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1. Expedition 370 Preliminary Report: Temperature Limit of the Deep Biosphere off Muroto

   https://doi.org/10.14379/iodp.pr.370.2017  

   Heuer, V.B.; Inagaki, F.; Morono, Y.; Kubo, Y.; Maeda, L. (January 2017, Preliminary report)

   null (Ed.)

   International Ocean Discovery Program (IODP) Expedition 370 aimed to explore the limits of life in the deep subseafloor biosphere at a location where temperature increases with depth at an intermediate rate and exceeds the known temperature maximum of microbial life (~120°C) at the sediment/basement interface ~1.2 km below the seafloor. Drilling Site C0023 is located in the vicinity of Ocean Drilling Program (ODP) Sites 808 and 1174 at the protothrust zone in the Nankai Trough off Cape Muroto at a water depth of 4776 m. ODP Leg 190 in 2000, revealed the presence of microbial cells at Site 1174 to a depth of ~600 meters below seafloor (mbsf), which corresponds to an estimated temperature of ~70°C, and reliably identified a single zone of higher cell concentrations just above the décollement at around 800 mbsf, where temperature presumably reached 90°C; no cell count data was reported for other sediment layers in the 70°–120°C range, because the limit of manual cell count for low-biomass samples was not high enough. With the establishment of Site C0023, we aimed to detect and investigate the presence or absence of life and biological processes at the biotic–abiotic transition with unprecedented analytical sensitivity and precision. Expedition 370 was the first expedition dedicated to subseafloor microbiology that achieved time-critical processing and analyses of deep biosphere samples by simultaneous shipboard and shore-based investigations. Our primary objectives during Expedition 370 were to study the relationship between the deep subseafloor biosphere and temperature. We aimed to comprehensively study the factors that control biomass, activity, and diversity of microbial communities in a subseafloor environment where temperatures increase from ~2°C at the seafloor to ~120°C at the sediment/basement interface and thus likely encompasses the biotic–abiotic transition zone. We also aimed to determine geochemical, geophysical, and hydrogeological characteristics in sediment and the underlying basaltic basement and elucidate if the supply of fluids containing thermogenic and/or geogenic nutrient and energy substrates may support subseafloor microbial communities in the Nankai accretionary complex. To address these primary scientific objectives and questions, we penetrated 1180 m and recovered 112 cores across the sediment/basalt interface. More than 13,000 samples were collected, and selected samples were transferred to the Kochi Core Center by helicopter for simultaneous microbiological sampling and analysis in laboratories with a super-clean environment. Following the coring operations, a temperature observatory with 13 thermistor sensors was installed in the borehole to 863 mbsf. 

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2. Expedition 370 Scientific Prospectus: T-Limit of the Deep Biosphere off Muroto (T-Limit)

   https://doi.org/10.14379/iodp.sp.370.2016  

   Hinrichs, K.-U.; Inagaki, F.; Heuer, V.B.; Kinoshita, M.; Morono, Y.; Kubo, Y. (May 2016, Scientific prospectus)

   null (Ed.)

   Determining factors that limit the biomass, diversity, and activity of subseafloor microbial communities is one of the major scientific goals to be addressed by scientific ocean drilling. In the International Ocean Discovery Program (IODP) T-Limit Project, we will drill and core at new boreholes in the immediate vicinity of Ocean Drilling Program (ODP) Sites 1173, 1174, and 808 off Cape Muroto, Japan, in the central Nankai Trough, where anomalously high heat flow regimes result in temperatures of 110° to 140°C at the sediment/basement interface. Because of their location in the trench (Site 1173) and landward protothrust zone of the Nankai Trough accretionary prism (Sites 808 and 1174), the sites have different geotectonic and thermal histories that have resulted in contrasting (bio)geochemical modes of hydrocarbon gas production and consumption. Although the upper temperature limit appears well constrained at relatively energy-rich hydrothermal vent systems at just above 120°C, it is unknown in energy-starved sedimentary subseafloor settings but is generally presumed to be lower and thus expected to be covered by our target sites. During the IODP T-Limit Project, we aim to • Comprehensively study the factors that control biomass, activity, and diversity of microbial communities in a subseafloor environment where temperatures increase from ~30° to ~130°C and thus likely encompasses the biotic–abiotic transition zone; and • Determine geochemical, geophysical, and hydrogeological characteristics in sediment and the underlying basaltic basement and elucidate if the supply of fluids containing thermogenic and/or geogenic nutrient and energy substrates may support subseafloor microbial communities in the Nankai accretionary complex. Because of the D/V Chikyu’s schedule, these scientific objectives cannot be achieved within a single expedition. During the first T-Limit expedition (370), we will drill and retrieve core samples from sedimentary sections (200–1210 m below seafloor) and basement basalt (1210–1260 m below seafloor) at the protothrust site near ODP Site 1174 and measure temperatures in situ. 

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3. Data report: calcareous nannofossils from the middle Miocene to Pleistocene, IODP Expedition 370 Site C0023

   https://doi.org/10.14379/iodp.proc.370.201.2018  

   Hagino, K. (April 2018, Proceedings of the International Ocean Discovery Program)

   null (Ed.)

   International Ocean Discovery Program Expedition 370 recovered marine subsurface sediments from 189 to 1126 meters below seafloor (mbsf) at Site C0023, which was established on the Nankai Trough, off Muroto Peninsula, Japan. Biostratigraphic studies based on calcareous nannofossil assemblages revealed the ages of sediments are from the middle Miocene to Pleistocene. Samples from 1126.885 to 796.530 mbsf yielded Miocene taxa such as Coccolithus miopelagicus, Cyclicargolithus floridanus, Catinaster calyculus, and Discoaster quinqueramus. The interval 790.285–774.984 mbsf spans the late Miocene to the early Pliocene, as it corresponds to bioevents spanning from the last occurrence (LO) of D. quinqueramus to the first occurrence of Ceratolithus cristatus. Samples from 769.440 to 639.982 mbsf are referred to the Pliocene based on occurrences of C. cristatus, Reticulofenestra pseudoumbilicus (≥7 µm), and/or Discoaster surculus. Sediments from 630.440 to 190.483 mbsf are dated to the Pleistocene, as an interval above the LO of Discoaster pentaradiatus. Preservation of calcareous nannofossils is poor or moderate in the middle Miocene samples and moderate to good in the samples from the upper Miocene to Pleistocene. 

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4. Expedition 395E Preliminary Report: Complete South Atlantic Transect Reentry Systems

   https://doi.org/10.14379/iodp.pr.395E.2021  

   Williams, T.; Estes, E.R.; Rhinehart, B.; Coggon, R.M.; Sylvan, J.B.; Christeson, G.L.; Teagle, D.A.H. (December 2021, Preliminary report)

   International Ocean Discovery Program (IODP) Expeditions 390C and 395E were implemented in response to the global COVID-19 pandemic and occupied sites proposed for the postponed Expeditions 390 and 393, South Atlantic Transect 1 and 2. Expedition 395E completed most of the preparatory work that Expedition 390C did not have time to complete. The overall objective of Expeditions 390C and 395E was to core one hole at each of the South Atlantic Transect sites with the advanced piston corer/extended core barrel (APC/XCB) system to basement for gas safety monitoring and to install a reentry system with casing through the sediment to a few meters into basement in a second hole. Expedition 395E started in Cape Town, South Africa, and ended in Reykjavík, Iceland, after 20 days of on-site operations. We cored to basement at two new sites, U1560 and U1561, and completed reentry systems at three sites, U1556, U1557, and U1560. These operations will expedite basement drilling during the rescheduled Expeditions 390 and 393. Hole U1560A (Proposed Site SATL-25A) lies in ~15.2 Ma crust and is composed of carbonate-rich sediments to 120 meters below seafloor (mbsf) and 2.5 m of underlying basalt. A reentry system was deployed in Hole U1560B to 122.0 mbsf. We then moved to the sites at the western end of the transect on ~61 Ma crust. In Hole U1557D, 10¾ inch casing was deployed to 571.6 mbsf to deepen the 16 inch casing that was deployed during Expedition 390C, and in Hole U1556B, a reentry system was deployed to 284.2 mbsf. The remaining operations time was insufficient to install a reentry system at the originally planned site, Proposed Site SATL-33B. Instead, we cored Hole U1561A (Proposed Site SATL-55A) to 47 mbsf. It is composed of red clay and carbonate ooze overlying 3 m of basalt. The six primary sites of the South Atlantic Transect lie perpendicular to the Mid-Atlantic Ridge on the South American plate, overlying crust ranging in age from 7 to 61 Ma. Basement coring will increase our understanding of how crustal alteration progresses over time across the flanks of a slow/intermediate-spreading ridge and how microorganisms survive in deep subsurface environments. Sediment will be used in paleoceanographic and microbiological studies. 

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5. Expedition 390 Preliminary Report: South Atlantic Transect 1

   https://doi.org/10.14379/iodp.pr.390.2022  

   Coggon, R.M.; Sylvan, J.B.; Teagle, D.A.H.; Reece, J.; Christeson, G.L.; Estes, E.R.; Williams, T.J. (October 2022, Preliminary report)

   The South Atlantic Transect (SAT) is a multidisciplinary scientific ocean drilling project that comprises four International Ocean Discovery Program (IODP) expeditions: engineering Expeditions 390C and 395E as well as Expeditions 390 and 393. Altogether, the expeditions aim to recover complete sedimentary sections and the upper 100–350 m of the underlying oceanic crust along a slow/intermediate spreading rate Mid-Atlantic Ridge crustal flow line at ~31°S. 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. Given dramatic advances in drilling technology and analytical capabilities since Leg 3, many high-priority scientific objectives can be addressed by revisiting the transect. The SAT expeditions target six primary sites on 7, 15, 31, 49, and 61 Ma ocean crust, which 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 is required to investigate the history of the low-temperature hydrothermal interactions between the aging ocean crust and the evolving South Atlantic Ocean and quantify past hydrothermal contributions to global biogeochemical cycles. Samples from the transect of the previously unexplored sediment- and basalt-hosted deep biosphere beneath the South Atlantic Gyre are essential to refining global biomass estimates and examining 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, providing access to 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. Engineering Expeditions 390C and 395E cored a single hole through the sediment/basement interface with the advanced piston corer/extended core barrel system at five of the six primary proposed SAT sites and installed a reentry system with casing 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% recovery) over 30.3 days of on-site operations. Sediment coring, basement drilling, and logging were conducted at two sites on 61 Ma crust, and sediment coring was completed at the 7 Ma crust site. At Site U1557 on 61 Ma crust, the drill bit was deposited on the seafloor prior to downhole logging, leaving Hole U1557D available for future deepening and to establish a legacy borehole for basement hydrothermal and microbiological experiments. Expedition 390 scientists additionally described, and analyzed data from, 792 m of core collected during Expeditions 390C and 395E. Expedition 393 plans to operate at four sites, conducting basement drilling and downhole logging at the 7 Ma site, in addition to sediment coring, basement drilling, and logging at the sites intermediate in age. 

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