Please consider submitting your abstract to Goldschmidt Session 15b: Hydrothermal Biogeochemistry and GeobiologyConvenors: Christopher German, Wolfgang Bach, Costantino Vetriani, Donato Giovannelli. Keynote: Ken Takai (JAMSTEC). Abstract: Hydrothermal systems are increasingly recognized to involve biological, particularly microbial, aspects to their geochemical cycles – whether in the case of subseafloor water-rock interactions or in terms of the fate of their export products released into the overlying water column. Both the depth of hydrothermal systems and their geologic setting can play an important role in the nature of the systems that arise and their impact on the oceans – up to and including the photic zone. In the limit, such systems can also provide new insights to the origins of life on Earth and the potential for life-hosting habitats on other Ocean Worlds. This session will seek to bring together researchers interested in sharing their newest findings from a wide range of seafloor hydrothermal settings, from understudied shallow hydrothermal vents and other previously under-represented settings – ranging from the ultra-slow spreading Arctic ridges to subduction-related venting in the SW Pacific and from intra-plate volcanic hotspots to tectonically controlled fracturing of the ocean crust. We welcome contributions on the biogeochemistry and geobiology of hydrothermal systems throughout Earth’s oceans, as well as comparative studies ranging from continental geothermal studies to putative submarine venting beyond Earth. Abstract deadline: April 1, 2017.