ThermoSill - Effects of local heating due to sill intrusion on subsurface sedimentary microbial activity

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The recent IODP Exp. 385 (Guaymas Basin Tectonics and Biosphere, Sept.-Nov. 2019) drilled eight sites in Guaymas Basin, a young marginal rift basin in the Gulf of California, characterized by active seafloor spreading and rapid sediment deposition, including organic-rich sediments derived from biologically highly productive overlying waters and terrigenous sediments from nearby continental margins. The expedition cored sequences of organic-rich sediments with sill intrusions. Microbiology was one of the key research topics of this expedition. A large sample set for quantification of sulfate reduction rates, the quantitatively most important electron acceptor process in marine sediments, was collected and is now at GFZ. The proposed ThermoSill project will initially focus on the general characteristics of microbial sulfate reduction in these cores as well as the effects of pressure and temperature. The drill cores have experienced vastly different geothermal gradients, ranging from ca. 200 to over 800 °C km-1, and the on-board geochemical data already indicate a great diversity of biogeochemical processes taking place. In deep sea sediments, especially those buried to great depth and therefore temperature, organic substrates like short-chain organic acids are a generally a limiting factor. Because the sediments in Guaymas Basin are hydrothermally influenced, thermogenic cracking of macromolecular sedimentary organic matter provides abundant microbial substrates. ThermoSill will investigate whether this abundance of substrate has an effect leads to a preferential degradation of specific substrates and therefore shed light onto processes at the upper temperature limit of life. Particular attention will be paid to anaerobic oxidation of methane. In the final part of the project, the emplacement of sills and concomitant temperature increase will be simulated by heating experiments. These experiments will be accompanied by pyrolysis experiments and kinetic modelling to test whether such short-time heating can have an effect on the microbial community over geologic timescales. The proposed project is directly relevant to the objectives of the expedition. It is also complementary to an ongoing project that studies the effects of pressure and temperature on microbial sulfate reduction in non-hydrothermally influenced sediments from Nankai Trough (IODP. Exp. 370).