Scars in the Abyss: Reconstructing sequence, location and temporal change of the 78 plough tracks of the 1989 DISCOL deep sea disturbance experiment in the Peru Basin

High-resolution optical and hydroacoustic seafloor data acquired in 2015 enabled the reconstruction and exact localization of disturbance tracks of a past deep sea re-colonization experiment (DISCOL) that was conducted in 1989 in the Peru Basin during a German environmental impact study associated with manganese nodule mining. Based on this information, the disturbance level of the experiment regarding the direct plough-impact and 45 distribution and re-deposition of sediment from the evolving sediment plume was assessed qualitatively. The compilation of all available optical and acoustic data sets available from the DISCOL Experimental Area (DEA) and the derived accurate positions of the different plough marks facilitate the analysis of the sedimentary evolution over the last 26 years for a sub-set of the 78 disturbance tracks. The results highlight the remarkable difference between natural sedimentation in the deep-sea and sedimentation of a resettled sediment plume; most 50 of the blanketing of the plough tracks happened through the resettling of plume sediment from later created plough tracks. Generally sediment plumes are seen as one of the important impacts associated with potential Mn-nodule mining. For


Ecological risks associated with Mn-nodule mining from the deep seafloor
For several years, mining of manganese (Mn) nodules from the deep seafloor is again considered a worthwhile option to meet future resource demands. Several nations secure exploration contracts in areas beyond any national jurisdiction as they seek economic benefits and/or aim for technological leadership in terms of deep sea 70 mining. Current plans for a future mining scenario involve collectors that will move on the seafloor gathering Mn-nodules from the top 10 to 30 cm of the sediment, most likely using a hydraulic collection mechanism (Kuhn et al., 2011. This principle implies considerable consequences for the benthic environment in the mined area. Besides the removal of the Mn nodules as an important hard-substrate habitat on the abyssal plains (Purser et al., 2016;Vanreusel et al., 2016, Thiel et al., 1993, the mining activities will 75 completely re-work the top sediment layers and re-suspend large amounts of sediment into the water column. Depending on the plume properties such as particle size, flocculation behavior, sediment mass per liter, and the prevailing current conditions these sediment particles might be transported outside the mined area. The deposition of this material will cause a secondary impact on the environment by clogging of filter feeders and burial of the sessile fauna, which are both adapted to the low sedimentation rates in the deep sea (Thiel and 80 Schriever, 1989). Re-sedimentation of this material can also lead to differences in local geochemical gradients and consequently might influence the recolonization processes of the primary and secondary disturbed areas. To evaluate these effects on the environment, several benthic impact experiments (BIE) and one Recolonization Experiment, the German Research Project "Disturbance and Recolonization Experiment-DISCOL" (http://www.discol.de), have been conducted in the past within different large Mn-nodule areas, including the 85 Peru Basin (Thiel and Schriever, 1989), the Central Equatorial Pacific (e.g. Burns, 1980;Fukushima, 1995) or the Indian Ocean Basin (Desa, 1997). Information about the sediment plume dispersal during the different largescale disturbances are compiled in section 1.2. A review of the biological responses to such BIEs was recently presented by Jones et al. (2017) and studies by Simon-Lledó et al. (2019) in the DISCOL Experimental Area (DEA) show that colonization pattern-differences still exist between the disturbed and undisturbed areas even 90 after 26 years

Summary of plume dispersal results of past benthic impact experiments (BIE's)
In the late 1970's, the first so called 'mining test' operations were conducted in the central North Pacific as part of the DOMES project (Ozturgut et al. , 1980) that used a Suction dredge towed on skis to create a disturbance for illustrating potential mining impacts. Here, the experimental area was surveyed before, during 95 and after the experiment, with each disturbance lasting for several hours (see Table A1 for details on location, duration, monitoring techniques and impacted area). For the first three tests in spring 1978, operated by Ocean Mining Inc. (OMI), detailed data about the induced sediment plume were derived from different sampling methods including sediment coring and sediment traps (see Burns, 1980, and details in Table A1) and results indicate a plume dispersal of up to 16 km downstream of the created disturbance (Table A1). Model results 100 based on the OMI experiment indicate a sediment blanketing thickness of ≤1 mm beyond 400 m distance to single disturbance tracks. An extrapolation of these results for a potential mining scenario was performed and predicted a distribution of re-suspended sediment particles of up to 160 km distance (Lavelle et al., 1981).
Another 'mining test' phase in November 1978 focused on the distribution of a surface discharge plume (Ozturgut et al., 1980). During an 18 hour lasting operation by Deepsea Ventures Inc. / Ocean Mining Associates (OMA). A a second seafloor mining test was conducted in November 1978 by the Ocean Mineral Company (OMCO) using a Remote Controlled self-propelled Miner (RCM) (Chung, 2009). This vehicle removed approximately 4 cm of the upper sediment layer (Khripounoff et al., 2006) creating a track of 1.5 m width (Miljutin et al., 2011). The aim of this experiment was mainly to test the mining technology and not to monitor the benthic impact of the plume. Hence, detailed information regarding the sediment plume dispersal right after 110 the impact is missing. In 2004, the disturbed area was revisited and investigated for its ecological recovery (Mahatma, 2009;Miljutin et al., 2011) indicating only a near-track influence of re-deposited sediment.
Chronologically the next and largest ever created disturbance was conducted in the DISCOL Experimental Area in the Peru Basin. For creating the disturbance, a plough-harrow (8 m width) was towed 78 times crisscrossing through a circular area of 2 nautical miles in diameter (Thiel and Schriever, 1989). Due to technical problems the 115 deployed nephelometers at that time did not detect the sediment plume and the amount of suspended material remains largely uncertain. Nevertheless, the presence of a plume in the water column about 6 hours after the last plough deployment was confirmed by visual observations (Thiel and Schriever, 1989). Numerical modeling predicted a dispersal of the suspended sediment for several kilometers with coverages of resettled material of >100 gm -2 up to a distance of 2 km (Jankowski et al., 1996; Table A1). The effects of the disturbance were 120 investigated just after the experiment (RV SONNE cruise SO61, Thiel and Schriever, 1989) as well as 0.5 (cruise SO64, Schriever, 1990), 3 (cruise SO77, Schriever and Thiel, 1992), 7 (cruise SO106; Schriever et al., 1996) and finally 26 years later (cruise SO 242; Boetius, 2015;Greinert, 2015) to document the environmental impact, the recolonization and sediment geochemical equilibration of the disturbed sites in comparison to a number of undisturbed reference sites in the vicinity.

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Again north of the equator, the first large-scale benthic disturbance experiment in the eastern Clarion-Clipperton Fracture Zone (CCFZ) conducted by the United States was the Benthic Impact Experiment II (BIE-II) in 1993, using the "Deep Sea Sediment Resuspension System" (DSSRS) (Brockett and Richards, 1994;Tsurusaki, 1997) as disturbance tool (Trueblood and Ozturgut, 1997). The initiated sediment plume was monitored with camera systems, sediment traps and transmissiometers, which were moored in different distances from the tow zone in 130 order to estimate the distribution areas of re-settled sediment and the plume dispersal in the water column. The studies revealed an area of strong sediment blanketing within the first 50 m downstream of the disturbance and a decreasing blanketing thicknesses with increasing distance. Moorings located 400 m away still detected suspended material passing by and also deployed sediment trap samples indicated a maximum "blanketing" thickness of 1 mm. In contrast to these data, camera observations suggested a sediment blanketing thickness of 135 1-2 cm close to the disturbance zone (Jones, 2000) already indicating that the sediment traps might have missed the additional sediment transport of initiated gravity flows just above the seafloor.
One year after the American experiment, the Metal Mining Agency of Japan (MMAJ) carried out another disturbance study within the CCFZ, the "Japan Deep sea Impact Experiment" (JET) in 1994 (Fukushima, 1995).
The disturbance was again created with the DSSRS (Tsurusaki, 1997). The distribution of the initiated sediment 140 plume was analyzed using two different approaches. One approach measured the thickness of the blanketing sediment layer using sediment traps and spatially interpolated the results using Kriging. A dispersal of 2.5km in length and approximately 1km in width was calculated and a maximum blanketing thickness of 2.6 mm was determined (Barnett and Suzuki, 1997). The second approach used visual data from deep-towed camera surveys to estimate the extent of the sediment blanketing that covered the Mn-nodules. Respective results show that the 145 'heavy' re-sedimentation area, defined by a thickness > 0.26 mm did not extend for more than 100 m away from the disturbance track. Thinner blanketing < 0.26mm was observed over an area of ~3km length and ~2.5km width around the disturbance (Yamazaki and Kajitani 1999), covering a much wider area compared to the Kriging approach.
In 1995, the InterOceanMetal (IOM) Joint Organization conducted a benthic disturbance experiment (IOM-BIE) 150 over an area of 2000 x 1500m also in the eastern CCFZ, once more using the DSSRS (Kotlinski and Stoyanova, 1999;Radziejewska, 2002). Studies focused on the physical and chemical properties of the re-suspended and resettled sediments rather than on the spatial distribution of the material; this leads to only limited information on the amount of re-suspended material. Radziejewska (2002) estimated the volume of re-suspended material to be approximately 1800 m 3 over the entire duration of the experiment, but the actual volume is not known.

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In 1997, the "Indian Deep sea Environment Experiment" (INDEX) was carried out in the Central Indian Ocean Basin. For the fourth time, the DSSRS was used to create the disturbance during 9 days of operation (Desa, 1997;Sharma and Nath, 1997). Results from sediment traps distributed up to 800 m away from the track show an increase in average particle fluxes from 48 to 150 mg m -2 d -1 during the disturbance phase. The flux decreased to 95 mg m -2 d -1 within the first six days after the disturbance stopped (Sharma, 2001). Based on visual 160 observations, most of the sediment particles re-settled already within 150 m from the edge of the disturbance area , with the major part of material settling within approximately 100 m distance (Sharma, 2000).
The last large-scale BIE was conducted in 1997 by MMAJ within the area of the Marcus-Wake Seamounts in the North Pacific Ocean (Yamada and Yamazaki, 1998). The induced sediment plume was visually monitored 165  and data revealed a sediment blanketing thickness on top of Mn-nodules of up to 0.2 mm (Yamazaki et al., 2001). Due to the different geological setting (seamount in 2200 m water depth) and different sediment properties (calcareous sediments, coarser sediment particles, stronger currents), these results are not directly comparable to the results from most of the other BIEs mentioned above.
Reviewing the different large-scale BIEs and pilot mining tests conducted between the late 70's and late 90's it 170 becomes obvious that the different experimental setups and the missing uniform definition of 'a' plume (grain size distribution, flocculation behavior, total mass per liter, settling velocity etc.) make it impossible to use the presented information for a meaningful prediction of the behavior of a sediment plume created during a real deep sea mining operation (Peukert et al., 2018). As basis for sample interpretation Tthus reconstructing the initial disturbance of 1989 in the DISCOL area, which is considered as the most extensively sampled and monitored 175 BIE site, might help to gain new and more conclusive insights in terms of the distribution of re-suspended and re-deposited sediment during and shortly after conducting the disturbance and consequently be used as a basis for sample interpretation from this area.
This study presents new data from the DEA, which were acquired in 2015 during RV SONNE cruise SO242-1 with state-of-the-art AUV multibeam and side scan sonar systems, cameras and under water navigation technology (Greinert, 2015).

DISCOL revisited in 2015 and objectives of this study
Since 1989, major technological advancements improved deep sea investigations with regard to data acquisition technologies and positioning accuracy. In 1989 GPS for example was not as sophisticated and high-resolution acoustic seafloor mapping with multibeam echosounder systems (MBES) was not as developed as it is today 185 (e.g. 59 beams compared to 432 beams; single swath compared to dual swath; Lurton, 2017). AUV-based technologies did not even exist.
To acquire most accurate data of the old plough tracks, the entire DISCOL area was re-mapped using ship-and AUV-based hydroacoustic MBESs with different resolution (Boetius, 2015;Greinert, 2015). This provided new information for reconstructing the extent and impact of the initial disturbance experiment, the different 190 geological settings within and next to the DEA and related varying and habitats. The results presented in this study mainly focus on the data collected by GEOMAR's AUV ABYSS (Linke and Lakschewitz, 2016, http://dx.doi.org/10.17815/jlsrf-2-149). The AUV was deployed in three different modes running either MBES, side scan sonar (SSS) or a photo camera system enabling autonomous mapping with a resolution of 2 m for bathymetric data, 0.5 m for SSS data and a few mm per pixel for photo surveys. All systems show clear evidence 195 of the disturbance tracks created by the plough-harrow 26 years before.
This study presents the best georeferenced data set of the study area through a combined processing of the available ship-and AUV-obtained acoustic and optical data. In addition to this mapping exercise the succession of the disturbance tracks as well as their correct location is are reconstructed, as this could not accurately be documented in 1989. Although the 78 plough tracks were created over a period of only 4 weeks (Thiel and 200 Schriever, 1989) a more detailed understanding of their sequence is relevant regarding faunal differences from within or close to plough tracks in strong or weaker disturbed parts of the DEA. Furthermore for the understanding of varying down-core geochemical gradients the spatial thickness-change of the resettled sediment, the "blanketing", needs to be understood. This thickness distinctly differs between the plough tracks depending on if they were created in an earlier or later stage of the disturbance, which highlights the difference 205 between high plume-sedimentation rates and natural deep-sea low sedimentation rates. Next to this an unbiased and correct comparison between areas that have not been impacted by any re-settled sediment with areas that have been impacted to various amounts, should be performed. Interpreting biological or geochemical results correctly requires a very precise knowledge of the exact and absolute sample or footage location on the seafloor and their spatial relation to the tracks which are only a few meters wide and apart from each other. Thus a correct 210 georeferencing of all different data layers was a significant task of this study and although highly developed positioning systems were used in 2015, uncertainties and deviations of tens to a few hundreds of meters occurred. This task became even more important for georeferencing legacy data from 1989 for conclusively defining changes between 1989 and 2015 and spatial sediment re-settling differences established already during the plough experiment.

Digitizing and archiving of DISCOL legacy data
Until 2015, the location and path of the disturbance tracks as well as the position of video and photo material of the past OFOS (Ocean Floor Observation System) surveys only existed as a vast collection of analogue (i.e. cruise reports, printed large navigational charts, video cassettes and slide films) and some digital records (i.e.

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OFOS annotation files, sample analysis as text or EXCEL files, e.g. Bluhm, 1994, Bluhm and Thiel 1996, Thiel and Schriever 1989, Schriever 1990, Schriever and Thiel 1992, Schriever et al., 1996. In preparation for the 2015 cruise, these records were digitized and compiled in a database, also including all other available sampling stations (i.e. BC, MUC, moorings, baited traps see e.g. Drazen et al., 2019) that were done during the first four expeditions to the DEA. This database was used for station planning prior to the SO242 cruises and allows 225 comparing past and present disturbance levels and seafloor and ecosystem conditions at their best possible correct location.

Data description / working area
Generally the area is located about 800 km West of the Peruvian coast and about 700 km South of the Galapagos Islands ( Figure 2A). N-S striking graben and horst structures can be seen throughout the entire area, corresponding to the highest slope angles of up to 36° (Fig. B3B); they are related to the tectonic setting of the 270 study site being located on the Nazca Plate which originates from the East Pacific Rise (Devey et al., in reviewMelchior, 2017). Within the working area the water depth varies between 4300 m and 3850 m ( Figure   2B), with the minimum water depth corresponding to the summit of a rough sloping (>30° slope angle, Fig.   B3B) seamount (rising ~200 m) north of the DEA (Devey et al., in reviewMelchior, 2017). West of the summit the terrain drops along one of the NS-striking graben structures with two lower sea mounts of about 100 m 275 height. About 18 km to the SE of the DEA another larger seamount rises up to 3980 m water depth showing pit structures of tens of meters in depth and width as has been recently described for the wider region to be generally associated with hill crests (Devey et al., in reviewMelchior, 2017). In the very west of the working area a NSstriking narrow ridge highlights again a tectonic nature of the area with another element of the graben and horst fault system in the area. Besides of these dominating bathymetric features, the rest of the terrain shows smooth 280 undulating elevations and basins of several tens of meters depth and few kilometers width, with slope angles of <10°.
The finer structure of these flatter parts is much better resolved in AUV-acquired MBES data (Abyss192-194).
The gently sloping terrain exhibits up to 15 m high hill/ridge and basin structures in the DEA and the western part of the mapped area ( Figure 2C, Figure B4A). In 99 % of the DEA the maximum slope of the terrain is only 285 3° ( Figure B4B) with generally NNW-SSE striking morphological features. Parallel to these, < 1 m high and 20-40 m wide ripple structures extending from the center of the DEA towards the North. The appearance of these features remind of ripple structures oriented parallel to the predominant bottom current direction in this area (Thiel andSchriever, 1989, Greinert, 2015) and are further described below within this section. In the NE the terrain rises distinctly forming up to 50 m high summits ( Figure   Using the AUV side scan sonar (SSS) an area of 4 x 3.5 km with the DEA in the center was mapped ( Figure 3).
The acoustic signals captured a significant amount of the plough tracks, which appear darker in the SSS map, 300 representing a lower backscatter. Three dark distinct patches between 140 and 200 m in size are apparent within the side scan data, indicating softer substrate within these structures that bathymetrically represent sediment filled local basins of ~5 m depth with a rather horizontal seafloor. The MBES backscatter and side scan sonar data of the NNW-SSE-striking channel structures, indicating deposition of softer sediment within the depressions. The channel structures are oriented parallel to the prevailing strong bottom current direction within 305 the area towards the NNW (Thiel and Schriever, 1989;Schriever and Thiel, 1992) and the undulating shape of the structures indicate a generation by a flow regime and not by tectonic activities, which would appear straighter. We assume that bottom currents are channelized through the local trough around the rising terrain towards the NE and may cause turbulent flows which eventually cause furrowing. This process has been described also in the deep ocean with a dominant strong bottom current flow between 5-20 cm -s -1 (Flood, 310 1983), which is given in the DEA and hence this process could have formed these structures.

Geo-referencing of AUV data sets
AUV operation in great water depth suffers from inaccurate positioning of acquired data sets. Underwater positioning is typically determined using hydro acoustic techniques as ultra-short base-line (USBL, measurement between the ship and the AUV) or long-base-line systems (LBL, triangulation of the AUV using seafloor 315 deployed transponders). AUV "Abyss" navigates autonomously using a combination of different navigational methods (Linke and Lakschewitz, 2016). During our studies LBL navigation was only used to set an accurate starting position of the AUV at the beginning of each survey after arriving at the seafloor. No additional LBL fixes were considered as this often results in abrupt track corrections that cause unwanted artifacts, particularly in SSS data. Instead, navigation after the initial LBL fix relied on a Doppler Velocity Log (DVL) data, inertial 320 navigation sensors and dead reckoning data fusion as supplied by the AUV system (Linke and Lakschewitz, 2016). Typically, such kind of navigation is prone to slow drifts, which over the course of an entire mission (up to 20 hours operation time) can add up to several tens or hundreds of meters offsets. These navigational shifts need to be derived and corrected during processing when comparing or combining several different data sets as MBES, SSS, and imagery of the AUV, imagery of OFOS and ship-based bathymetry 325 To achieve the best possible alignment and absolute geo-referencing, the ship-based EM122 bathymetric data with a spatial resolution of 38 m were taken as absolute reference layer (Fig. C1). The AUV bathymetric data with a spatial resolution of 2 m was resampled to match the 38 m resolution enabling a direct grid comparison (e.g. grid subtraction, Fig. E1) and correction of vertical and lateral offsets of the AUV bathymetric grid relative to the ships data layer. Using 5 m contour lines to visualize morphological features in the area, the 38 m AUV 330 bathymetry was shifted/stretched manually onto the EM122 data ( Figure 4). Subsequently, the high-resolution AUV bathymetric digital terrain model was shifted in the same way using the ArcGIS 10.2 Georeferencing Toolbox for geographic corrections (contour lines were derived with the Spatial Analyst Toolbox and grids were subtracted to see z-offsets using the Raster Calculator function). Fig. 4 shows the high-resolution AUV bathymetry with the contour lines derived from the ship-based bathymetric grid to visualize the accordance of 335 both data sets after the alignment.

Position and age sequence of disturbance tracks
Disturbance tracks visible in SSS data ( Figure 5D) were manually digitized using functionalities of ArcGIS.
Each track was given a unique identifier and was assigned to one of four classes reflecting the general 355 orientation of the respective track: H for E-W orientated tracks, D for NW-SE and NE-SW orientated tracks, V for N-S orientated tracks and P for non-continuous tracks and track segments (H=horizontal, D=diagonal, V=vertical, P=parts of tracks; Figure 6; Table 1). The track IDs were arbitrary given during the digitizing and were not renamed after the sequencing, thus the numbers do not reflect the age sequencing. During the digitizing groups (V, H, D)", due to subsequent extension after further investigations. Because of this there is no clear definition when tracks are labeled P; however, typically "P" tracks are typically shorter than 1200 m. Generally during the ploughing in 1989 several tracks were undertaken during one deployment of the plough (station name PFEG-1 to PFEG-11; PFEG1 was a gear handling test a few nautical miles south of the DEA).
After the first two groups PFEG2 and PFEG3 OFOS dive OFOS009 and OFOS010 were conducted during SO61 and the photo and video material collected during these two OFOS dives could be examined for track 370 occurrences. The track orientation was determined from each seafloor image and matched to the track orientation on the SSS map considering the course over ground (COG) and heading of the OFOS (Figure 7) to distinguish the correct plough track. This way and considering the log files from cruise SO61, which gave an idea about rough course and location of the ploughs, the tracks corresponding to the first two plough groups (15 tracks) could be identified. Considering photo and video material from ROV (http://dx.doi.org/10.17815/jlsrf-3-160),

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AUV and OFOS surveys and the SSS data, intersections were visually examined to establish a relative age succession between the investigated tracks ( Figure 8). For some intersections the sequence could not directly be established and was inferred considering the relative track age information of other intersecting tracks ( Figure 8). Unfortunately, this workflow could not be applied for the later groups of tracks (PFEG4 to PFEG11) since they were not directly followed by an OFOS survey.

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Thus the track density until the next OFOS observation got too high and considering the navigation uncertainties, an unambiguous assignment of the tracks is not possible. The reconstruction of the age succession of all tracks was finally done using a 84x84 matrix (including 60 identified tracks and 24 track segments, Table   F1) where all observed crossings were included. Logical process of elimination and cross-referencing of individual tracks relative to all other tracks in combination with their position and the reconstructed ships 390 navigation during the time of the experiment (Fig. 1A) was performed. Based on this, the tracks were assigned to their respective PFEG and to track ID's (Table 1).

Figure 8: Establishing the relative age sequence based on intersections betw een tw o tracks from the SSS map (A) and seafloor photographs (B). Absolute age infor mation can be derived from
cross-referencing relative age infor mation of more than tw o individual tracks (B). Here the age sequence of all show n tracks is V02 < D12 < D14 < D10 < H12.

Reconstructing the impact of the re-settled plume
The initial impact of the plough tracks is given through the mixing (ploughing) of the top 20 to 30 cm of the sediment and the related suspension of sediment into the bottom water (Foell et al., 1990). Nodules were not 400 removed from the seafloor but ploughed under (Thiel and Schriever, 1989). The re-sedimentation of the initiated sediment plume is considered the secondary impact. For reconstructing the initial impact and the proximal (in images visible) sediment blanketing, the course of the plough tracks were used in combination with bottom current information recorded during the time of the experiment to establish a disturbance intensity map (including initial and secondary impact). Considering also the plume deposition information from other BIEs 405 (Table A1) and the recent study by Peukert et al. (2018), the qualitative sediment blanketing thickness within the DEA was determined based on the following assumptions and set parameters. Each track was assumed to have a width of 8 m, not considering the possible handling problems with the plough-harrow (e.g. being towed only on the side, short loss of bottom contact, Thiel and Schriever, 1989). The intensity of the disturbance was assumed to be the highest within and close to the tracks and the sediment blanketing thickness to decrease with increasing 410 distance off the track. Studies from other BIEs showed visual sediment blanketing distances between 70 m and 150 m in current direction away from the track. It is assumed that the majority of the re-suspended sediment (about 90 %) resettled over this distance (Lavelle et al., 1981;Peukert et al., 2018).
The main factor controlling the re-deposition are current speed/direction and particle settling velocity with the latter being describable as a function of the particle size according to Stoke's Law and the method described by

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McCave (McCave, 1984;Jankowski et al., 1996). The sediments within the DEA are composed of layered clayey silts or silty clays, with a sand fraction of ca. 5 % consisting of foraminiferous residues and shell fragments . According to Lavelle et al. (1981), Schriever et al. (1996) and Becker et al. (2001) the stirred up sediment mixture induced flocculation and aggregation of particles causing a very rapid resedimentation (≥ 1 cm s -1 ) of the plume within the first 20 m away from the track. Latest research on mining-The sediment blanketing decreases as a function of reduced particle settling velocities as finer particles dominate the plume composition and stay longer in suspension (Lavelle et al., 1981).
Bottom current direction and velocity determine the direction of the re-sedimentation area and sediment spreading (Lavelle et al., 1981;Jankowski et al., 1996;Greinert, 2015). Bottom currents in the Central Pacific 425 are reported to be distinctly different even at locations only a few kilometers apart (Robinson and Kupferman, 1985). Several measurements in the DISCOL area revealed a predominantly northern to northwestern direction with maximum current speeds of 17 cm s -1 (Thiel and Schriever, 1989;Schriever and Thiel, 1992) indicating a transport of the re-suspended particles primarily in this direction. The undertaken measurements showed that the currents in the DEA alternate between strong (> 5 cm s -1 ) and quasi unidirectional currents towards the NNW 430 and weaker currents (< 1-3 cm s -1 ) with greater directional variability (Klein, 1993;Klein, 1996). This variability has also been observed during the first cruise SO61 to the DISCOL area (Thiel and Schriever, 1989), with the "strong" current regime occurring during the first leg (February 1989) and the creation of PFEG1 to 7 and the weaker currents towards the end of the second leg (March 1989) and the creation of PFEG8 to 11, where the currents showed semidiurnal change of current direction from predominantly NNE to predominantly SSE. This 435 certainly affected the sediment plume dispersal.
Since no information about the amount of re-suspended material is available, the impact is reconstructed qualitatively using values resembling disturbance intensity between 1 within the disturbance tracks and 0.1, representing the deposition of 90% of the re-suspended material at the maximum distance of the proximal disturbance. With regards to other impact monitoring results from large-scale disturbances (e.g. Lavelle et al.,  (Greinert, 2015), and SO242/2 (Boetius, 2015), the maximum distance affected by sediment blanketing was assumed to be 120 m with, and 20 m against the current direction for the "strong" current regime. These distribution values and a distribution direction of 334° was set for PFEG 2 to 7 and all recognized parts of plough tracks, which could not be assigned to a distinct PFEG. To 445 account for the changing conditions during weaker bottom currents (PFEG 8 to 11), the distances were set to 100 m with, and 30 m against the current direction. Based on the statistics of the current directions (Thiel and Schriever, 1989) during the creation of 31 recognized tracks of that period, the plough tracks were divided in two groups, one considering a NNE-current (towards 18°, 19 tracks) and the other group considered a SSE-current (towards 143°; 12 tracks). Considering the semidiurnal current direction change, the assignment of the tracks to For calculating the sediment plume deposition down-current and up-current (due to turbulences) the following simple function was used: with y representing the relative sediment thickness at distance x from the disturbance track.
An exponential function was chosen to account for the effects of flocculation and aggregation of the resuspended sediment closer to the track. The factor R was introduced to meet the assumption that 10% of the re-suspended material remains in the water column and being re-deposited at greater distances (Lavelle et al., 1981;Jankowski et al., 1996). This factor was considered for the particle transport with the prevailing bottom currents.
Against the bottom currents the re-suspended material was assumed to completely resettled either within the first 20 m for strong currents or 30 m for weak currents. The relative sediment thickness was calculated in 0.8 m steps away from each disturbance track considering the above mentioned current directions. The final blanketing map was produced by adding all relative sediment thicknesses within each square meter of the DEA area using the 465 blockmean command in GMT (argument -Ss to get the sum; Wessel et al., 2013) and producing an interpolated grid using the nearneighbor command. It is assumed that the plough intensity and sediment re-suspension did not change during each plough track was created.

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Navigational offsets were detected between the different AUV missions with lateral offset between AUV-and ships data of 30 m to 80 m ( Figure 4). As AUV data sets from four different MBES surveys are used, a good geo-referencing of completely compiled AUV data set on the ships bathymetry was not possible. Therefore a focus for the best possible alignment was set to the DEA region with only three AUV data sets. To check for the improvement of the geo-referencing the AUV-bathymetry was subtracted from the ship-obtained dataset at 475 identical resolution (Fig. E1). Prior to shifting and stretching of the AUV grid, the depth differences showed a mean offset of -9 m. Thus 9 m were added to the entire AUV bathymetric grid to account for this absolute zoffset and after shifting/stretching, the difference between the AUV and ships bathymetry showed the mean to be at 0 m depth-difference with only +-0.5 m median range (Fig. E1).
As for the MBES data, the lateral offset between different SSS data surveys was not constant but varied between 480 40 m and 50 m. Geo-referencing the combined SSS map onto the AUV-bathymetry showed offsets between 30 and 80 m that were corrected (Fig. G1). The photo mosaics, which could be aligned to the SSS map very accurately, show sampling locations that we compared to the USBL position during the time of sampling for validating the geo-referencing results ( Figure 9; Table G1). The mean difference between the georeferenced photomosaic sampling locations and those from the USBL navigation is 14 m (Table G1)

Plough tracks and their sequence
Within the SSS map a total of 60 continuous tracks were identified and assigned to three different classes: V

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(11), H (28) and D (21) (Figure 6). In addition, 24 track segments were found and represent the fourth class P ( Figure 6; Table 1). Some of these partial tracks were assigned to other track identifiers (Table 1), based on the same course, but this could not be accomplished for all of the segments.

Initial plough impact
Based on the detected plough tracks (including the track segments) the directly impacted area is 1.9 km 2 , 505 corresponding to approximately 19 % of the DEA (10.81 km 2 ) assuming a width of each individual disturbance track of 8 m (Thiel and Schriever, 1989) and a length of approximately 3 km within the DEA. This area agrees with the original estimate (ca. 20%; Thiel and Schriever, 1989). However, this represents only an approximation of the disturbed area as the length of the tracks is variable and individual ones reach a length of up to 5 km and not all of the tracks could be identified to their complete extent. The disturbance tracks can clearly be observed 510 to continue outside the target area of the 2 nmi in diameter DEA target-circle and the created impact on the ecosystem extends beyond the limits of the DEA and even beyond the area covered by the SSS data ( Figure 10).
The total plough area from 1989 is thus not exactly known.
In comparison, the previously reported disturbance track locations and the observations from 2015 generally show the same trend with a high density of E-W oriented tracks and less tracks with N-S orientation (Figure 10).

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The locations of individual disturbance tracks do not agree well most likely because the plough tracks from 1989 were reconstructed from the ships position only (with a much lower accuracy than today) and an almost unknown layback of the plough-harrow behind the ship (Thiel and Schriever, 1989).

Secondary sediment deposition impact
The derived sediment disturbance map of the DEA (Fig. 11) indicates the highest levels of disturbance within the center (C-sectors in Fig. 11) of the DEA coinciding with high densities of plough tracks and in the easternmost peripheral (P-sectors in Fig. 11) sectors.

Geo-referencing
The quality of geo-referencing different data layers towards each other highly depends on prominent morphological features that are detectable in all available data layers. The depth differences of more than 5 m between the ship-and AUV-based MB data after geo-referencing (red marked in Fig. E1) are related to two different AUV surveys, which seem to be inconsistent. However, the range of vertical depth deviation is still 540 within the given depth-resolution of the EM122 ship system (Kongsberg, 2007)

Plough tracks and age succession
About 77 % of the reported disturbance tracks (60 out of 78) could be identified, most of them based on the SSS data (section 2.4). The 24 track segments of class P might account for the missing 18 tracks (e.g. P04 has been assigned to D21, Table 1).
The high-resolution MBES data did not fully capture the disturbance tracks due to the small morphological 565 differences between plough-tracks and the surrounding seafloor (circa 15-30 cm; Boetius, 2015) and the internal structure of the plough-marks. The reconstruction of the initial disturbance was mainly based on the SSS mapping because of the higher along-swath resolution of the SSS compared to the MBES data. The penetration depth of the plough-harrow in combination with its very characteristic pattern facilitates the detection of the disturbance tracks. Morphological changes that are ensonified perpendicular (tracks parallel to the AUV flight 570 path) cause higher reflections of the emitted signal compared to perpendicular tracks for which the small ridges and valleys of a plough track are ensonified parallel (Lurton, 2017;Beunaiche, 2017). Thus some tracks can be seen more clearly in the SSS data and than others, which also causes that the sequence at some crossings could not be finally determined. The very first disturbance tracks are clearly visible within the SSS data, again indicating that the amplitude of the signal reflectance cannot be used as an indicator for their relative age. This 575 becomes even more evident when comparing acoustic and optical data of the AUV. Some tracks that were barely visible in the SSS image (resolution: 0.5 m) could be clearly detected in seafloor photographs. Following this, the most reliable data source to establish the relative age sequence is the image and video material recorded by the various devices (AUV, ROV, OFOS) deployed during SO242 and the OFOS data from the previous cruises.
The different survey altitudes and operation plans influence the area that was covered by each instrument and the 580 quality of the images (Greinert, 2015). The AUV photo mosaics turned out to show the best results in resolving the age relation of multiple tracks even in highly disturbed areas within the DEA. There were a total of 9 AUV, 18 ROV, and 57 OFOS surveys conducted within the DEA between 1989 and 2015. However, since the DEA was not entirely covered by visual investigations, it is possible that some tracks which were not detected by the SSS were also not seen with the optical devices.

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In general, the age reconstruction was successful, where more than one dataset was available. The plough tracks could be reconstructed with the highest amount of certainty for the very first and second set of disturbance tracks (PFEG 2 and PFEG 3). The uncertainties within the sequence regarding the absolute ages especially with later sets of tracks (PFEG 4-11) increase since they are mainly based on statistical information and logical method of elimination (see section 2.4).

Disturbance levels in the DEA
Meso-scale numerical sediment distribution modeling by Jankowski et al. (1996Jankowski et al. ( , 2001 considering all plough tracks of the DEA experiment predicted blanketing of resettled material of >100 gm -2 up to a distance of 2 km. Due to the lack of data that measured the amount of re-suspended sediment, actual mass values of the blanketing cannot be given for our near field estimate approach. The settling velocity is also highly dependent on the 595 sediment plume concentration (Gillard et al. 2019). As said, only 90% of the sediment is assumed to settle immediately due to flocculation and aggregation causing resettling of particles within proximal distances , Gillard et al. 2019). The changing current conditions over the course of the plough experiment, especially in the later phases of the disturbance with a clear semi-diurnal signal (Thiel and Schriever, 1989), combined with the residence time of the re-suspended particles in the water column for more 600 than 10 hours (Thiel and Schriever, 1989;Greinert, 2015) indicate that these remaining 10% were most likely spread across the entire DEA and beyond. The sediment blanketing map should thus be considered as the minimum impact, with the SE sector being least impacted as already suggested by Thiel and Schriever in 1989.
The sectors with the highest sediment blanketing are CSE and CW (Fig. 11A), where also a high density of disturbance tracks occurs (Fig. 11B). X-ray studies aiming at measuring the deposition thickness were performed 605 on selected MUC samples during SO077 (Fig. 11); results imply that sectors CS, CN, PSE and CNE are most heavily influenced with thicknesses between 5 and 30 mm (Schriever and Thiel, 1992, Table H1). In the disturbance map for example within sector PSE only low disturbance is indicated, due to the very low density of tracks. A sample taken in sector CW (SO077_110MC_358, Fig. 11, Table H1) only shows a thin re-sedimented layer (1-2 mm), despite it is located in one of the most heavily disturbed areas ( varied sometimes by more than 100 m (Greinert, 2015). In 1992, this distance might have even been greater as the old RV SONNE did not have dynamic positioning systems. Considering the high blanketing thicknesses proximal to the tracks a sample location offset of several tens of meters could considerably change the result.
Samples during SO077 could have been taken within or next to a track or from one punctual location within a disturbed area, where not much sediment has been deposited (Fig. 11A). This again highlights the importance 620 being aware of the exact sampling positions and thus the need for detailed geo-referencing for the interpretation of the data.
However the generation of the disturbance intensity map is based on simplifications, not considering the specific sediment settling parameters as particle sizes, density of particles and water turbulence. It also didn't include the local morphology, which has been proven to influence the sediment plume distribution (Peukert et al., 2018).

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Furthermore the micro-relief of ripple-crests and furrows within the track will also have an influence on the sediment blanketing thickness results from sediment cores, which again requires detailed position knowledge for accurate sample interpretation. These factors could also be a reason for the deviating results from the SO77 x-ray studies compared to the disturbance map of this study. For more detailed investigations, this should be considered and implemented into calculations and further sampling methods to allow an appropriate comparison 630 of the results.

Sediment cover evolution through time
The numerous optical data acquired by OFOS, AUV and ROV during all expeditions to the DEA facilitate a comparison of the impact and its evolution over the 26 years that passed between the first and the most recent visit to the DISCOL area. Due to the explained navigation uncertainties especially during the early visits to the 635 DEA (SO061, SO064) a direct comparison of exactly the same square meter of the seafloor is difficult but the comparison of different locations within an about 150 m long section of one track seems more reasonable ( Figure 12). Generally, the fine morphology of the disturbance tracks appears to be smoothed out over time by currents and natural sedimentation, although the characteristic sequence of alternating crests and valleys is still clearly visible after 26 years (Boetius, 2015;Greinert, 2015).

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Track V02 is one of the first tracks that have been created during PFEG 2 (Table 1) and could still be detected in different OFOS surveys in 1989 (SO61_OFOS10), 1990 (SO64_OFOS19) and 2015 (SO242_1_OFOS05; Figure   12A). Figure 12B shows the track only a few hours after it has been created. The characteristic plough structures are very prominent and the freshly broke up sediment lumps appear brighter than the surrounding sediments.
Half a year later, the track appears distinctly smoothed and covered by sediment ( Figure 12C) due to the re-  Track H15 was one of the last tracks, created during PFEG 11 (Table F1).This track could be captured in OFOS dives from different deployments in the center of the DEA; H15 was further covered by the AUV photo mosaic 665 of SO242 ( Figure 13A). Fig. 13B of SO61 (OFOS17 from 1989) shows the freshly ploughed sediment within the disturbance track comparable to Fig. 12B. During SO106 in 1996 the track morphology is smoothed but broken up sediment lumps are still visible (Fig. 13C). The sediment cover within this track appears less than for track V02. The smoothing continued until 2015 but the track ripple structures are still apparent. At one location captured in the photo mosaic the H15 track crosses the V02 track (Fig. 13A), which allows a direct comparison 670 of two tracks from different PFEGs. In 1992 V02 appears already much less distinct (Fig. 13E) than track H15 four years later (Fig. 13C), again pointing at strong re-sedimentation initiated by the plough activities after PFEG2. In 2015 the track ripples appear even weaker for V02 (Fig. 13F). This illustrates that the still observable levels of the secondary disturbance through the sediment plume need to be interpreted with respect to their sequential age and respective PFEG deployments. It underlines the importance of careful interpretations of the 675 disturbance state of samples inside and near tracks. Furthermore, the track orientation with regard to the bottom current direction plays an essential role in terms of estimating the ecological impact coming along with the sediment plume; V02 runs parallel to the prevailing current direction causing higher sedimentation within and

690
Results of our combination of legacy data from 1989 to 1996 with data from 2015 clearly indicate that underwater navigation and determining the accurate position of a seafloor sampling or observation location has been and still is difficult even using state-of-the-art technology. The common approach used in this study that utilizes multiple hydroacoustic data sets of different resolution that are referenced against an absolute GPS-based data set (ships bathymetry) improved the overall accuracy. This is a pre-requisite for effective monitoring of 695 deep sea impacts from deep sea mining or other spatial impact. Modern USBL and LBL systems linked with DVL and INS navigation on ROVs and AUVs can result in an absolute location accuracy of < 5m., which should be at. High resolution visual and acoustic data from AUV surveys emerged as a very resourceful tool for deep sea surveys in general and monitoring impact experiments or even deep sea mining long-term effects in particular.

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The re-georeferenced plough mark-positions and the estimated sediment plume distribution allow a more precise evaluation of the primary and secondary disturbance. With respect to uncertainties in under water navigation of up to hundreds meters this knowledge is essential for a correct interpretation of physical and optical samples. here are not unconditionally comparable to the impact of such a large-scale and long-lasting operation (Gollner et al., 2017). The absolute deposition will be much more as the top 10 (or more) cm of the sediment will be 710 suspended, gravity flows will most likely be generated. The amount of fine grained material remaining in the water column might be more as well and sediment blanketing most likely occurs up to tens of kilometers beyond the mined area (Boetius and Haeckel, 2017).
Detailed investigations are needed in coming impact experiments that should quantify the amount of sediment that is being re-suspended to enable a conclusive interpretation of the quantitative results for sediment blanketing 715 analyses (be it through visual, sedimentological or chemical means). Knowing bottom currents and the local bathymetry in high spatial and temporal resolution are a fundamental pre-requisite for future impact experiments.
Technologies exist and workflows are in place for conclusive assessments.

Data availability
The final referenced hydroacoustic maps and photo mosaics (as GeoTIFF), as well as the disturbance tracks (as 720      smoothed. The time of sampling and as a consequence the position was determined using the "wire tension" of the cable (Fig. D1).
Appendix G: Offset between before and after the alignment of the different data sets

Authors contribution
FG has digitized the analogue available data from the initial DISCOL impact cruises, collected the entire set of study-relevant data from all DISCOL cruises from 1989-2015 and did the post-processing of navigation data from SO242 deployments. He substantially contributed to the methodology and the document writing. AH 835 contributed with literature review and to the methodology. She also created the Figures and was substantially involved in the document writing. TS and KK contributed with the acquisition, processing, curation and data analysis of optical data. KK created the mosaics of the AUV-acquired photographs. JG was the supervisor and the initiator of this study, developed the study design and contributed to methodology strategies He also substantially contributed to the document writing.