Tracer studies of faunal organic matter transformation in marine sediments, and the wider role of seafloor faunal communities in C cycling

Burial of organic matter (OM) in marine sediments is a major factor in the global C cycle, and thus climate. Organisms living on the sea floor play major roles in determining the amount and composition of OM ultimately buried in sediments. The organisms depend on the OM reaching the sea floor as food, and also influence its fate by mixing and irrigating the sediments through feeding and burrowing activities. However, these diverse processes remain poorly understood, and the roles of benthic organisms in sedimentary OM cycling remain a major gap in current understanding of the marine C cycle. A new approach is required, at the interface between biology and geochemistry, that will combine tracer experiments with state-of-the-art analytical tools. The proposed work will investigate the links between the digestive processes of seafloor fauna and the organic geochemistry of the sediments. The overall role of fauna in the cycling and burial of OM will be investigated in a range of contrasting environments, from shallow estuarine to deeper continental margin sites. The work will be accomplished through feeding experiments performed on important individual species, as well as on whole, intact benthic faunal communities. The first part of the study will involve controlled feeding experiments on two large, ecologically important and contrasting species from Molenplaat in the Scheldt estuary in The Netherlands (in collaboration with scientists at NIOO). Specimens will be fed with algae labelled with the stable isotope 13C for up to 4 weeks. Using novel techniques, samples will be analysed for the 13C-labelled biochemicals they contain. Amino acids, carbohydrates and lipids, will for the first time be traced, from the food into gut contents, faunal tissues, faecal materials and surrounding sediments. The study of this range of biochemicals will yield a critical understanding of OM alteration brought about by faunal digestive process, the detail and completeness of which are unprecedented. The relatively long duration of experiments will allow the first direct links to be made between faunal digestion, sediment geochemistry and OM preservation. The second part of the study will involve similar experiments, but with whole, intact benthic communities under natural conditions. A primary aim will be to construct quantitative C budgets for contrasting coastal sites with different communities. These C budgets will be constructed by tracking the 13C label. The whole-community experiments will firstly be conducted in contrasting estuarine settings; Loch Etive (western Scotland), and the Ythan estuary (eastern Scotland). The two sites also have strongly contrasting benthic communities and physical conditions, but include fauna targeted in the preliminary microcosm experiments, thus providing a valuable ground-truthing. The Loch Etive experiment has already been carried out, and the samples are in hand. Sufficient samples are available to allow both the quantitative tracing of the 13C label, and analysis of some specimens for labelled amino acids, carbohydrates and lipids. The Ythan estuary experiment will be conducted in situ, using mesocosm technology available at the University of Aberdeen. Finally, the above experiments will be complimented by a suite of in situ experiments conducted on the continental margin of the Arabian Sea, as part of a Japan-led international collaboration. Among these will be 13C tracer studies of C cycling by benthic communities at sites ranging in depth from ca. 150m to 1850m, and with dramatically different sediment OM content, bottom-water oxygen levels and benthic communities. State-of-the-art seafloor experimentation will be made possible by the availability of a submersible and a Remotely Operated. I will contribute to the running of experiments, and also to sampling, but most significantly through the inclusion of 13C tracing into the full suite of major biochemicals.