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International Ocean Discovery Program (IODP) Expedition 385T aimed to take advantage of a transit of the R/V JOIDES Resolution from Antofagasta, Chile, to San Diego, California (USA), to accomplish new sampling and data collection from legacy borehole observatories in Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) Holes 504B and 896A on the southern flank of the Costa Rica Rift. In addition, the US Science Support Program organized the participation of 3 Outreach Officers to evaluate the performance of the JOIDES Resolution Outreach Officer program as well as 2 educators and 12 undergraduate students for a shipboard “JR Academy.” Our scientific objectives were to collect (1) new Formation MicroScanner logs from Hole 504B for improving lithologic interpretations of crustal architecture at this archetype deep oceanic crust hole and (2) fluid samples from both holes for evaluating the crustal deep biosphere in deep and warm oceanic crust. These operations in Holes 504B and 896A have direct relevance to Challenges 5, 6, 9, 10, 13, and 14 of the IODP 2013–2023 Science Plan. Accomplishing both of these scientific objectives required the removal of old wireline CORK observatories, including associated inflatable packers that were installed in the cased boreholes in 2001. The fluid sampling plan also included testing a new Multi-Temperature Fluid Sampler. Despite successfully removing the CORK wellhead platforms from both holes, we were unable to remove the packers stuck in casing at both locations after 48 h of milling operations in Hole 504B and 2 h of milling operations in Hole 896A, thus precluding accomplishing any of the scientific objectives of the expedition. We provide an assessment of the final state of the holes and recommendations for possible future operations. 

International Ocean Discovery Program (IODP) Expedition 385T will revisit two Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) legacy sites—Holes 504B and 896A on the Costa Rica Rift flank—to advance lithostratigraphic, hydrogeological, and deep biosphere studies of upper oceanic crust. Hole 504B has served as a standard reference site for upper oceanic crust for decades despite low core recovery during drilling operations. Hole 896A serves as an analog site of crustal alteration for examining biogeography in the crustal deep biosphere. During Expedition 385T, we will advance lithostratigraphic records of in situ crustal architecture through Formation MicroScanner (FMS) logging, with priority for these operations in Hole 504B. The new logs from Hole 504B will reveal whether unrecovered intervals are highly fractured and/or brecciated and whether alteration style and intensity are correlated to volcanic architecture, which will allow for assessment of the hypothesis that hydrothermal alteration and mineralization style are spreading-rate dependent. We will also advance crustal hydrogeological and deep biosphere research through temperature logging and water sampling in both holes, with priority for these operations in Hole 896A. The new FMS-based lithostratigraphy coupled with new fluid assessment will also allow for improvements on the thermal limits of microbial life and seawater-basalt reactions. These operations in Holes 504B and 896A have direct relevance to Challenges 5, 6, 9, 10, 13, and 14 of the IODP 2013–2023 Science Plan. To achieve these data and sample recoveries from these legacy sites, existing wireline observatories installed in both holes will be removed and the remaining cased holes will be left open for possible future installation of next-generation observatories. The expedition will be implemented as an abbreviated (10 operational days) expedition with no new coring. 

Seventeen holes were drilled at nine sites across Atlantis Massif. Use of seabed drills enabled collection of more than 57 m of core, with borehole penetration ranging from 1.30 to 16.44 meters below seafloor and core recoveries as high as 74.76% of total penetration. This high level of recovery of shallow mantle sequences is unprecedented in the history of ocean drilling. The cores recovered along the southern wall of Atlantis Massif have highly heterogeneous lithologies, types of alteration, and degrees of deformation. The ultramafic rocks are dominated by harzburgites with intervals of dunite and minor pyroxenite veins, as well as gabbroic rocks occurring as melt impregnations and veins, all of which provide information about early magmatic processes and the magmatic evolution in the southernmost portion of Atlantis Massif. Dolerite dikes and basaltic rocks represent the latest stage of magmatic activity. Overall, the ultramafic rocks recovered during Expedition 357 reveal a high degree of serpentinization, as well as metasomatic talc-amphibole-chlorite overprinting and local rodingitization. Metasomatism postdates an early phase of serpentinization but predates late-stage intrusion and alteration of dolerite dikes and the extrusion of basalt. The intensity of alteration is generally lower in the gabbroic and doleritic rocks. Chilled margins in dolerite intruded into talc-amphibole-chlorite schists are observed at the most eastern Site M0075. Deformation in Expedition 357 cores is variable and dominated by brecciation and formation of localized shear zones; the degree of carbonate veining was lower than anticipated. All types of variably altered and deformed ultramafic and mafic rocks occur as components in sedimentary breccias and as fault scarp rubble. The sedimentary cap rocks include basaltic breccias with a carbonate sand matrix and/or fossiliferous carbonate. Fresh glass on basaltic components was observed in some of the breccias. The expedition also successfully applied new technologies, namely (1) extensively using an in situ sensor package and water sampling system on the seabed drills for evaluating real-time dissolved oxygen and methane, pH, oxidation-reduction potential (ORP), temperature, and conductivity during drilling; (2) deploying a borehole plug system for sealing seabed drill boreholes at four sites to allow access for future sampling; and (3) proving that tracers can be delivered into drilling fluids when using seabed drills. The rock drill sensor packages and water sampling enabled detection of elevated dissolved methane and hydrogen concentrations during and/or after drilling, with “hot spots” of hydrogen observed over Sites M0068–M0072 and methane over Sites M0070–M0072. Shipboard determination of contamination tracer delivery confirmed appropriate sample handling procedures for microbiological and geochemical analyses, which will aid all subsequent microbiological investigations that are part of the science party sampling plans and will verify this new tracer delivery technology for seabed drill rigs. Shipboard investigation of biomass density in select samples revealed relatively low and variable cell densities, and enrichment experiments set up shipboard reveal growth. Thus, we anticipate achieving many of the deep biosphere–related objectives of the expedition through continued scientific investigation in the coming years. Finally, although not an objective of the expedition, we were serendipitously able to generate a high-resolution (20 m per pixel) multibeam bathymetry map across the entire Atlantis Massif and the nearby fracture zone, MAR, and eastern conjugate, taking advantage of weather and operational downtime. This will assist science party members in evaluating and interpreting tectonic and mass-wasting processes at Atlantis Massif. 

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 to study the links between serpentinization processes and microbial activity in the shallow subsurface of highly altered ultramafic and mafic sequences that have been uplifted to the seafloor along a major detachment fault zone. The primary goals of this expedition were to (1) examine the role of serpentinization in driving hydrothermal systems, sustaining microbial communities, and sequestering carbon; (2) characterize the tectonomagmatic processes that lead to lithospheric heterogeneities and detachment faulting; and (3) assess how abiotic and biotic processes change with variations in rock type and progressive exposure on the seafloor. To accomplish these objectives, we developed a coring and sampling strategy based around the use of seabed rock drills—the first time that such systems have been used in the scientific ocean drilling programs. This technology was chosen in hopes of achieving high recovery of the carbonate cap sequences and intact contact and deformation relationships. The expedition plans also included several engineering developments to assess geochemical parameters during drilling; sample bottom water before and after drilling; supply synthetic tracers during drilling for contamination assessment; gather downhole electrical resistivity and magnetic susceptibility logs for assessing fractures, fluid flow, and extent of serpentinization; and seal boreholes to provide opportunities for future experiments. Seventeen holes were drilled at nine sites across Atlantis Massif, with two sites on the eastern end of the southern wall (Sites M0068 and M0075), three sites in the central section of the southern wall north of the Lost City hydrothermal field (Sites M0069, M0072, and M0076), two sites on the western end (Sites M0071 and M0073), and two sites north of the southern wall in the direction of the central dome of the massif and Integrated Ocean Drilling Program Site U1309 (Sites M0070 and M0074). Use of seabed rock drills enabled collection of more than 57 m of core, with borehole penetration ranging from 1.3 to 16.44 meters below seafloor and core recoveries as high as 75% of total penetration. This high level of recovery of shallow mantle sequences is unprecedented in the history of ocean drilling. The cores recovered along the southern wall of Atlantis Massif have highly heterogeneous lithologies, types of alteration, and degrees of deformation. The ultramafic rocks are dominated by harzburgites with intervals of dunite and minor pyroxenite veins, as well as gabbroic rocks occurring as melt impregnations and veins, all of which provide information about early magmatic processes and the magmatic evolution in the southernmost portion of Atlantis Massif. Dolerite dikes and basaltic rocks represent the latest stage of magmatic activity. Overall, the ultramafic rocks recovered during Expedition 357 revealed a high degree of serpentinization, as well as metasomatic talc-amphibole-chlorite overprinting and local rodingitization. Metasomatism postdates an early phase of serpentinization but predates late-stage intrusion and alteration of dolerite dikes and the extrusion of basalt. The intensity of alteration is generally lower in the gabbroic and doleritic rocks. Chilled margins in dolerite intruded into talc-amphibole-chlorite schists are observed at the most eastern Site M0075. Deformation in Expedition 357 cores is variable and dominated by brecciation and formation of localized shear zones; the degree of carbonate veining was lower than anticipated. All types of variably altered and deformed ultramafic and mafic rocks occur as components in sedimentary breccias and as fault scarp rubble. The sedimentary cap rocks include basaltic breccias with a carbonate sand matrix and/or fossiliferous carbonate. Fresh glass on basaltic components was observed in some of the breccias. The expedition also successfully applied new technologies, namely (1) extensively using an in situ sensor package and water sampling system on the seabed drills for evaluating real-time dissolved oxygen and methane, pH, oxidation-reduction potential, temperature, and conductivity during drilling; (2) deploying a borehole plug system for sealing seabed drill boreholes at four sites to allow access for future sampling; and (3) proving that tracers can be delivered into drilling fluids when using seabed drills. The rock drill sensor packages and water sampling enabled detection of elevated dissolved methane and hydrogen concentrations during and/or after drilling, with “hot spots” of hydrogen observed over Sites M0068–M0072 and methane over Sites M0070–M0072. Shipboard determination of contamination tracer delivery confirmed appropriate sample handling procedures for microbiological and geochemical analyses, which will aid all subsequent microbiological investigations that are part of the science party sampling plans, as well as verify this new tracer delivery technology for seabed drill rigs. Shipboard investigation of biomass density in select samples revealed relatively low and variable cell densities, and enrichment experiments set up shipboard reveal growth. Thus, we anticipate achieving many of the deep biosphere–related objectives of the expedition through continued scientific investigation in the coming years. Finally, although not an objective of the expedition, we were serendipitously able to generate a high-resolution (20 m per pixel) multibeam bathymetry map across the entire Atlantis Massif and the nearby fracture zone, Mid-Atlantic Ridge, and eastern conjugate, taking advantage of weather and operational downtime. This will assist science party members in evaluating and interpreting tectonic and mass-wasting processes at Atlantis Massif. 

International Ocean Discovery Program (IODP) Expedition 357 will be implemented as a Mission Specific Platform (MSP) expedition that will address two exciting discoveries in mid-ocean-ridge research: off-axis, serpentinite-hosted hydrothermal activity exemplified by the Lost City hydrothermal field (LCHF) and the significance of tectono-magmatic processes in forming and exposing heterogeneous mafic and variably serpentinized ultramafic lithosphere that are key components of slow- and ultraslow-spreading ridges. Serpentinization is a fundamental process that controls rheology and geophysical properties of the oceanic lithosphere and has major consequences for heat flux, geochemical cycles, and microbial activity in a wide variety of environments. However, we currently have no constraints on the nature and distribution of microbial communities in ultramafic subsurface environments. Our planned drilling focuses on (1) exploring the extent and activity of the subsurface biosphere in young ultramafic and mafic seafloor; (2) quantifying the role of serpentinization in driving hydrothermal systems, in sustaining microbiological communities, and in the sequestration of carbon in ultramafic rocks; (3) assessing how abiotic and biotic processes change with aging of the lithosphere and with variations in rock type; and (4) characterizing tectono-magmatic processes that lead to lithospheric heterogeneities and the evolution of hydrothermal activity associated with detachment faulting. This expedition will be the first IODP expedition to utilize seafloor drill technology (MeBo and BGS Seafloor Rockdrill 2) to core a series of shallow (50–80 m) holes across Atlantis Massif—an oceanic core complex (30°N, Mid-Atlantic Ridge), where detachment faulting exposes mafic and ultramafic lithologies on the seafloor. We aim to recover in situ sequences of sediments, hydrothermal deposits/veins, and basement rocks that comprise a broad zone of detachment faulting across (1) a spreading-parallel (east–west) profile along the southern wall and at varying distances from the LCHF and (2) a ridge-parallel (north–south) profile into the center of the massif, where the dominant rock type changes from ultramafic to mafic. Drilling the east–west profile will allow us to evaluate how microbial communities evolve with variations in hydrothermal activity and with age of emplacement on the seafloor. We aim to compare microbial activity and diversity in areas of diffuse, H2-rich fluid flow and carbonate precipitation with communities in areas away from the active hydrothermal system and with variable substrates and crustal ages. By quantifying the extent and evolution of carbonate precipitation we will evaluate the potential for natural CO2 sequestration in serpentinizing peridotites. Drilling the north–south profile will allow us to evaluate the nature of the deep biosphere in varying lithologies and to assess the role of the differing rheologies of gabbros and serpentinized ultramafic rocks in localizing detachment faults. This expedition will also include engineering developments to sample bottom waters before and after drilling and to monitor methane, dissolved oxygen, redox, conductivity, temperature, and depth while drilling. In addition, seafloor operations will include deploying borehole plugs and swellable packers to seal the holes at high-priority sites after drilling to provide opportunities for future hydrogeological and microbiological experiments.