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1. Guaymas Basin Tectonics and Biosphere

   https://doi.org/10.14379/iodp.proc.385.2021  

   Teske, A.; Lizarralde, D.; Höfig, T.W. (September 2021, Proceedings of the International Ocean Discovery Program)

   null (Ed.)

   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 sill-sediment 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 half-length 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. 

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2. Expedition 385 Preliminary Report: Guaymas Basin Tectonics and Biosphere

   https://doi.org/10.14379/iodp.pr.385.2020  

   Teske, A.; Lizarralde, D.; Höfig, T.W. (December 2020, Preliminary report)

   null (Ed.)

   International Ocean Discovery Program (IODP) Expedition 385 drilled organic-rich sediments with sill intrusions on the flanking regions and in the northern axial graben in Guaymas Basin, a young marginal rift basin in the Gulf of California. Guaymas Basin is characterized by a widely distributed, intense heat flow and widespread off-axis magmatism expressed by a dense network of sill intrusions across the flanking regions, which is in contrast to classical mid-ocean ridge spreading centers. The numerous off-axis sills provide multiple 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 the flanking regions of Guaymas Basin, covering a distance of ~81 km from the from the northwest to the southeast. Adjacent Sites U1545 and U1546 recovered the oldest and thickest sediment successions (to ~540 meters below seafloor [mbsf]; equivalent to the core depth below seafloor, Method A [CSF-A] scale), one with a thin sill (a few meters in thickness) near the drilled bottom (Site U1545), and one with a massive, deeply buried sill (~356–430 mbsf) that chemically and physically affects the surrounding sediments (Site U1546). Sites U1547 and U1548, located in the central part of the northern Guaymas Basin segment, were drilled to investigate a 600 m wide circular mound (bathymetric high) and its periphery. The dome-like structure is outlined by a ring of active vent sites called Ringvent. It is underlain by a remarkably thick sill at shallow depth (Site U1547). Hydrothermal gradients steepen at the Ringvent periphery (Holes U1548A–U1548C), which in turn shifts the zones of authigenic carbonate precipitation and of highest microbial cell abundance toward shallower depths. The Ringvent sill was drilled several times and yielded remarkably diverse igneous rock textures, sediment–sill interfaces, and hydrothermal alteration, reflected by various secondary minerals in veins and vesicles. Thus, the Ringvent sill became the target of an integrated sampling and interdisciplinary research effort that included geological, geochemical, and microbiological specialties. The thermal, lithologic, geochemical, and microbiological contrasts between the two deep northwestern sites (U1545 and U1546) and the Ringvent sites (U1547 and U1548) form the scientific centerpiece of the expedition. These observations are supplemented by results from sites that represent attenuated cold seepage conditions in the central basin (Site U1549), complex and disturbed sediments overlying sills in the northern axial trough (Site U1550), terrigenous sedimentation events on the southeastern flanking regions (Site U1551), and hydrate occurrence in shallow sediments proximal to the Sonora margin (Site U1552). 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 of authigenic mineral formation in sediments, and (4) yield new insights into many geochemical and geophysical aspects of both architecture and sill–sediment interaction in a nascent spreading center. The generally high quality and high degree of completeness of the shipboard datasets present opportunities for interdisciplinary and multidisciplinary collaborations during shore-based studies. In comparison to Deep Sea Drilling Project Leg 64 to Guaymas Basin in 1979, sophisticated drilling strategies (for example, the advanced piston corer [APC] and half-length APC systems) and numerous analytical innovations have greatly improved sample recovery and scientific yield, particularly in the areas of organic geochemistry and microbiology. For example, microbial genomics did not exist 40 y ago. However, these technical refinements do not change the fact that Expedition 385 will in many respects build on the foundations laid by Leg 64 for understanding Guaymas Basin, regardless of whether adjustments are required in the near future. 

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3. Data report: concentration and carbon isotopic composition in pore fluids from IODP Expedition 385

   https://doi.org/10.14379/iodp.proc.385.201.2022  

   Torres, M.E.; Kim, J.H. (October 2022, Proceedings of the International Ocean Discovery Program)

   During International Ocean Discovery Program (IODP) Expedition 385, sediments from Guaymas Basin were sampled with the goal of understanding the role of sill emplacement and fluid flow, as well as its associated temperature and fluid circulation regimes, on subsurface carbon mobilization and preservation of organic-rich sediments. We report on the concentration and isotopic composition of dissolved inorganic carbon (DIC) in pore fluids from Sites U1545 and U1546, which were drilled in the northern basin; Sites U1547 and U1548, which sampled an active hydrothermal vent site; Sites U1549 and U1552, which targeted cold seeps; and Site U1550, which was drilled in the axial trough. There is large variability in the DIC concentrations. The highest values were recorded at Sites U1549, U1550 (up to ~75 mM), and U1552 (~169 mM). The isotopic composition of the DIC (δ13CDIC) ranges −23.50‰ to 22.64‰ referenced to Vienna Peedee belemnite. At all locations outside Ringvent Site U1547, depletions in δ13CDIC values typically coincide with the sulfate–methane transition zone (SMTZ). Enrichment in δ13CDIC above seawater values, indicative of ongoing microbial methanogenesis, was recorded below the SMTZ at all locations except Ringvent. 

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4. Data report: detailed lithologic columns for IODP Expedition 385 and DSDP Leg 64 sites in the Guaymas Basin, Gulf of California, Mexico

   https://doi.org/10.14379/iodp.proc.385.202.2023  

   Persad, L.D.; Marsaglia, K.M. (June 2023, Proceedings of the International Ocean Discovery Program Expedition reports)

   The Guaymas Basin, in the central Gulf of California, is a marginal ocean basin characterized by active seafloor spreading and high sedimentation rates. It has been the focus of two drilling expeditions, Deep Sea Drilling Project (DSDP) Leg 64 and International Ocean Discovery Program (IODP) Expedition 385. Expedition 385 recovered over 4 km of middle Pleistocene to Holocene core at eight drill sites, providing only simplistic stratigraphic columns that were broadly divided into as many as four lithostratigraphic subunits largely based on diagenetic modifications of sediments (authigenic carbonate and silica). For this study, shipboard sedimentologic descriptions of these subunits were used to create new, more detailed lithostratigraphic columns at an approximately decimeter (core) scale for correlation purposes and sedimentary interpretation. This was accomplished through examination of slabbed core images, visual core description sheets, and a shipboard lithologic database. The new columns provide more detailed downhole variability in lithology. The lithologic classification scheme for Expedition 385 was then integrated with that of sites previously drilled during Leg 64 to translate published visual core descriptions so as to uniformly generate comparable stratigraphic columns for both sets of drill holes. These newly compiled and tabulated data provide a more detailed picture of stratigraphic variation of lithology on a core by core basis across the basin. 

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5. Mid-Norwegian Margin Magmatism and Paleoclimate Implications

   https://doi.org/10.14379/iodp.proc.396.2023  

   Planke, S.; Berndt, C.; Alvarez Zarikian, C.A. (April 2023, Proceedings of the International Ocean Discovery Program Expedition reports)

   The opening of the northeast Atlantic, starting around 56 My ago, was associated with the emplacement of the North Atlantic Igneous Province, including the deposition of voluminous extrusive basaltic successions and intrusion of magma into the surrounding sedimentary basins. The mid-Norwegian Margin is a global type example of such a volcanic rifted margin and is well suited for scientific drilling with its thin sediment cover and good data coverage. During International Ocean Discovery Program Expedition 396, 21 boreholes were drilled at 10 sites in five different geological settings on the mid-Norwegian Margin. The boreholes sampled a wide variety of igneous and sedimentary settings ranging from lava flow fields to hydrothermal vent complexes, along with thick successions of Upper Paleocene and Lower Eocene strata. A comprehensive suite of wireline logs was collected in eight boreholes. These data provide new constraints for geodynamic models to explain the rapid emplacement of large igneous provinces and will also allow us to test the hypothesis that the Paleocene–Eocene Thermal Maximum (PETM) was caused by hydrothermal release of carbon in response to magmatic intrusions and/or flood basalt eruption. Successful drilling and high core recovery of target intervals at all nine primary sites and one additional alternate site will allow us to achieve these goals during postcruise work. Expedition 396 highlights include (1) drilling and coring a unique, multihole transect across a supra-sill hydrothermal system and crater that was filled in during the PETM, (2) drilling and coring all the major lithofacies at each of the component parts of a volcanic rifted margin from terrestrial to deep marine, and (3) acquiring excellent petrophysical data and imaging support for core analyses of complex and diverse volcanic and volcaniclastic intervals across the terrestrial to marine transition. 

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