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

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.