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

A Unified Program of Argon Dark Matter Searches: DarkSide-50, DarkSide-20k and The Global Argon Dark Matter Collaboration

<p>This presentation was used for the Low-Radioactivity Underground Argon Workshop held at Pacific Northwest National Laboratory in Richland, Washington on March 19 - 20, 2018.</p

Kinematic and paleomagnetic restoration of the Semail ophiolite (Oman) reveals subduction initiation along an ancient Neotethyan fracture zone

The archetypal Semail ophiolite of Oman has inspired much thought on the dynamics of initiation of intra-oceanic subduction zones. Current models invoke subduction initiation at a mid-oceanic ridge located sufficiently close to the Arabian passive margin to allow initiation of continental subduction below the ophiolite within ∼10–15 Myr after the 96–95 Ma age of formation of supra-subduction zone crust. Here, we perform an extensive paleomagnetic analysis of sheeted dyke sections across the Semail ophiolite to restore the orientation of the supra-subduction zone ridge during spreading. Our results consistently indicate that the ridge was oriented NNE–SSW, and we infer that the associated trench, close to the modern obduction front, had the same orientation. Our data are consistent with a previously documented ∼150° clockwise rotation of the ophiolite, and we reconstruct that the original subduction zone was WNW-ward dipping and NNE–SSW striking. Initial subduction likely occurred in the ocean adjacent and parallel to a transform margin of the part of the Arabian continent now underthrust below Iran that originally underpinned the nappes of the Zagros fold-thrust belt. Subduction thus likely initiated along an ancient, continental margin-parallel fracture zone, as also recently inferred from near-coeval ophiolites from the eastern Mediterranean and NW Arabian regions. Subduction initiation was therefore likely induced by (WN)W–(ES)E contraction and this constraint may help the future identification of the dynamic triggers of Neotethyan subduction initiation in the Late Cretaceous

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

&lt;p&gt;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, &amp;#8220;Guaymas Basin Tectonics and Biosphere.&amp;#8221;&lt;/p&gt;&lt;p&gt;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&lt;sup&gt;9&lt;/sup&gt; cells/cm&lt;sup&gt;3&lt;/sup&gt;) was roughly 1000 times higher than the bottom seawater (~10&lt;sup&gt;6&lt;/sup&gt; cells/cm&lt;sup&gt;3&lt;/sup&gt;) 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 75&amp;#186;C, and we detected microbial cells even at hot horizons above 100&amp;#186;C.&lt;/p&gt;&lt;p&gt;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.&lt;/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

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 (&lt; 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 &lt;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

Heat flow and thermal regime in the Guaymas Basin, Gulf of California: Estimates of conductive and advective heat transport

International audienc

Heat flow and thermal regime in the Guaymas Basin, Gulf of California: Estimates of conductive and advective heat transport

International audienc