Izu-Bonin-Mariana Rear Arc: The Missing Half of the Subduction Factory

4GT) lies in the western part of the Izu fore-arc basin, ~60 km east of the arc-front volcano Aogashima, ~170 km west of the axis of the Izu-Bonin Trench, 1.5 km west of Ocean Drilling Program (ODP) Site 792, and at 1776 meters below sea level (mbsl). It was drilled as a 150 m deep geotechnical test hole for potential future deep drilling (5500 meters below seafloor [mbsf]) at proposed Site IBM-4 using the D/V Chikyu. Core from Site U1436 yielded a rich record of Late Pleistocene explosive volcanism, including distinctive black glassy mafic ash layers that may record large-volume eruptions on the Izu arc front. Because of the importance of this discovery, Site U1436 was drilled in three additional holes (U1436B, U1436C, and U1436D), as part of a contingency operation, in an attempt to get better recovery on the black glassy mafic ash layers and enclosing sediments and to better constrain the thickness of the mafic ash layers. IODP Site U1437 is located in the Izu rear arc, ~330 km west of the axis of the IzuBonin Trench and ~90 km west of the arc-front volcanoes Myojinsho and Myojin Knoll, at 2117 mbsl. The primary scientific objective for Site U1437 was to characterize “the missing half of the subduction factory”; this was because numerous ODP/Integrated Ocean Drilling Program sites had been drilled in the arc to fore-arc region (i.e., ODP Site 782A Leg 126), but this was the first site to be drilled in the rear part of the Izu arc. A complete view of the arc system is needed to understand the formation of oceanic arc crust and its evolution into continental crust. Site U1437 on the rear arc had excellent core recovery in Holes U1437B and U1437D, and we succeeded in hanging the longest casing ever in the history of R/V JOIDES Resolution scientific drilling (1085.6 m) in Hole U1437E and cored to 1806.5 mbsf

Mineralization kinetics of biosiliceous sediments in hot subseafloors

Temperature affects the timing of the transformation of amorphous silica (opal-A) into crystalline (opal-CT) exponentially. Thus, in hot subseafloor environments opal-A is expected to convert into opal-CT at relatively shallow burial depths, where in situ temperatures do not exceed ∼56 °C, as it has been previously observed at various deep-sea sites and in onshore rock outcrops as well as assessed during lab experiments. The response of biosilica (biogenic opal-A) diagenesis to steep geothermal gradients (∼224–529 °C/km) at extremely high sedimentation rates (∼1 m/kyr) was examined in cores from off-axis boreholes drilled by the International Ocean Discovery Program (IODP) Expedition 385 in the actively spreading, intrusive sill-riddled Guaymas Basin at the Gulf of California (Mexico) rifted margin. At three sites drilled by IODP Expedition 385 (U1545, U1546, and U1547), the conversion from amorphous opal (−A) to crystalline opal (−CT) occurs in relatively deep (up to ∼330 mbsf) and unexpectedly hot (in situ temperatures of ∼74–79 °C) subseafloor conditions. This observation indicates a significantly slower reaction kinetics of biosilica transformation than previously reported. A compilation of empirical data that include biosiliceous basins with a similarly hot subseafloor (Sea of Japan and Bering Sea) yield new kinetic parameters that account for the slower rates of silica transformation. Thus, current kinetic models for the prediction of opal-A to −CT conversion face limitations when burial rates exceed those typical of biogenic sedimentation in open-ocean conditions. At Guaymas Basin Site U1545, where there is no evidence of sill-related metamorphic overprint, the d-spacing of the opal-CT (101) peak correlates linearly with in situ temperature between ∼75 and 110 °C throughout the opal-CT zone, thus, providing a local silica paleothermometry proxy that can be used to calculate the maximum temperature to which opal-CT sediment has been subjected

Nitrogen isotope homogenization of dissolved ammonium with depth and 15N enrichment of ammonium during incorporation into expandable layer silicates in organic-rich marine sediment from Guaymas Basin, Gulf of California

Sedimentary nitrogen isotopic ratios are used as a proxy for ancient biogeochemical cycles on Earth's surface. It is generally accepted that sediment hole tops record primary signatures because organic nitrogen (ON) is predominant in this part of the hole. In contrast to such early to middle diagenetic stages, it is well known that heavier nitrogen isotope 15N tends to enrich in sedimentary rocks during later diagenetic and metamorphic stages. However, there are some critical gaps in our understanding of nitrogen isotopic alteration associated with abiotic processes during early-middle diagenesis. In this study, we examined the isotope ratios of ammonium nitrogen in interstitial water (IW) and total nitrogen (TN), including exchangeable ammonium and mineral nitrogen, in the solid-phase of organic-rich-sediment recovered by International Ocean Discovery Program (IODP) Expedition 385 cores drilled in the Guaymas Basin, Gulf of California, that contained ammonium-rich IW. The isotopic ratios (δ15N value) of TN are the most variable with depth compared to any other type of nitrogen. This variation can be interpreted as reflecting changes in the water mass environment in the basin caused by glacial–interglacial climate changes, modifying the δ15N values of the marine primary producers. Thus, the δ15N value of TN is a proxy for environmental change in the basin, while each component of TN shows different trends. The δ15N values of IW and exchangeable ammonium did not exhibit significant changes with depth, but the latter values are about 3 ‰ enriched in 15N. This may be due to advective transport of solute into adjacent layers followed by the formation of an isotopic equilibrium between IW and exchangeable ammonium in the case of fast sediment accumulation rate. The δ15N value of exchangeable ammonium is the highest among the other types of nitrogen with one exception, where the δ15N value of TN is the highest. The calculated δ15N values of ON based on mass balance are almost the same as those of associated TN in the shallow sediment layers (< 150 m below seafloor), but the difference in the δ15N values of TN and ON are significant in the deeper layers, where proportions of ON contents are <50%. In particular, in the layer where the δ15N value of TN is the highest, that of ON shows an even higher value and the difference reaches 3.5 ‰. The δ15N values of mineral nitrogen are similar to that of IW ammonium except the surface layers. Under such conditions, when δ15N value of TN is intermediate between those of mineral nitrogen and exchangeable ammonium, calculated δ15N value of ON is close to that of TN. On the other hand, if δ15N value of TN is out of the range between mineral nitrogen and exchangeable ammonium, it causes further difference in δ15N value of ON. It means that the fluctuation of δ15N values of TN is reduced relative to those of ON through depth. It has been considered that δ15N value of TN in sediment is similar to that of ON, and changes in the δ15N value of TN due to diagenesis are limited, but in such environment ON fluctuations over depth may be slightly underestimated

Assessment of the diagenetic overprint of sediments within the deep subseafloor biosphere in the Nankai Trough, Japan – IODP Expedition 370

IODP Expedition 370 established Site C0023 down to 1180 mbsf in the Nankai Trough, Japan, to explore the upper temperature limit of microbial life in deep subseafloor sediments. The area is characterized by strongly changing sedimentation rates. The diagenetic iron (Fe) cycling is an important process within the deep biosphere. However, microbial and abiotic alterations of Fe phases in deep subseafloor sediments are poorly understood. Fe (oxyhydr)oxides are important carriers of sedimentary magnetic signals. Diagenetic cycling, especially the reductive dissolution of Fe (oxyhydr)oxides driven by microbial degradation of organic matter and/or by reactions with hydrogen sulfide, may lead to transformations of primary ferrimagnetic Fe (oxyhydr)oxides to secondary Fe sulfides, and thus, to modification of rock magnetic properties. In this study, we aim at assessing the alteration of the primary sedimentary record at Site C0023, including authigenic formation of secondary minerals, pyritization as well as effects on rock magnetic properties. To investigate the Fe mineralogy, sequential extractions of Fe (oxyhydr)oxides and sulfides are combined with rock magnetic analyses and SEM-EDS observations. The reactive Fe pool includes Fe carbonates and Fe (oxyhydr)oxides. Amorphous Fe oxides are the dominant reactive Fe fraction at Site C0023. Fe sulfides, mainly pyrite, are present in all investigated intervals. However, pyritization only affects 5 to 15% of the reactive Fe pool. Rock magnetic properties also show the presence of metastable Fe sulfides in the upper interval between 200 and 450 mbsf. Their preservation might be caused by limited hydrogen sulfide availability, an increase in sedimentation rates, which prevents a complete pyritization by decreasing the time the interval was located in the sulfidic zone, or by recent authigenesis. Combining our geochemical and rock magnetic data improves the understanding of iron cycling in subseafloor sediments and the role of iron minerals in maintaining life in the deep biosphere

Microbial cell distribution in the Guaymas Basin subseafloor biosphere, a young marginal rift basin with rich organics and steep temperature gradient

<p>Guaymas Basin is a young marginal rift basin in the Gulf of California characterized by active seafloor spreading and rapid sediment deposition, including organic-rich sediments derived from highly productive overlying waters and terrigenous sediments from nearby continental margins. The combination of active seafloor spreading and rapid sedimentation within a narrow basin results in a dynamic environment where linked physical, chemical, and biological processes regulate the cycling of sedimentary carbon and other elements. This continuum of interrelating processes from magma to microbe motivated International Ocean Discovery Program Expedition 385 and is reflected in its title, “Guaymas Basin Tectonics and Biosphere.”</p><p>During IODP Expedition 385, organic-rich sediments with sill intrusions on the flanking regions and in the northern axial graben of Guaymas Basin (in eight sites) were drilled and core samples were recovered. Those cored samples were examined for their microbial cell abundance in a highly sensitive manner by density-gradient cell separation at the super clean room of Kochi Core Center, Japan, followed by direct counting on fluorescence microscopy. Cell abundance in surficial seafloor sediment (~10<sup>9</sup> cells/cm<sup>3</sup>) was roughly 1000 times higher than the bottom seawater (~10<sup>6</sup> cells/cm<sup>3</sup>) and gradually decreased with increasing depth and temperature. In contrast to the cell abundance profile observed at Nankai Trough (IODP Exp. 370), the gradual decrease of cell abundance was observed up to around 75ºC, and we detected microbial cells even at hot horizons above 100ºC.</p><p>We will present the overview of the microbial cell distribution in the Guaymas Basin and discuss its relation to the current and past environmental conditions, e.g., temperature and sill-intrusion, etc.</p&gt

Mineralization kinetics of biosiliceous sediments in hot subseafloors

International audienceTemperature affects the timing of the transformation of amorphous silica (opal-A) into crystalline (opal-CT) exponentially. Thus, in hot subseafloor environments opal-A is expected to convert into opal-CT at relatively shallow burial depths, where in situ temperatures do not exceed ~56 ◦C, as it has been previously observed at various deep-sea sites and in onshore rock outcrops as well as assessed during lab experiments. The response of biosilica (biogenic opal-A) diagenesis to steep geothermal gradients (~224–529 ◦C/km) at extremely high sedimentation rates (~1 m/kyr) was examined in cores from off-axis boreholes drilled by the International Ocean Discovery Program (IODP) Expedition 385 in the actively spreading, intrusive sill-riddled Guaymas Basin at theGulf of California (Mexico) rifted margin. At three sites drilled by IODP Expedition 385 (U1545, U1546, and U1547), the conversion from amorphous opal (-A) to crystalline opal (-CT) occurs in relatively deep (up to ~330 mbsf) and unexpectedly hot (in situ temperatures of ~74–79 ◦C) subseafloor conditions. This observation indicates a significantly slower reaction kinetics of biosilica transformation than previously reported. A compilation of empirical data that include biosiliceous basins with a similarly hot subseafloor (Sea of Japan and Bering Sea) yield new kinetic parameters that account for the slower rates of silica transformation. Thus, current kinetic models for the prediction of opal-A to -CT conversion face limitations when burial rates exceed thosetypical of biogenic sedimentation in open-ocean conditions. At Guaymas Basin Site U1545, where there is no evidence of sill-related metamorphic overprint, the d-spacing of the opal-CT (101) peak correlates linearly with in situ temperature between ~75 and 110 ◦C throughout the opal-CT zone, thus, providing a local silica paleothermometry proxy that can be used to calculate the maximum temperature to which opal-CT sediment has been subjected

Paleomagnetism of the Guaymas Basin, Gulf of California: New Results from IODP Expedition 385

International audienceSill-driven hydrothermal circulation results in geochemical exchanges between ocean crust, sediments, and seawater. IODP Expedition 385 drilled sill/sediment sequences at eight sites in the Guaymas Basin to better understand the interactions between magmatic, hydrothermal, and sedimentary systems in a highly active marine environment. We focus on the magnetic data of two adjacent sites with similar sedimentation history, but different alteration degree due to an intruding sill. Site U1546 recovered a 540m sedimentary sequence disrupted by a 80m wide sill between 350/430 meters below seafloor (mbsf). The sill terminates towards Site U1545 which recovered 507m of an undisrupted sedimentary sequence. These sites allow us to directly compare paleomagnetic measurements and changes related to the different heat system in both sites.Detailed paleomagnetic measurements were conducted, including anisotropy of magnetic susceptibility (AMS), thermal demagnetization, and alternating field demagnetization on both discrete samples and section halves . Both sites feature a decrease of magnetic intensity (NRM) and magnetic susceptibility between 50-80 mbsf, associated with a change in coercivity (averaging from 24 to 10 mT from MDF). This correlates with the sulphate-methane transition zone (SMTZ) where magnetic mineral assemblages change because of biogeochemical processes. A stable magnetic component was identified in most samples after removal of a low coercivity magnetic drilling overprint (generally removed after AF 5-10 mT), resulting in a mean inclination of ~46°, close to the GAD inclination of 46.3°. Cores were assigned to the C1n Brunhes chron. In sill-bearing Site U1546, magnetic intensity is found to gradually increase toward the sill and to decrease below the mafic sill (basaltic, doleritic, and gabbroic textures), suggesting authigenesis of magnetic minerals in the contact zone, likely triggered by the heat of the igneous intrusion in combination with hydrothermal fluid circulation.Here we will present an overview of the paleomagnetic and rock magnetic data used to identify the magnetic mineralogy through the oceanic crustal sections of the Guaymas Basin, and to compare these two adjacent sites to assess the effect of heat and hydrothermal circulation on the paleomagnetic record in the Guaymas Basin

Nitrogen isotope homogenization of dissolved ammonium with depth and 15N enrichment of ammonium during incorporation into expandable layer silicates in organic-rich marine sediment from Guaymas Basin, Gulf of California

International audienceSedimentary nitrogen isotopic ratios are used as a proxy for ancient biogeochemical cycles on Earth’s surface. It is generally accepted that sediment hole tops record primary signatures because organic nitrogen (ON) is predominant in this part of the hole. In contrast to such early to middle diagenetic stages, it is well known that heavier nitrogen isotope 15N tends to enrich in sedimentary rocks during later diagenetic and metamorphic stages. However, there are some critical gaps in our understanding of nitrogen isotopic alteration associated with abiotic processes during early-middle diagenesis. In this study, we examined the isotope ratios of ammonium nitrogen in interstitial water (IW) and total nitrogen (TN), including exchangeable ammonium and mineral nitrogen, in the solid-phase of organic-rich-sediment recovered by International Ocean Discovery Program (IODP) Expedition 385 cores drilled in the Guaymas Basin, Gulf of California, that contained ammonium-rich IW. The isotopic ratios (δ15N value) of TN are the most variable with depth compared to any other type of nitrogen. This variation can be interpreted as reflecting changes in the water mass environment in the basin caused by glacial–interglacial climate changes, modifying the δ15N values of the marine primary producers. Thus, the δ15N value of TN is a proxy for environmental change in the basin, while each component of TN shows different trends. The δ15N values of IW and exchangeable ammonium did not exhibit significant changes with depth, but the latter values are about 3 ‰ enriched in 15N. This may be due to advective transport of solute into adjacent layers followed by the formation of an isotopic equilibrium between IW and exchangeable ammonium in the case of fast sediment accumulation rate. The δ15N value of exchangeable ammonium is the highest among the other types of nitrogen with one exception, where the δ15N value of TN is the highest. The calculated δ15N values of ON based on mass balance are almost the same as those of associated TN in the shallow sediment layers (< 150 m below seafloor), but the difference in the δ15N values of TN and ON are significant in the deeper layers, where proportions of ON contents are <50%. In particular, in the layer where the δ15N value of TN is the highest, that of ON shows an even higher value and the difference reaches 3.5 ‰. The δ15N values of mineral nitrogen are similar to that of IW ammonium except the surface layers. Under such conditions, when δ15N value of TN is intermediate between those of mineral nitrogen and exchangeable ammonium, calculated δ15N value of ON is close to that of TN. On the other hand, if δ15N value of TN is out of the range between mineral nitrogen and exchangeable ammonium, it causes further difference in δ15N value of ON. It means that the fluctuation of δ15N values of TN is reduced relative to those of ON through depth. It has been considered that δ15N value of TN in sediment is similar to that of ON, and changes in the δ15N value of TN due to diagenesis are limited, but in such environment ON fluctuations over depth may be slightly underestimated