During the first research cruise (SO061) in the DISCOL project in 1989, a circular area within the Peru Basin (DEA) was artificially disturbed to simulate the impact of a possible manganese nodule mining operation. This was realized by dragging a specially designed “plough-harrow” device behind the ship over the seafloor. The blades mounted on the device penetrated into and reworked the upper 10 to 15 centimeters of the seafloor sediments and thus buried the present nodules.
The DEA, in which the largest disturbance was created, is circular with a diameter of 2 nautical miles (nmi), covering an area of 3.14 nmi2 or 10.8 km2. A circular area was chosen to provide a central, highly disturbed and a peripheral, less disturbed area. In total, 78 so-called “disturbance tracks” were created on diametrical courses by the “plough-harrow” device and as a result about 20 % of the seafloor sediments inside the DEA were disturbed directly. The locations of these "disturbance tracks" were determined from digitalized maps (see figure below; "disturbance tracks" plotted on the latest bathymetric data, which were acquired during cruise SO242/1 in August 2015) and one objective of especially cruise SO242/1 within the framework of the JPI Oceans Pilot Action "Ecological Aspects of Deep-Sea Mining" was to find and identify the old impact structures and assess the current level of recovery.
However, the area indirectly impacted by re-sedimentation from sediment plumes caused by the resuspension of particles during the ploughing of the seafloor was much larger and thus the impact was categorized into 4 different disturbance levels (after Bluhm et al., 1995):
The indirectly impacted areas were covered by an up to 30 millimeter thick layer of resettled particles from the respective sediment plumes, whereas most of the material was re-deposited within the first 2 miles from the impact area (Schriever and Thiel, 1992; Thiel et al., 2001).
The appearance and the contours of the disturbance tracks were targeted during each cruise to the DISCOL area afterwards and inspected with the OFOS system to assess the recovery of the ecosystem within the DEA. It was observed that the surface contours of the disturbance tracks were smoothened after a few years and the sharp edges were flattened, but that overall the tracks were still clearly visible, even 26 years later during the cruises SO242/1 and SO242/2. The figure below shows example photographs of some disturbance tracks taken during each cruise to the DISCOL area - SO061 (A), SO064 (B), SO077 (C) and SO106 (D) - and illustrate the very slow recovery rate of the disturbed seabed over the first seven years after the initial impact in 1989. For further reading and the latest results from the most recent cruises (JPI Oceans "Ecological Aspects of Deep-Sea Mining") to the DISCOL area please see the publications listed on this website or the respective cruise reports (SO242/1 and SO242/2).