Novel high-pressure culture experiments on deep-sea benthic foraminifera - Evidence for methane seepage-related δ13C of Cibicides wuellerstorfi

In field studies of active hydrocarbon seeps the carbon isotopic composition of Rose Bengal stained benthic foraminiferal tests (delta C-13(test)) and bottom water DIC (delta C-13(DIC)) deviates from their normal marine ratios. This circumstance led to ongoing discussions on whether aerobic foraminifers like Cibicides wuellerstorfi are capable of living at seepage sites and, more importantly, if their tests reflect the low delta C-13 values of emanating methane. To evaluate the discrepancy between delta C-13(DIC) and delta C-13(test), we conducted methane seepage-emulating culture experiments on undepressurized sediments from the Halton Mosby Mud Volcano, a modern methane seepage structure that hosts living C. wuellerstorfi with distinct negative delta C-13 values. The collected sediments were cultured at a site-alike pressure and mean bottom water methane concentration using newly developed high-pressure aquaria. Over an experimental period of 5 months our novel technology enabled a successful reproduction of all calcareous deep-sea benthic foraminiferal species living at that site, notably the first C wuellerstorfi cultured in the laboratory. To show the influence of methane on delta C-13(test), we ran parallel experiments with >99% C-12- and 99% C-13-methane in the experimental "bottom water". During the experimental running time methanotrophs in the water column obviously converted the experimentally added methane source to delta C-13-enriched and -depleted DIC, respectively. Since whole sediment cores were cultured, it was impossible to keep delta C-13(DIC) constant over the 5-month duration, which is reflected in a variability of delta C-13(test) in foraminiferal shells. Irrespective of that, the methane source is reflected in delta C-13(test) of foraminiferal shells, and for the natural seep-conditions simulating C-12-experiment the mean delta C-13(DIC) and delta C-13(test) in C wuellerstorfi were equal. Although for future culturing experiments improvements of the experimental conditions are advisable, our first results are evidence that persistent methane emanation impacts the carbon isotopic composition of deep-sea benthic foraminifera. (C) 2015 The Authors. Published by Elsevier B.V.