In this study, ship- and autonomous underwater vehicle (AUV)-based multibeam
data from the German ferromanganese-nodule (Mn-nodule) license area in the
Clarion–Clipperton Zone (CCZ; eastern Pacific) are linked to ground-truth
data from optical imaging. Photographs obtained by an AUV enable
semi-quantitative assessments of nodule coverage at a spatial resolution in
the range of meters. Together with high-resolution AUV bathymetry, this
revealed a correlation of small-scale terrain variations (
AUV imagery was also successfully employed to map the distribution
of resettled sediment following a disturbance and sediment cloud
generation during a sampling deployment of an epibenthic
sledge. Data from before and after the “disturbance” allow
a direct assessment of the impact. Automated image processing
analyzed the nodule coverage at the seafloor, revealing nodule
blanketing by resettling of suspended sediment within 16 h after
the disturbance. The visually detectable impact was spatially
limited to a maximum of 100
These results highlight the importance of detailed terrain knowledge
when engaging in resource assessment studies for nodule abundance
estimates and defining mineable areas. At the same time, it shows
the importance of high-resolution mapping for detailed benthic
habitat studies that show a heterogeneity at scales of 10 to
100
The deep ocean is an area of economic interest due to its potential reserve
of metal resources. Before deep-sea mining can be conducted, a better
understanding is required of the ecological role of the deep sea as the
largest habitat on Earth. One focus lies on impacts of ferromanganese-nodule
(Mn-nodule) mining which recently has been studied in international projects
like MIDAS (FP7 project 603 418) and Mining Impact (JPI Oceans project). Mn
nodules form a hard substrate for sessile fauna
Several studies correlate bathymetry and nodule occurrence, revealing
a complex/non-coherent interrelation which mainly depends on the considered
spatial scale. Most studies have focused on nodule occurrence variability
between very different terrain settings such as seamounts, valleys, plains
and undulating terrain
The study analyzes ship-based bathymetric data for large-scale
background information together with autonomous underwater vehicle (AUV)-obtained high-resolution
multibeam (MB) and optical data to reveal detailed nodule coverage
patterns within a 12
Sessile benthic organisms depending on manganese nodules as a hard substrate habitat. Images are from the German claim area in the Clarion–Clipperton Zone (photos: remotely operated vehicle (ROV) Kiel 6000, GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany).
An equivalent approach was applied for an environmental impact study on
sediment blanketing during a simulated “mining operation”. Mn-nodule mining
will affect the seafloor and benthic fauna in several ways. A removal of the
uppermost sediment layer (5–20
Various benthic impact experiments (BIEs) have been conducted to study
sediment resuspension and the distribution of sediment plumes in
Mn-nodule areas
The use of different disturbance gear for different duration per BIE leads to
inconsistent interpretations
Plume model results are based on several assumptions to include
parameters describing the environment. Particle sizes and settling
velocities are key factors in modeling plume distribution distances
The study presented here focuses on an area within which the resettled sediment was visually observable in deep-sea photographs of the seafloor. Two AUV photo surveys over the same area were conducted before and after the deployment of an epibenthic sledge (EBS), that created a sediment plume. The two data sets are directly compared to determine the scale of the visible disturbance. The correlation of the photo data with AUV-obtained bathymetry data reveals the influence of the local terrain variability on the sediment blanketing pattern and thus the sediment plume spreading.
The study area is part of the Prospective Area 1 (PA1) within the
German license area in the eastern Clarion–Clipperton Zone (CCZ). The
wider area (Fig.
Overview maps of PA1 showing
High-resolution studies using data from several AUV deployments were carried
out within the mineable plateau (black square in Fig.
Overview maps for the geological setting of the AUV study
area.
All ship- and AUV-based surveys were conducted in March 2015 during the
EcoResponse cruise SO239 with R/V
The AUV camera system DeepSurveyCam
As a side product of benthic sampling using a B-EBS type sledge
The water depth within the AUV-mapped area ranges from 4110 to 4143
Bathymetric map obtained by the AUV, with the black line
indicating the track of AUV Abyss Dive 168 prior to the EBS
deployment. The black arrow marks the tow track of the EBS
deployment.
The two AUV photo surveys provide visual data from within the
high-resolution MBES map (black lines, Fig.
In the following examinations, the threshold between “low” and
“high” Mn-nodule coverage is set at 12.5 %, which is the
analyzed mean coverage value of the considered range. In the eastern
A2 sub-area, a greater proportion of higher coverage values
(13–16 %) can be observed. A positive correlation was found
between Mn-nodule coverage and median size of the nodules
(Fig.
Scatterplots indicating relationships between Mn-nodule
percent coverage (%) and eight other nodule and terrain values:
median nodule size in
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.
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.
Figure
No further correlation between Mn-nodule coverage and bathymetric derivatives
was found, and no relation to absolute water depth could be observed
(Fig.
Distribution of derivative values in sub-areas A1 and A2.
A lower Mn-nodule coverage (
In sub-area A1 (
Comparing the terrain statistics of areas A1 and A2
(Fig.
To evaluate sediment plume resettling, results of the automated
image-based Mn-nodule detection before the EBS disturbance
(SO239_019_Abyss168 with 6061 usable photos) and after the EBS
disturbance (SO239_028_Abyss169 with 10 783 usable photos) were
compared (Fig.
The AUV tracks of the photo surveys run perpendicular to the EBS
track. A strong sediment blanketing can be observed close to the
disturbance track (Figs.
The combination of AUV-obtained bathymetry and imagery reveals
a distinct blanketing pattern depending on the small-scale morphology
(Fig.
Section of the photo mosaic along one survey track line, with
calculated nodule coverage values, indicated by the color-coded dots
(representative of the center of each individual photo which are
approximately 15
Seafloor photographs have been used for Mn-nodule occurrence studies
for almost two decades
Photographs only provide information of the sediment surface and thus
will not be able to detect buried/sediment-covered Mn nodules
In general, properties such as sedimentation rate
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
With respect to the large scale of the ship-based bathymetry in
Fig.
According to the study by
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
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
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
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.
The observations made on a broad scale (several hundreds of meters; grid
cell size of 55
The approach of conducting a photo mosaic survey before and after a seafloor disturbance proved successful for detecting sediment blanketing visually, offering the possibility to accurately map the area of strongest plume impact. This area is characterized by the sediment plume transport direction and resettling of the majority of the sediment. Very fine particles within the sediment plume might be dispersed much further; more detailed biological studies need to evaluate which sediment concentrations and grain sizes will impact benthic organisms on long timescales (cumulative effects) outside the visually clearly detectable impacted areas.
The thickness of the resettled sediments could not be determined from
the AUV-based images or ROV-based video footage during the
cruises. Video observations from other, similar areas point towards
a sediment cover on millimeter or sub-millimeter range that can still
be detected in images
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
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
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
(
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
In our very small-scale experiment, the EBS created a local impact with
clearly visible sediment blanketing within 100
The actual scenario of disturbance will be different during real-case
mining during which the top 10–20
We conclude that, for both of our study topics, the Mn-nodule distribution to
terrain comparison as well as the redeposition of sediments indicate that
Mn-nodule coverage and sediment blanketing vary measurably on a very small
scale (several tens to hundreds of meters), even if the seafloor terrain
changes are minor (less than 1
With respect to the sediment plume study, it became obvious that
a visible blanketing occurs in a limited distance (here
On the technical side, the study showed that we have the needed tools and
techniques at hand to map the seafloor for Mn-nodule resource assessments and
a better understanding of Mn-nodule distribution, as well as for assessing
mining impacts visually. It became clear that without such high-resolution
techniques valid assessments cannot be carried out. Areas that appeared
suitable for mining (slopes
Source code for the automated nodule delineation
is available in Pangaea
The slope was calculated using the algorithm included in the spatial analyst
toolbox
For calculating aspect, BPI and terrain
ruggedness, the ArcGIS benthic terrain modeler (BTM) add-in
The aspect is defined as the inclination direction of the maximum rate
of change in depth from each cell to its neighbors, the slope
inclination
The BPI describes the relative topographic variability of a central
grid cell to a circular annulus with an inner and outer radius; both
are manually defined (Table
The terrain ruggedness was calculated for the AUV bathymetric data set
(Fig.
Considering a potential error in correctly detecting nodules by the CoMoNoD
algorithm, the application of quantiles of the size distribution allows
a more robust interpretation of the data. It is suggested not to use size
values of the smallest and largest 1 % of the quantile calculation due to
the abovementioned error source. The graph in Fig.
Two example images (bottom) which clearly differ in nodule size and coverage. The graph shows the size distribution as calculated by the CoMoNoD algorithm. The most significant difference is observed in the 75 % quantile.
Scatterplots indicating the relation between Mn-nodule
coverage and total curvature
ADCP data obtained by a DOS lander deployment during
SO239 indicating a SSW current flow during the EBS deployment (grey
shaded box; ensemble 196 to 204). Box plots show the mean and standard deviation of
the time series of the entire deployment. Mean currents between 10
and 30
Example for how the program identifies nodules in the
image.
Metadata of the created maps including raster cell sizes for the considered regions and subregions.
Classification dictionary with upper and lower bounds for the classification of the PA1 area used with the BTM.
Classification dictionary with upper and lower bounds for the classification of the working area used with the BTM.
Algorithms and ArcGIS tools applied for the calculation of the bathymetric derivatives.
Classification dictionary for the classification of the AUV-mapped study area used with the BTM to reveal areas of possible lower Mn-nodule coverage.
Statistics summary of the derivatives derived from the AUV-obtained bathymetry for the A1 and A2 survey areas.
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 the captain and crew of RV