Abstract
Abstract. High-resolution optical and hydro-acoustic sea floor data acquired in 2015 enabled the reconstruction and exact localization of disturbance tracks of a past deep-sea recolonization 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 distribution and redeposition 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 of the blanketing of the plough tracks happened through the resettling of plume sediment from plough tracks created later. Generally sediment plumes are seen as one of the important impacts associated with potential Mn-nodule mining. For enabling a better evaluation and interpretation of particularly geochemical and microbiological data, a relative age sequence of single plough marks and groups of them was derived and is presented here. This is important as the thickness of resettled sediment differs distinctly between plough marks created earlier and later. Problems in data processing became eminent for data from the late 1980s, at a time when GPS was just invented and underwater navigation was in an infant stage. However, even today the uncertainties of underwater navigation need to be considered if a variety of acoustical and optical sensors with different resolution should be merged to correlate accurately with the absolute geographic position. In this study, the ship-based bathymetric map was used as the absolute geographic reference layer and a workflow was applied for geo-referencing all the other data sets of the DISCOL Experimental Area until the end of 2015. New high-resolution field data were mainly acquired with sensors attached to GEOMAR's AUV Abyss and the 0.5∘ × 1∘ EM122 multibeam system of RV Sonne during cruise SO242-1. Legacy data from the 1980s and 1990s first needed to be found and compiled before they could be digitized and properly geo-referenced for our joined analyses.
Highlights
1.1 Ecological risks associated with Mn-nodule mining from the deep-sea floorFor several years, mining of manganese (Mn) nodules from the deep-sea floor is again considered a worthwhile option to meet future resource demands
A review of the biological responses to such benthic impact experiments (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 after 26 years
A focus for the best possible alignment was set to the DEA region with only three AUV data sets
Summary
1.1 Ecological risks associated with Mn-nodule mining from the deep-sea floorFor several years, mining of manganese (Mn) nodules from the deep-sea floor is again considered a worthwhile option to meet future resource demands. Depending on the plume properties such as particle size, flocculation behaviour, sediment mass per litre 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 filter feeders and burying the sessile fauna, which are both adapted to the low sedimentation rates in the deep sea (Thiel and Schriever, 1989). Resedimentation of this material can lead to differences in local geochemical gradients and might influence the recolonization processes of the primary and secondary disturbed areas. 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 after 26 years
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