Please consider submitting your abstract to Goldschmidt Session 15h: Geobiology of the Modern Convenors: Fumio Inagaki, Kai-Uwe Hinrichs, Chuanlun Zhang, Brian Hedlund, Fengping Wang, Stefan Sievert, Karen Lloyd, Benedicte Menez. Keynote: Victoria Orphan (Caltech). Abstract: The deep ocean and subseafloor biosphere is vast and diverse, harboring many uncultured clades of microorganisms. Energy and nutrients are supplied primarily by transformations of biologically and/or abiotically produced organic matter and the redox chemistry of water-rock interactions. Recent technological breakthroughs in biogeochemistry, geomicrobiology and molecular biology, as well as in obtaining pristine samples from the hadal zone of the ocean and the deep-subsurface biosphere enable us to address essential questions about microbial community composition, biogeochemical contribution, and limits to microbial ecosystems in the deep ocean and subseafloor biosphere. In this session, we would like to highlight studies broadly focusing on the triangular relationship between microbiology, geochemistry, and geophysics in (but not limited to) diverse oceanic and subseafloor biosphere settings. Given the slow pace of deep life activity and the associated challenges for detecting biosignatures in the most extreme sections of the Earth’s microbial ecosystems, we also encourage submissions addressing the exploration of biomarkers. Abstract deadline: April 1, 2017.