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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. Data report: wavelength dispersive X-ray fluorescence in sediments from all IODP Expedition 385 sites in the Guaymas Basin

   Pastor, L. ; Boissier, A. ; Burin, C. ( June 2023 , Proceedings of the International Ocean Discovery Program Expedition reports)

   International Ocean Discovery Program (IODP) Expedition 385 focused on the impact of sill emplacement and fluid flow on subsurface biogeochemical processes in organic-rich sediments of the Guaymas Basin. It resulted in the drilling of eight sites with contrasted settings; Sites U1545 and U1546 were drilled in the northern basin and allowed the recovery of a deep sill at Site U1546; Sites U1547 and U1548 were drilled on an active hydrothermal vent called Ringvent; Sites U1549 and U1552 were drilled on the Sonora margin where cold seeps and hydrates were identified; Site U1550 was sampled as a replicate of Deep Sea Drilling Project (DSDP) Leg 64 Site 481 on an axial trough; and Site U1551 was drilled ~29 km southeast of the axial graben and was mainly terrigenous. The quantitative X-ray fluorescence data presented here were measured on squeeze cakes retrieved after squeezing of whole-round core segments for pore water extraction. Major elements (Al, Ca, Cl, Fe, K, Mg, Mn, Na, P, Si, Ti, and S) as well as minor/trace elements (As, Ba, Br, Ce, Co, Cr, Cu, La, Mo, Nb, Ni, Pb, Rb, Sc, Sr, V, Y, Zn, and Zr) are provided in this data report. 

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

   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. Expedition 393 Preliminary Report: South Atlantic Transect 2

   Teagle, D.A.H. ; Reece, J. ; Coggon, R.M. ; Sylvan, J.B. ; Christeson, G.L. ; Williams, T.J. ; Estes, E.R. ( January 2023 , Preliminary report)

   The South Atlantic Transect (SAT) is a multidisciplinary scientific ocean drilling experiment designed to investigate the evolution of the oceanic crust and overlying sediments across the western flank of the Mid-Atlantic Ridge. This project comprises four International Ocean Discovery Program expeditions: fully staffed Expeditions 390 and 393 (April–August 2022) built on engineering preparations during Expeditions 390C and 395E that took place without science parties during the height of the Coronavirus Disease 2019 (COVID-19) pandemic. Through operations along a crustal flow line at ~31°S, the SAT recovered complete sedimentary sections and the upper ~40–340 m of the underlying ocean crust formed at a slow to intermediate spreading rate at the Mid-Atlantic Ridge over the past ~61 My. 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. The SAT expeditions targeted six primary sites on 7, 15, 31, 49, and 61 Ma ocean crust that 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 was required to investigate the history, duration, and intensity of the low-temperature hydrothermal interactions between the aging ocean crust and the evolving South Atlantic Ocean. This knowledge will improve the quantification of past hydrothermal contributions to global biogeochemical cycles and help develop a predictive understanding of the impacts of variable hydrothermal processes and exchanges. Samples from the transect of the previously unexplored sediment- and basalt-hosted deep biosphere beneath the South Atlantic Gyre are essential to refine global biomass estimates and examine 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, which provides a baseline for records of carbonate chemistry and deepwater mass properties across the western South Atlantic through key Cenozoic intervals of elevated atmospheric CO2 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. During engineering Expeditions 390C and 395E, a single hole was cored through the sediment cover and into the uppermost rocks of the ocean crust with the advanced piston corer (APC) and extended core barrel (XCB) systems at five of the six primary proposed SAT sites. Reentry systems with casing were then installed 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%) over 30.3 days of on-site operations. Sediment coring, basement coring, and wireline logging were conducted at two sites on 61 Ma crust (Sites U1556 and U1557), and sediment coring was completed at the 7 Ma Site U1559. Expedition 393 operated at four sites, drilling in 12 holes to complete this initial phase of the SAT. Complete sedimentary sections were collected at Sites U1558, U1583, and U1560 on 49, 31, and 15 Ma crust, respectively, and together with 257.7 m of sediments cored during earlier operations, more than 600 m of sediments was characterized. The uppermost ocean crust was drilled at Sites U1558, U1560, and U1583 with good penetration (~130 to ~204 meters subbasement), but at the youngest ~7 Ma Site U1559, only ~43 m of basement penetration was achieved in this initial attempt. Geophysical wireline logs were aquired at Sites U1583 and U1560. Expeditions 390 and 393 established legacy sites available for future deepening and downhole basement hydrothermal and microbiological experiments at Sites U1557, U1560, and U1559 on 61, 15, and 7 Ma crust, respectively. 

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5. Expedition 385 Scientific Prospectus: Guaymas Basin Tectonics and Biosphere

   https://doi.org/10.14379/iodp.sp.385.2018  

   Teske, A. ; Lizarralde, D. ; Höfig, T.W. ( November 2018 , Scientific prospectus)

   null (Ed.)

   The Guaymas Basin in the Gulf of California is a young marginal rift basin characterized by active seafloor spreading and rapid deposition of organic-rich sediments from highly productive overlying waters. The high sedimentation rates in combination with an active spreading system produce distinct oceanic crust where the shallowest magmatic emplacement occurs as igneous intrusion into overlying sediments. The intrusion of magma into organic-rich sediments creates a dynamic environment where tightly linked physical, chemical, and biological processes regulate the cycling of sedimentary carbon and other elements, not only in a narrow hydrothermal zone at the spreading center but also in widely distributed off-axis venting. Heat from magmatic sills thermally alters organic-rich sediments, releasing CO2, CH4, petroleum, and other alteration products. This heat also drives advective flow, which distributes these alteration products in the subsurface and may also release them to the water column. Within the sediment column, the thermal and chemical gradients created by this process represent environments rich in chemical energy that support microbial communities at and below the seafloor. These communities may play a critical role in chemical transformations that influence the stability and transport of carbon in crustal biospheres. Collectively, these processes have profound implications for the exchange of heat and mass between the lithosphere and overlying water column and may determine the long-term fate of carbon accumulation in organic-rich sediments. The fate of carbon deposited in Guaymas Basin, throughout the Gulf of California, and more broadly within similar marginal seas throughout the world, depends on the relative efficiencies of interacting physical, chemical, and microbial processes, some working to sequester carbon and others working to release carbon back to the ocean and the atmosphere. Drill core samples from Expedition 385 to Guaymas Basin will enable us to study these processes, their interactions, and their ultimate effects on carbon cycling. Samples obtained from scientific drilling are crucial to these goals, which include Quantifying the sedimentary and elemental inputs to the system through time and their variation with oceanographic and climatic conditions; Sampling igneous sills and the surrounding sediments to determine the products and efficiency of alteration and key hydrologic factors such as sediment type, faulting, and permeability evolution; and Studying subsurface microbial communities hosted by alteration products to determine their efficiency at capturing carbon-bearing alteration products and to further our understanding of the conditions that limit life in the deep biosphere. 

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