RRS "Charles Darwin" Cruise 145, 12 Mar - 09 Apr 2003. Benthic ecology and biogeochemistry of the Pakistan Margin

Abstract

RRS Charles Darwin cruise 145 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 establish a series of some five sites on a transect spanning the Arabian Sea oxygen minimum zone 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 provide a general characterization of the seabed in the area of these sites using acoustic remote sensing (EM12 and 3.5 kHz ) and seabed imagery (WASP ); d) to initiate a programme of detailed seabed sampling at these sites to determine a suite of biological, chemical and biogeochemical parameters using a range of coring devices (multicorer, Megacorer, box corer); and e) to assess and sample the megabenthos of these sites by the combined use of trawling (Agassiz trawl) and seabed photography (WASP).

Despite being beset by a number of difficulties, this was a very successful cruise that very largely achieved its planned objectives: 1) A series of five sites on a transect spanning the Arabian Sea oxygen minimum zone were established in the Pakistan Margin work area, with a sixth deep-water site also successfully studied; 2) CTD sensor profiles and chemical determinations on water bottle samples from both the CTD and BBLS were successfully undertaken at all six sites; 3) A substantial tranche of EM12 swath bathymetric mapping, and supporting 3.5 kHz seabed profiling, was achieved in the Pakistan Margin work area; 4) An extensive coring programme, delivering a wealth of samples, was successfully carried out at all six sites.; and 5) Good quality seabed imagery was obtained with the WASP system at all six primary sites and a further six additional sites. Supporting work with the trawl was also undertaken at five of the six study depths and additional depths in the lower boundary of the oxygen minimum zone where rapid changes in the composition of the megabenthic fauna appears to occur. In summary, this cruise laid a firm foundation on which forthcoming cruises (see CD146, 150 and 151) will build.

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