Search

International Ocean Discovery Program Expedition 385 drilled organic-rich sediments and intruded sills in the off-axis region and axial graben of the northern spreading segment of Guaymas Basin, a young marginal seafloor spreading system in the Gulf of California. Guaymas Basin is characterized by high heat flow and magmatism in the form of sill intrusions into sediments, which extends tens of kilometers off axis, in contrast with the localized volcanism found at most mid-ocean ridge spreading centers. Sill intrusions provide transient heat sources that mobilize buried sedimentary carbon, in part as methane and other hydrocarbons, and drive hydrothermal circulation. The resulting thermal and geochemical gradients shape abundance, composition, and activity of the deep subsurface biosphere of the basin. Drill sites extend over a broad region of Guaymas Basin. Adjacent Sites U1545 and U1546, located ~52 km northwest of the northern Guaymas Basin axial graben, recovered sediment successions to ~540 meters below seafloor (mbsf) (equivalent to the core depth below seafloor, Method A [CSF-A] scale), including a thin sill (a few meters thick) drilled near the bottom of Site U1545 and a massive sill (~355-430 mbsf) at Site U1546 that chemically and physically affects the surrounding sediments. Sites U1547 and U1548, located ~27 km northwest of the axial graben, were drilled to investigate an active sill-driven hydrothermal system evident at the seafloor as an 800 m wide, circular bathymetric high called Ringvent because of its outline of a ring of active vent sites. Ringvent is underlain by a thick sill at shallow depth (Site U1547). Geothermal gradients steepen toward the Ringvent periphery (Holes U1548A-U1548C), and the zones of authigenic carbonate precipitation and of highest microbial cell abundance correspondingly shallow toward the periphery. The underlying sill was drilled several times and yielded diverse igneous rock textures, sediment/sill interfaces, and alteration minerals in veins and vesicles. The Ringvent sill became the target of an integrated, interdisciplinary sampling and research effort that included geological, geochemical, and microbiological components. The thermal, lithologic, geochemical, and microbiological contrasts between the northwestern sites (U1545 and U1546) and the Ringvent sites (U1547 and U1548) form the core scientific observations informing the direct influence of sillsediment interaction. These observations are supplemented by results from sites that exhibit persistent influence of thermally equilibrated sill intrusions, including supporting long-lived methane cold seeps, as observed at off-axis Sites U1549 and U1552, and the persistent geochemical record of hydrocarbon formation near the sill/sediment contact, as observed at the northern axial trough Site U1550, which confirms observations from Deep Sea Drilling Project (DSDP) Leg 64. Drilling at Site U1551 ~29 km southeast of the axial graben was not successful due to unstable shallow sands, but it confirmed the dominant influence of gravity-flow sedimentation processes southeast of the axial graben. The scientific outcomes of Expedition 385 will (1) revise long-held assumptions about the role of sill emplacement in subsurface carbon mobilization versus carbon retention, (2) comprehensively examine the subsurface biosphere of Guaymas Basin and its responses and adaptations to hydrothermal conditions, (3) redefine hydrothermal controls on authigenic mineral formation in sediments, and (4) yield new insights into the long term influence of sill-sediment interaction on sediments deposited at the earliest stages of seafloor spreading, that is, when spreading centers are proximal to a continental margin. The generally high quality and high degree of completeness of the shipboard data sets present opportunities for inter- and multidisciplinary collaborations during shore-based studies. In comparison to DSDP Leg 64 to Guaymas Basin in 1979, continuous availability of sophisticated drilling strategies (e.g., the advanced piston corer [APC] and halflength APC systems) and numerous analytical innovations greatly improved sample recovery and scientific yield, particularly in the areas of organic geochemistry and microbiology. For example, microbial metagenomics did not exist 40 y ago. However, these technical refinements do not change the fact that Expedition 385 in many respects builds on the foundations of understanding laid by Leg 64 drilling in Guaymas Basin. [ABSTRACT FROM AUTHOR]

International Ocean Discovery Program Expedition 363 sought to document the regional expression and driving mechanisms of climate variability (e.g., temperature, precipitation, and productivity) in the Indo-Pacific Warm Pool (IPWP) as it relates to the evolution of Neogene climate on millennial, orbital, and geological timescales. To achieve our objectives, we selected sites with a wide geographical distribution and variable oceanographic and depositional settings. Nine sites were cored during Expedition 363, recovering a total of 6956 m of sediment in 875-3421 m water depth with an average recovery of 101.3% during 39.6 days of on-site operations. Two moderate sedimentation rate (~3-10 cm/ky) sites are located off northwestern Australia at the southwestern maximum extent of the IPWP and span the late Miocene to present. Seven of the nine sites are situated at the heart of the Western Pacific Warm Pool (WPWP), including two sites on the northern margin of Papua New Guinea with very high sedimentation rates (>60 cm/ky) spanning the past ~450 ky, two sites in the Manus Basin (north of Papua New Guinea) with moderate sedimentation rates (~4-14 cm/ky) recovering upper Pliocene to present sequences, and three sites with low sedimentation rates (~1-3 cm/ky) on the southern and northern Eauripik Rise spanning the early Miocene to present. The wide spatial distribution of the cores, variable accumulation rates, exceptional biostratigraphic and paleomagnetic age constraints, and mostly excellent or very good foraminifer preservation will allow us to trace the evolution of the IPWP through the Neogene at different temporal resolutions, meeting the primary objectives of Expedition 363. Specifically, the high-sedimentation rate cores off Papua New Guinea will allow us to better constrain mechanisms influencing millennial-scale variability in the WPWP, their links to high-latitude climate variability, and implications for temperature and precipitation in this region under variable mean-state climate conditions. Furthermore, the high accumulation rates offer the opportunity to study climate variability during previous warm periods at a resolution similar to that of existing studies of the Holocene. With excellent recovery, Expedition 363 sites are suitable for detailed paleoceanographic reconstructions at orbital and suborbital resolution from the middle Miocene to Pleistocene and thus will be used to refine the astronomical tuning, biostratigraphy, magnetostratigraphy, and isotope stratigraphy of hitherto poorly constrained intervals within the Neogene timescale (e.g., the late Miocene) and to reconstruct the history of the Asian-Australian monsoon and the Indonesian Throughflow on orbital and tectonic timescales. Results from high-resolution interstitial water sampling at selected sites will be used to reconstruct density profiles of the western equatorial Pacific deep water during the Last Glacial Maximum. Additional geochemical analyses of interstitial water samples in this tectonically active region will be used to investigate volcanogenic mineral and carbonate weathering and their possible implications for the evolution of Neogene climate. [ABSTRACT FROM AUTHOR]