Greinert, Jens, ed. (2015) RV SONNE Fahrtbericht / Cruise Report SO242-1 [SO242/1]: JPI OCEANS Ecological Aspects of Deep-Sea Mining, DISCOL Revisited, Guayaquil - Guayaquil (Equador), 28.07.-25.08.2015. . GEOMAR Report, N. Ser. 026 . GEOMAR Helmholtz-Zentrum für Ozeanforschung, Kiel, Germany, 290 pp. DOI 10.3289/GEOMAR_REP_NS_26_2015.

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Abstract

Cruise SO242/1 ran from 28 July to 25 August 2015 starting and ending in Guayaquil, Ecuador. In
total, 40 scientists from five European countries took part in this cruise of the JPIO project ‚Ecological
Aspects of Deep‐Sea Mining‘ to study the ecological long‐term impact of deep sea disturbances. The
working area, the DISCOL area in the Peru Basin, was ploughed in 1989 and thoroughly studied in the
years thereafter, the last cruise, SO106, happened in 1996.
SO242/2 aimed at mapping the DISCOL Experimental Area (DEA) in great detail using ship‐based
and AUV‐based hydroacoustic and optical methods. To see changes between differently disturbed
areas and study the possible recovery of the ecosystem, biological sampling occurred with TV‐guided
multi‐coring (MUC), box coring (BC) and epi‐benthic sledge tracks (EBS). Additional biological
sampling for scavenging animals occurred with baited Amphipod‐Traps within and further outside
the DEA. For geochemical sampling, multi‐, box and gravity coring (BC) was used. Two lander systems
equipped with physical sensors such as ADCPs and CTDs were used for current measurements and to
monitor sediment plume dispersal created by the EBS. Additional visual studies of the fauna
distribution occurred with camera tows (OFOS).
Five main sampling areas were selected, two within the DEA targeting heavily disturbed
(ploughed) locations and three reference areas 3 to 4 nmi outside the DEA. All five areas had been
sampled in the past and can be directly compared concerning ecological changes. Despite a four‐day
break due to medical reasons the work program could almost be completed. Four of the working
areas were at least sampled with five BCs and MUCs each, and one GC. Box coring could not be
performed in the western reference area. In total, 5 CTDs, 27 MUCs, 25 BCs, 7 GCs, 8 EBSs, 5
Amphipod‐Traps, 6 lander deployments and 6 OFOS tracks were successfully undertaken and one
thermistor mooring was deployed.
In addition, 16 AUV dives clearly showed that plough marks are still well visible after 26 years.
There is a slight sediment cover next to the plough tracks, but first analyses of the faunal distribution
show that the sessile fauna has not yet recolonised the tracks. Stalked sponges, corals and anemones
can be found outside the tracks but still within the DEA. Their distribution patterns inside the DEA do
not vary clearly from those on reference sites. The Mn‐nodule distribution is not homogenous; there
are areas inside the DEA that do not have nodules at the seafloor surface; they are typically linked to
depressions that show low backscatter intensity in the AUV side scan sonar data pointing towards
less dense sediment infill. In gravity cores, nodules could be recovered even in 9m sediment depth,
finding several more or less intact nodules throughout the entire sediment column was common.
Water current measurements show slow currents (max. 6cm/s) and a strong tide‐influenced current
direction, whereas no general direction could be observed. Two ‘disturbance experiments’
demonstrated that sediment plumes can be monitored using high frequency ADCPs (1200 kHz). The
disturbance by the EBS created a sediment plume that stayed close to the seafloor. First analyses of
current trajectories showed that the sediment resettled rather quickly. It became clear that plume
behaviour during large‐scale mining cannot be extrapolated from these small‐scale and short‐term
experiments. In resume, cruise SO242/1 was very successful and research should continue in the DEA
area, which is undoubtedly the best studied long‐term deep sea disturbance site in the ocean.

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