Expedition 357 summary

International Ocean Discovery Program (IODP) Expedition 357 successfully cored an east–west transect across the southern wall of Atlantis Massif on the western flank of the Mid-Atlantic Ridge (MAR) to study the links between serpentinization processes and microbial activity in the shallow subsurface of highly altered ultra- mafic and mafic sequences that have been uplifted to the seafloor along a major detachment fault zone. The primary goals of this ex- pedition were to (1) examine the role of serpentinization in driving hydrothermal systems, sustaining microbial communities, and se- questering carbon; (2) characterize the tectonomagmatic processes that lead to lithospheric heterogeneities and detachment faulting; and (3) assess how abiotic and biotic processes change with varia- tions in rock type and progressive exposure on the seafloor. To ac- complish these objectives, we developed a coring and sampling strategy centered on the use of seabed drills—the first time that such systems have been used in the scientific ocean drilling pro- grams. This technology was chosen in the hope of achieving high recovery of the carbonate cap sequences and intact contact and de- formation relationships. The expedition plans also included several engineering developments to assess geochemical parameters during drilling; sample bottom water before, during, and after drilling; sup- ply synthetic tracers during drilling for contamination assessment; acquire in situ electrical resistivity and magnetic susceptibility mea- surements for assessing fractures, fluid flow, and extent of ser- pentinization; and seal boreholes to provide opportunities for future experiments. (...)