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Biogeosciences An interactive open-access journal of the European Geosciences Union
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Volume 15, issue 8 | Copyright

Special issue: Assessing environmental impacts of deep-sea mining...

Biogeosciences, 15, 2525-2549, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 27 Apr 2018

Research article | 27 Apr 2018

Understanding Mn-nodule distribution and evaluation of related deep-sea mining impacts using AUV-based hydroacoustic and optical data

Anne Peukert1, Timm Schoening1, Evangelos Alevizos1, Kevin Köser1, Tom Kwasnitschka1, and Jens Greinert1,2 Anne Peukert et al.
  • 1GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
  • 2Christian-Albrechts University Kiel, Kiel, Germany

Abstract. 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 ( < 5m horizontally,  < 1m vertically) with nodule coverage. In the presented data set, increased nodule coverage could be correlated with slopes  > 1.8° and concave terrain. On a more regional scale, factors such as the geological setting (existence of horst and graben structures, sediment thickness, outcropping basement) and influence of bottom currents seem to play an essential role for the spatial variation of nodule coverage and the related hard substrate habitat.

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 16h after the disturbance. The visually detectable impact was spatially limited to a maximum of 100m distance from the disturbance track, downstream of the bottom water current. A correlation with high-resolution AUV bathymetry reveals that the blanketing pattern varies in extent by tens of meters, strictly following the bathymetry, even in areas of only slightly undulating seafloor ( < 1m vertical change).

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 100m. Terrain knowledge is also needed to determine the scale of the impact by seafloor sediment blanketing during mining operations.

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Manganese nodules are a deep-sea mineral resource considered for mining. This paper provides insights into measuring the distribution of manganese nodules at meter resolution. Nodule abundance was determined by autonomous robots using cameras and echo sounders. Based on the meter-scale abundance measurements, environmental impacts of simulated deep-sea mining were assessed. The spatial extent of a sediment plume was determined and showed correlation to small variations in seafloor topography.
Manganese nodules are a deep-sea mineral resource considered for mining. This paper provides...