RRS "Charles Darwin" Cruise 150, 22 Aug - 15 Sep 2003. Benthic ecology and biogeochemistry of the Pakistan Margin

Abstract

RRS Charles Darwin cruise 150 forms part of a larger programme of research (“Benthic processes in the Arabian Sea: interrelationships between benthos, sediment, biogeochemistry and organic matter cycling”, NER/A/S/2000/01280), focusing on the benthic biogeochemistry of the Pakistan Margin, that includes four cruises in total (CD145, 146, 150 and 151). The primary objectives of the present cruise were: a) to revisit a series of five previously established study sites (A140, A300, A950, A1200 and A1850) spanning the Arabian Sea oxygen minimum zone (OMZ) as it impinges on the seabed at the Pakistan Margin; b) to assess the chemical oceanography of the water column overlying these sites, through CTD sensor profiles and chemical determinations on water bottle samples from both the CTD and BBLS; c) to initiate a programme of detailed seabed sampling at these sites to determine a suite of biological, chemical and biogeochemical parameters using a Megacorer and a multicorer; d) to assess and sample the megabenthos of these sites by the combined use of trawling (Agassiz trawl) and seabed photography (WASP); e) as possible, to carry out similar operations at a site located at a depth between A300 and A950; f) as possible, to provide additional general characterization of the seabed in the area of these sites using acoustic remote sensing (EM12 and 3.5 kHz ) and seabed imagery (WASP ).

The cruise successfully achieved all of the planned objectives. The effort of assessing the science of the cruise will take many months of work ashore. Of those parameters that could be initially assessed onboard there was little indication of major change between cruises 145 and 150, other than in the OMZ itself. Minimum oxygen values encountered during cruise 145 were around 400m, but during the present cruise were in the 150-200m range. If a value of 0.5 ml/l is used as a boundary, then it had shallowed from c. 180m (CD145) to c. 80m (CD150). There also appeared to be some elevation of the lower boundary of the OMZ, although this was less marked.

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