Culture experiments on the gamma 13C values recorded in tests of benthic foraminifera from methane sources at high latitudes
Final Report Abstract
Methane is one of the most potent greenhouse gases. However, the potential role of marine methane sources on climate changes is still under debate. Derived from depleted δ13C ratios in benthic foraminifers of the geological records sudden methane emanations have/had the potential to trigger sudden climate change, whereas, in field studies the δ13C ratios in benthic foraminifers are more often unrelated to methane seepages. The contradicting findings of fossil and field foraminifera led to the basic questions of this project: 1. Do foraminifers live and reproduce under the influence of elevated methane concentrations? 2. Is the methane concentration (its isotopic composition) of pore and bottom waters at seep sites waters reflected in the δ13C ratios of DIC and calcareous benthic foraminifers? 3. Are there constant relationships between the methane concentration, the δ13C ratios of DIC and calcareous benthic foraminifers? 4. If not, to which extend are the δ13C ratios of calcareous benthic foraminifera influenced by their nutrition? The contradicting data situation required culture experiments to answer this questions. However, we started this project, no one had ever succeeded in the reproduction of barophilic deep-sea benthic foraminifers, notably Cibicides wuellerstorfi, which are pressuredependent and regarded as relevant for palaeoceanographic reconstructions. Technologically speaking, this project started from zero, Within this project we constructed high-pressure aquaria that can be filled and closed at the seafloor by aid of an remotely operated vehicle (ROV). We repeatedly changed this basic construction, developed a specific interior and exterior to construct an efficient high-pressure aquarium. Fully transparent push-corers with mechanical valve systems were developed and improved to allow for the transfer of undisturbed sedimentcores into the high-pressure aquaria. We ran long trials with varying combinations of high-pressure pumps, peristaltic pumps and different sets of valve systems, as well as varying methane enrichment and injection methods. Finally we established the first laboratory that carried out high-pressure culture experiments on benthic deep-sea foraminifers, and the first that succeeded in the reproduction of Cibicides wuellerstorfi, our most important paleodeepwater recorder. With this new technique we carried out multiple methane seepage emulating experiments. Whenever possible, we collected undepressurised sediments with aid of a ROV during other expeditions transferred sediments from multiple corer casts in our aquaria, always succeeding in a reproduction of all taxa common at that specific site. Yet, regarding the conversion of methane to DIC, we found that the only satisfying experiments we ran on undepressurised sediments derived from the Håkon Mosby Mud Volcano (HMMV), a modern methane seepage structure that hosts living C. wuellerstorfi with distinct negative δ13C ratios and the methanotrophic microbiota necessary for the conversion of methane to DIC. At the HMMV, we collected sediments that we cultured at a site-alike pressure and mean bottom water methane concentration. To show the influence of methane on δ13C ratio tests, we ran parallel experiments with >99% 12C- and 99% 13C-methane in the experimental “bottom water”. During the run time of the experiments, methanotrophs in the water column obviously converted the experimentally added methane source to δ13C-enriched and to δ13C-depleted DIC, respectively. The methane source was reflected in δ13Ctest ratios of foraminiferal shells, and for the natural seep-conditions simulating (12C –experiment), the mean δ13C DIC and δ13Ctest ratios in C. wuellerstorfi were equal. However, the δ13Ctest ratios of endobenthic foraminifers and the 99% 13C-methane experiments show that the bottom water methane concentration/isotope signal contributes little (2%) to the bottom water DIC, and the influence on the in δ13Ctest ratios is even less (<1%).
Publications
- Development of a high-pressure aquarium for use under transmitted light (patent AZ 10 2014 018 858.2)
Jutta Wollenburg and Erich Dunker
- Novel high-pressure culture experiments on deep-sea benthic foraminifera – evidence for methane seepage-related δ13C of Cibicides wuellerstorfi, Marine Micropaleontology 117, 47- 64, 2015
Jutta E. Wollenburg, Markus Raitzsch, Ralf Tiedemann
(See online at https://doi.org/10.1016/j.marmicro.2015.04.003)
