- and southward directions from the EBS track along slope A2E implies that the transportation of the suspension load follows the slope downhill. In subsection A2W, where the terrain is very smooth (the relief changes by 1 to 2 m), a dependency of the sediment blanketing extent to structures of the undulating seafloor could still be observed. At the western end of subsection A2W, the east-facing slopes act as barrier for an undisturbed migration of the sediment plume with the bottom current towards the south. The spreading of the sediment blanketing is wider in the east of subsection A2W where the seafloor is almost horizontal, before slightly dipping towards the east and into the Pit 1 structure. The slopes considered show angles of less than 2∘ and the morphological variability is sometimes less than 1 m. More distinct features, like Pit 1 (Fig. ), cause a more variable sediment plume dispersal. The sediment blanketing within this 2 m deep feature does not exceed the southward edge of the depression. The resuspended sediment seems trapped within this feature with possible additional suspension load coming from the neighboring eastward slope.

In a first approach, we estimated the plume height generated by the EBS by considering the extent of the observed sediment blanketing and measured bottom current velocities at the time of the EBS deployment (31 mms-1; measured by ADCP). Former models from the CCZ reported settling velocities of particles in a sediment plume in the range of 0.1 to 1 mms-1 derived from visual and experimental data . Preliminary results of particle size analysis from a comparable site within PA1 indicate a median grain size of 29 µm (Benjamin Gillard, Jacobs University Bremen, Germany, personal communication, 2016). Following Stokes' law and disregarding aggregation of the particles, the determined median particle size for the area would translate to sinking velocities of approximately 1 mms-1. Assuming an average dispersal width of 30–50 m downstream, as indicated for the A2W subsection (Fig. ), this would require a plume height of approximately 0.96 to 1.6 m. Aggregation processes leading to larger particle sizes are likely to occur, which, due to increased friction, would sink slower than similar-sized Stokes particles, but that would scavenge a substantial amount of very small particles (Laurenz Thomsen, Jacobs University Bremen, Germany, personal communication, 2016). As part of studies in the south Pacific DISCOL area, lander-based ADCP backscatter measurements detected a passing-by sediment plume induced by a similar EBS experiment as discussed here. These data indicate a plume height between 1.5 and 2 m .

It can be assumed that, due to the higher turbulence caused by the deployment of an industrial collector system and the continuous release of suspended material into the water column during mining, the dynamic behavior of the sediment plume could be altered and adjusted in such a way that the suspended sediment is resettling in the fastest possible way, keeping the dispersion to a minimum. Determining the dynamic behavior of the plume under different collector–dispersion scenarios by monitoring in situ and under real mining conditions is thus essential to improve our understanding and model capacity with regards to the near- and far-field plume distribution and finally to evaluate ecological short- and long-term impacts.

These ecological impacts can be significantly confined to a small area by reducing the height of the sediment plume, increasing the settling velocity and aggregation of particles (scavenging the very fine sediment fraction). Vertical discharge of sediment after its separation from the Mn nodules should be avoided; instead a horizontal discharge close to the bottom (<10 m from the bottom; below stable stratification above the well-mixed bottom boundary layer) with a velocity as slow as possible (speed of the collector) should be aimed for. One first implementation of this concept was the setup of the Deep-Sea Sediment Resuspension System (DSSRS) disturber deployed in a few large-scale benthic impact experiments (BIE-II, JET, IOM-BIE, INDEX).

As indicated by our results, a low-height sediment plume will be trapped in small depressions. Thus, detailed knowledge of the local morphology on small scales is a prerequisite to correctly determine the area and thickness of resettling sediment. This is also relevant in planning adjacent mining tracks from a miner's point of view, since strong sediment blanketing might bury adjacent nodules to be mined. According to our results, this impact will be highest in sediment accumulation sites, but even on flat areas with slopes of less than 3∘ the distribution of the sediment plume and the resulting sediment blanketing distance will vary on a range of several tens of meters. In areas with steeper slopes (e.g., 10∘), the sediment blanketing distance can be even wider.

In our very small-scale experiment, the EBS created a local impact with clearly visible sediment blanketing within 100 m downstream off the track. This localized impact is also the result of only partial resuspension of the surface sediments that was directly caused by the EBS (1.2 m in width). Observations of EBS tracks during another experiment revealed that a larger part of the sediment is compressed by the EBS and pushed aside with only a smaller (unknown) fraction being suspended . It can be speculated that resedimentation of particles outside the visible blanketing area is minor, will happen over longer time and thus might not have a significant effect on the benthic organisms and the ecosystem (short- and long-term cumulative impacts on specific fauna still need to be determined).

The actual scenario of disturbance will be different during real-case mining during which the top 10–20 cm of the sediment are removed, then filtered for nodules and then discharged at the seafloor. One single track will be about 17 m wide as, e.g., planned in a German concept , whereas the track width of the EBS was only 1.2 m. As not only one track will be mined, but the collector system will operate constantly in a lawn-mowing pattern of long tracks scraping off the seafloor surface, the entire mined area will see a strong impact . Considering local topography, bottom currents, optimizing particle settling for fast and effective flocculation by the collector, and the cleaning of the sediment plume from the water column by settling phytodetritus from plankton blooms (increased flocculation), the size of the impacted area and the impact itself caused by the sediment blanketing outside the mined area might be rather small (<10 km) and controllable. For a final validation, an experimental setup closer to the expected mining conditions is needed ; the presented study shows that we have the understanding, tools and the methodologies at hand to perform monitoring studies needed for such a realistic deep-sea mining experiment.