In field studies of active hydrocarbon seeps the carbon isotopic composition of Rose Bengal stained benthic foraminiferal tests (δ13Ctest) and bottom water DIC (δ13CDIC) 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 δ13C values of emanating methane. To evaluate the discrepancy between δ13CDIC and δ13Ctest, we conducted methane seepage-emulating culture experiments on undepressurized sediments from the Håkon Mosby Mud Volcano, a modern methane seepage structure that hosts living C. wuellerstorfi with distinct negative δ13C 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 δ13Ctest, we ran parallel experiments with > 99% 12C- and 99% 13C-methane in the experimental “bottom water”. During the experimental running time methanotrophs in the water column obviously converted the experimentally added methane source to δ13C-enriched and -depleted DIC, respectively. Since whole sediment cores were cultured, it was impossible to keep δ13CDIC constant over the 5-month duration, which is reflected in a variability of δ13Ctest in foraminiferal shells. Irrespective of that, the methane source is reflected in δ13Ctest of foraminiferal shells, and for the natural seep-conditions simulating 12C-experiment the mean δ13CDIC and δ13Ctest 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.
AWI Organizations > Biosciences > BioGeoScience