Geochemistry, faunal composition and trophic structure in reducing sediments on the southwest South Georgia margin

Abstract

Despite a number of studies in areas of focused methane seepage, the extent of transitional sediments of more diffuse methane seepage, and their influence upon biological communities is poorly understood. We investigated an area of reducing sediments with elevated levels of methane on the South Georgia margin around 250 m depth and report data from a series of geochemical and biological analyses. Here, the geochemical signatures were consistent with weak methane seepage and the role of sub-surface methane consumption was clearly very important, preventing gas emissions into bottom waters. As a result, the contribution of methane-derived carbon to the microbial and metazoan food webs was very limited, although sulfur isotopic signatures indicated a wider range of dietary contributions than was apparent from carbon isotope ratios. Macrofaunal assemblages had high dominance and were indicative of reducing sediments, with many taxa common to other similar environments and no seep-endemic fauna, indicating transitional assemblages. Also similar to other cold seep areas, there were samples of authigenic carbonate, but rather than occurring as pavements or sedimentary concretions, these carbonates were restricted to patches on the shells of Axinulus antarcticus (Bivalvia, Thyasiridae), which is suggestive of microbe–metazoan interactions.

Hydrothermal activity lowers trophic diversity in Antarctic hydrothermal sediments

“…Sediment methane production is thought to be accelerated by the heat flux associated with the mixing of hydrothermal fluid in sediment (Whiticar and Suess, 1990), and sediment and Siboglinum isotopic data suggest that the footprint of hydrothermal influence may be much larger than previously recognised, giving rise to transitional environments (Bell et al, 2016a;Levin et al, 2016). A clear contribution of methane-derived carbon to consumer diets was limited predominately to neotanaids, which is consistent with the relatively small population sizes (64-159 ind.…”

“…Differences across all samples were not significant (paired t test, δ 13 C: t = 2.10, df = 3, p = 0.126; δ 15 N: t = 1.14, df = 3, p = 0.337). Given the unpredictable response of isotopic signatures to preservation effects (which also cannot be extricated from within-site, intraspecific variation), it was not possible to correct isotopic data (Bell et al, 2016a). This contributed an unavoidable but generally quite small source of error in these measurements.…”

“…CNS measurements used the internal standards (MSAG2: (methanesulfonamide-gelatin and M1: methionine) and the international silver sulfide standards IAEA-S1, IAEA-S2, and IAEA-S3. All sample runs included samples of freezedried, powdered Antimora rostrata (ANR), an external reference material used in other studies of chemosynthetic ecosystems (Reid et al, 2013;Bell et al, 2016a) to monitor variation between runs and instruments (Supplement S1). Instrument precision (SD) for each isotope measured from ANR was 0.42, 0.33, and 0.54 ‰ for carbon, nitrogen, and sulfur, respectively.…”

Benthic carbon fixation and cycling in diffuse hydrothermal and background sediments in the Bransfield Strait, Antarctica

“…These hydrothermally influenced sediments are likely to be more spatially extensive than hard substrate vents, although their diffusive nature makes their extent hard to quantify. Sedimented hydrothermal vents have been shown to influence biological community composition and nutrition at adjacent sites which were otherwise characterised as "inactive" or "off-vent" (Levin et al, 2009;Bell et al, 2016aBell et al, , b, 2017a. However, the ecology of sedimented hydrothermal sites has received relatively little study.…”

Exploring the Ecology of Deep-Sea Hydrothermal Vents in a Metacommunity Framework

“…Sulfide is widespread in non-seep organic-rich sediments on margins, and many background species adapted to these conditions can occur at weakly seeping sites (Bell et al, 2016). Consequently, changes to geochemical gradients may be less disruptive for seep communities than fluid flow alterations are for vent communities.…”