Impact of temperature on microbial activity and carbon flow in deep hot sediments of the Guaymas Basin, Gulf of California (IODP Expedition 385)

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IODP Expedition 385 is investigating the tectonics and deep biosphere of the Guaymas Basin, a young marginal rift basin in the Gulf of California, at eight drilling sites. The geologic environment is characterized by active seafloor spreading and rapid deposition of sediments rich in organic material. Due to the high sedimentation rate, formation of new igneous crust involves the intrusion of sills into the thick overlying sediment burden. Sill emplacement presumably drives hydrothermal circulation, mobilizes carbon from sedimentary organic matter, and determines the distribution and activity of microbial communities. With the goal of investigating the interactions between tectonics and the deep biosphere, the expedition conducted in situ temperature measurements and collected numerous samples for microbiological and geochemical studies from deep sediments at temperatures as high as 120°C.Our goal with this project is to better understand microbial activity and carbon flux at the interface of the biological and geological carbon cycle. To do this, we are determining the relative abundance of microbial life in the form of dormant endospores, the production and turnover of the key metabolite acetate, and the sources and sinks of methane and higher hydrocarbon gases. Our investigations are focused on Sites U1545-U1548. Site U1545 and U1546 are located ~52 km northwest of the axial trench. The <0.44 million year old sediments are rich in organic material (up to 4 wt%) and hydrocarbon gases, and formation temperatures reach up to 120 °C. While Site U1546 contains a deep magma intrusion at a sediment depth of ~356-430 m, Site U1545 serves as a reference without sill intrusion. Sites U1547 and U1548 are located ~27 km northwest of the axial trench. They are located in the center and periphery of a circular hydrothermal hill called Ringvent. Both sites are affected by a slowly cooling sill intrusion encountered at ≥82 mbsf and ≥65 mbsf, respectively. The temperature range and high TOC content of the rapidly accumulating sediments are similar to those at Sites U1545 and U1546, but hydrothermal circulation has resulted in a geochemical regime dominated by sulfate-reducing conditions. This may provide a better energy supply for microorganisms compared to the northern sites. We will determine the distribution of endospores, acetate, and hydrocarbon gases in cores recovered from these sites and follow metabolic processes using stable isotope labeling experiments in the laboratory. The project aims to elucidate how microbial activity and carbon flux are determined by sediment temperature, geological and geochemical regimes.