The South Atlantic Transect (SAT) is a multidisciplinary scientific ocean drilling project that comprises four International Ocean Discovery Program (IODP) expeditions: engineering Expeditions 390C and 395E as well as Expeditions 390 and 393. Altogether, the expeditions aim to recover complete sedimentary sections and the upper 100-350 m of the underlying oceanic crust along a slow/intermediate spreading rate Mid-Atlantic Ridge crustal flow line at ~31°S. The sediments along this transect were originally spot cored more than 50 y ago during Deep Sea Drilling Project Leg 3 (December 1968-January 1969 to help verify the theories of seafloor spreading and plate tectonics. Given dramatic advances in drilling technology and analytical capabilities since Leg 3, many high-priority scientific objectives can be addressed by revisiting the transect. The SAT expeditions target six primary sites on 7, 15, 31, 49, and 61 Ma ocean crust, which fill critical gaps in our sampling of intact in situ ocean crust with regards to crustal age, spreading rate, and sediment thickness. Drilling these sites is required to investigate the history of the low-temperature hydrothermal interactions between the aging ocean crust and the evolving South Atlantic Ocean and quantify past hydrothermal contributions to global biogeochemical cycles. Samples from the transect of the previously unexplored sediment-and basalt-hosted deep biosphere beneath the South Atlantic Gyre are essential to refining global biomass estimates and examining microbial ecosystems' responses to variable conditions in a low-energy gyre and aging ocean crust. The transect is located near World Ocean Circulation Experiment Line A10, providing access to records of carbonate chemistry and deepwater mass properties across the western South Atlantic through key Cenozoic intervals of elevated atmospheric CO 2 and rapid climate change. Reconstruction of the history of the deep western boundary current and deepwater formation in the Atlantic basins will yield crucial data to test hypotheses regarding the role of evolving thermohaline circulation patterns in climate change and the effects of tectonic gateways and climate on ocean acidification.Engineering Expeditions 390C and 395E cored a single hole through the sediment/basement interface with the advanced piston corer/extended core barrel system at five of the six primary proposed SAT sites and installed a reentry system with casing either into basement or within 10 m of basement at each of those five sites. Expedition 390 (7 April-7 June 2022) conducted operations at three of the SAT sites, recovering 700 m of core (77% recovery) over 30.3 days of on-site operations. Sediment coring, basement drilling, and logging were conducted at two sites on 61 Ma crust, and sediment coring was completed at the 7 Ma crust site. At Site U1557 on 61 Ma crust, the drill bit was deposited on the seafloor prior to downhole logging, leaving Hole U1557D available for future deepening and to establish a legacy borehole for basement hydrothermal and mi...
