Expeditions 390 and 393 are a multidisciplinary and joint scientific ocean drilling project that aims to recover complete sedimentary sections and ~200 m of oceanic crust along a crustal age transect at ~31°S across the South Atlantic Ocean to: (1) investigate the history of low-temperature hydrothermal interactions between the aging ocean crust and the evolving South Atlantic Ocean; (2) quantify past hydrothermal contributions to global geochemical cycles; (3) investigate sediment and basement-hosted microbial community variation with substrate composition and age in the low energy South Atlantic Gyre subseafloor biosphere; and (4) investigate the responses of Atlantic Ocean circulation patterns and the Earth’s climate system to rapid climate change, including elevated CO2 during the Cenozoic. The expeditions will occur from 5 October to 5 December 2020 (Expedition 390) and 6 April to 6 June 2021 (Expedition 393). Opportunities exist for researchers (including graduate students) in all shipboard specialties, including but not limited to sedimentologists, petrologists, micropaleontologists, paleomagnetists, petrophysicists, geophysicists, inorganic and organic geochemists, and microbiologists. The deadline to apply is August 1, 2019.