Abstract
Abstract. First videographic indication of an Antarctic cold seep ecosystem was recently obtained from the collapsed Larsen B ice shelf, western Weddell Sea (Domack et al., 2005). Within the framework of the R/V Polarstern expedition ANTXXIII-8, we revisited this area for geochemical, microbiological and further videographical examinations. During two dives with ROV Cherokee (MARUM, Bremen), several bivalve shell agglomerations of the seep-associated, chemosynthetic clam Calyptogena sp. were found in the trough of the Crane and Evans glacier. The absence of living clam specimens indicates that the flux of sulphide and hence the seepage activity is diminished at present. This impression was further substantiated by our geochemical observations. Concentrations of thermogenic methane were moderately elevated with 2 μM in surface sediments of a clam patch, increasing up to 9 μM at a sediment depth of about 1 m in the bottom sections of the sediment cores. This correlated with a moderate decrease in sulphate from about 28 mM at the surface down to 23.4 mM, an increase in sulphide to up to 1.43 mM and elevated rates of the anaerobic oxidation of methane (AOM) of up to 600 pmol cm−3 d−1 at about 1 m below the seafloor. Molecular analyses indicate that methanotrophic archaea related to ANME-3 are the most likely candidates mediating AOM in sediments of the Larsen B seep.
Highlights
Ocean research of the last decade has provided evidence for a variety of fascinating ecosystems associated with fluid, gas and mud escape structures
Sediment subsampling for gases and DNA was performed with cut-off syringes (GC and multiple corer (MUC) cores) and for anaerobic oxidation of methane (AOM) and sulphate reduction (SR) rates with glass tubes (GC cores) and acrylic core liners (MUC cores) according to previously described methods (Treude et al, 2003; Niemann et al, 2006a)
In the easternmost depression (Fig. 1b), we examined the seafloor during two ROV and multi grab transects (Fig. 1c), of which the South-North ROV transect was previously surveyed with a video sledge by Domack et al (2005) in March 2005
Summary
Ocean research of the last decade has provided evidence for a variety of fascinating ecosystems associated with fluid, gas and mud escape structures These so-called cold seeps are often colonized by thiotrophic bacterial mats, chemosynthetic fauna and associated animals (Jørgensen and Boetius, 2007). Key agents of the anaerobic oxidation of methane (AOM) with sulphate are consortia of methanotrophic archaea (ANME) and sulphate-reducing bacteria (SRB) (Knittel and Boetius, 2009). These consortia effectively reduce the efflux of methane to the hydrosphere (Hinrichs and Boetius, 2002; Reeburgh, 2007), and are the source for sulphide which fuels the chemosynthetic seep fauna (Jørgensen and Boetius, 2007). Denitrifying bacteria of the candidate division “NC10” were found to mediate AOM
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