International Ocean Discovery Program

Abstract Three boreholes drilled during the International Ocean Discovery Program (IODP) Expedition 396 have yielded unexpected findings of altered granitic rocks covered by basalt flows, interbedded sediments and glacial mud near the continent‐ocean transition of the mid‐Norwegian margin. U‐Pb and K‐Ar geochronological analyses were conducted on both protolithic and authigenically formed K‐bearing minerals to determine the age of granite crystallisation and subsequent alteration episodes. The granite's crystallisation age based on 104 zircons is 56.3 ± 0.2 Ma, and subsequent exhumation along with alteration/weathering events took place between 54.7 ± 1 and 37.1 ± 1 Ma. This intrusion represents the youngest granite discovered in Norway and intruded at an extremely shallow crustal level before a rapid rift‐to‐drift transition. The shallow emplacement of granitic rock and its fast exhumation before and during the onset of volcanism holds significant implications for the syn‐ and post‐breakup tectonic evolution of volcanic margins.

Contents 1 Background and objectives 2 Operations 5 Lithostratigraphy 19 Igneous petrology 27 Micropaleontology 39 Paleomagnetism 45 Stratigraphic correlation 47 Chronostratigraphy 49 Geochemistry 55 Physical properties 62 Downhole measurements 68 References

International audience

Abstract The Sea of Japan is a marginal sea, connecting to adjacent seas by four shallow straits (water depths <130 m). Marginal seas are ideal for studying biotic responses to large‐scale environmental changes as they often are sensitive to glacial‐interglacial and stadial‐interstadial climatic cycles. However, only a limited number of studies cover time periods beyond the last two glacial‐interglacial cycles. Here we present a 700,000‐year record of benthic biotic response to paleoceanographic changes in the southern Sea of Japan, covering the past seven glacial‐interglacial cycles, based on ostracode assemblages at the Integrated Ocean Drilling Program (IODP) Site U1427. The results indicate that long‐term oxygen variability in the bottom water has been the major control impacting the marginal‐sea biota. Five local extirpation events were recognized as barren zones during glacial maxima immediately before terminations I, II, IV, V, and VII, which are probably caused by bottom‐water deoxygenation. Results of multivariate analyses indicated clear faunal cyclicity influenced by glacial‐interglacial oxygen variability with a succession from opportunistic species dominance through tolerant infauna dominance to barren zone during the deoxygenation processes and the opposite succession during the recovery processes. The Sea of Japan ostracode faunal composition showed distinct difference between the post‐MBE and pre‐MBE (Mid‐Brunhes Event) periods, indicating the MBE as a major disturbance event in marginal‐sea ecosystems. The MBE shortened the duration of the extirpation events, fostered dominance of warmer‐water species, and amplified the glacial‐interglacial faunal cyclicity. Our long‐term biotic response study clearly indicates that deep marginal sea ecosystems are dynamic and vulnerable to climate changes.

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 superclean 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.