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1. Temperature Limit of the Deep Biosphere off Muroto

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

   Heuer, V.B.; Inagaki, F.; Morono, Y.; Kubo, Y.; Maeda, L. (November 2017, Proceedings of the International Ocean Discovery Program)

   null (Ed.)

   International Ocean Discovery Program (IODP) Expedition 370 explored the limits of the biosphere in the deep subseafloor where temperature exceeds the known temperature maximum of microbial life (~120°C) at the sediment/basement interface ~1.2 km below the seafloor. Site C0023 is located in the protothrust zone in the Nankai Trough off Cape Muroto at a water depth of 4776 m, in the vicinity of Ocean Drilling Program (ODP) Sites 808 and 1174. In 2000, ODP Leg 190 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 elevated 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 detection limit of manual cell counting for low-biomass samples was not low 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 utilizing 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, conducting simultaneous shipboard and shore-based investigations. Our primary objective during Expedition 370 was to study the relationship between the deep subseafloor biosphere and temperature. We comprehensively studied the controls on biomass, activity, and diversity of microbial communities in a subseafloor environment where temperatures increase from ~2°C at the seafloor to ~120°C and thus likely encompasses the biotic–abiotic transition zone. This included investigating whether 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 of sediment and 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. In addition to microbiological measurements, we determined the geochemical, geophysical, and hydrogeological characteristics of the sediment and the underlying basement and installed a 13-thermistor sensor borehole temperature observatory into the borehole at Site C0023. 

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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. The recovery of the deep biosphere at the Chicxulub impact crater

   Coolen, M. (January 2018, American Geophysical Union Fall Meeting)

   The Chicxulub crater, Mexico, is the site of the asteroid impact that led to the end-Cretaceous mass extinction. While impact events are known to be able to cause severe disruption to surface-dwelling organisms, the effects of such catastrophic perturbations on the deep biosphere are not known. Deep ocean drilling into the peak ring of the Chicxulub impact crater (IODP expedition 364) in 2016 allowed us to study the modern deep biosphere within the (a) high-porosity melt-bearing impact breccia/suevite (617-740 mbsf) emplaced within a day or so of the Cenozoic, (b) the overlying low porosity post-impact marine Cenozoic carbonates (504-617mbsf), and the impacted and fractured granitic basement (740-1334 mbsf). The microbial biomass (~10 cells/g wet weight) was highest in the upper suevite, in underlying non-granitic subvolcanic pre-impact basanite, and at the intercalation of suevite and impact melt rock. Pre-impact sterile conditions of the uplifted granitic basement rocks and mineralogical evidence of impact-induced sterilization suggest that the basement rocks have only been amenable to microbial colonization for less than 66 Myr. Enrichments at in situ 50-60 °C show the presence of heterotrophic lifestyles in the suevite and bacterial sulfate reduction extending into the top of the granitic basement. Cultivation-independent 16S diversity profiling revealed the presence of heterotrophic (fermentative) as well as autotrophic C-fixing thermophilic bacteria in the organic-rich (up to 4 wt % total organic carbon; TOC) Cenozoic sediments. The organic-lean suevite (< 0.1% TOC) showed the unique presence of sequences related to thermophilic Synechococcus (cyanobacteria) and S-oxidizing green sulfur bacteria (chlorobi), and Chloroflexi often associated with organic-poor deep-sea sediments. Alphaproteobacteria, predominated in the upper part of the granitic basement (<1000 mbsf), while putative manganeseoxidising Bacilli (Firmicutes) predominated in the melt-rich granitic basement (>1200 mbsf). Our data suggest that the catastrophe that led to the end-Cretaceous mass extinction caused geological disruption and recolonization of microbial life in the deep subsurface biosphere at the Chicxulub impact site. 

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5. 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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