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1. Data report: 87Sr/86Sr in pore fluids from IODP Expeditions 372 and 375, Hikurangi margin

   https://doi.org/10.14379/iodp.proc.372B375.202.2020  

   Ayres, C. ; Torres, M.E. ; Haley, B. ; Luo, M. ( March 2020 , Proceedings of the International Ocean Discovery Program)

   null (Ed.)

   We report on the Sr isotopic compositions of pore fluids recovered during International Ocean Discovery Program (IODP) Expeditions 372 and 375. Pore fluid samples were acidified and loaded directly onto columns containing EICHROM Sr spec resin, followed by analyses using a NU multicollector inductively coupled plasma–mass spectrometer (MC-ICP-MS). The strontium systematics in this margin point to a series of processes that include carbonate diagenesis, ash alteration, and diffusive exchange with the underlying oceanic crust. The role of ash alteration is evidenced at all drilled sites by a decrease in the 87Sr/86Sr ratio in the upper 130 m of the sediment column to values as low as 0.70846. In the calcareous early Paleocene to late Miocene sediment from Site U1520, the pore fluid strontium concentration shows a marked increase to 1280 µM at 647.3 meters below seafloor (mbsf), but the corresponding 87Sr/86Sr ratios do not show a significant change is this unit. In the early Late Cretaceous sediment at this site, there is an increase in dissolved strontium concentration with a corresponding decrease in 87Sr/86Sr ratios to 0.706312 at the bottom of the cored section. 

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2. Authigenic Clays Versus Carbonate Formation as Products of Marine Silicate Weathering in the Input Sequence to the Sumatra Subduction Zone

   https://doi.org/10.1029/2022GC010338  

   Torres, M. E. ; Milliken, K. L. ; Hüpers, A. ; Kim, J. ‐H. ; Lee, S. ‐G. ( April 2022 , Geochemistry, Geophysics, Geosystems)

   We use geochemical and petrographic data from anoxic sequences of the Nicobar Fan to document extensive marine silicate weathering (MSiW) in the input sediment of the Sumatra subduction zone and the conditions that result in authigenic minerals originating from this reaction: precipitation of authigenic carbonate—which sequesters carbon—and formation of authigenic clay—which releases CO₂. Increase in⁸⁷Sr/⁸⁶Sr in pore fluids from International Ocean Discovery Program Expedition 362 (Site U1480 to 0.71376 and Site U1481 to 0.71296) reveals a radiogenic strontium contribution from alteration of the Himalayan continental sediment that dominates the Nicobar Fan. Peaks in the dissolved strontium isotope data coincide with zones of methane presence, consistent with MSiW reactions driven by CO₂generation during methanogenesis. Later‐stage fan sequences from 24 to 400 mbsf (meters below seafloor) contain only minor carbonate with⁸⁷Sr/⁸⁶Sr ratios that deviate only slightly from the co‐eval seawater values (0.70920–0.70930); geochemical data in this zone point to a contribution of authigenic clay formation. In contrast, microscopy and elemental mapping of the carbonate‐cemented zones in the earliest fan deposits (>780 mbsf) show replacement of feldspars and dense minerals by carbonate, which ranges in volume from a few percent of the grain to near total grain obliteration. This deeper authigenic carbonate is significantly enriched in radiogenic⁸⁷Sr (0.71136–0.71328). Thus, MSiW leads to distinct products, likely in response to a weathering‐derived supply of silica in the younger setting versus calcium enrichment via diffusion from oceanic basement in the older sequence.

    

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3. Data report: 87Sr/86Sr in pore fluids from IODP Expedition 355 Arabian Sea Monsoon

   https://doi.org/10.14379/iodp.proc.355.201.2017  

   Carter, S.C. ; Griffith, E.M. ; Scher, H.D. ( October 2017 , Proceedings of the International Ocean Discovery Program)

   null (Ed.)

   Here we report the strontium isotope ratios (87Sr/86Sr) from pore fluids collected during International Ocean Discovery Program (IODP) Expedition 355. Ratios from Sites U1456 (N = 21) and U1457 (N = 20) are reported. Pore fluid 87Sr/86Sr is a useful tool to establish fluid-rock reactions, sources of Sr, and fluid mixing. The measured 87Sr/86Sr of the pore fluid has significant variations at both sites, and three distinct zones are identified. At Site U1456, 87Sr/86Sr starts at values similar to that of modern seawater (~0.7092) from near the seafloor down to ~100 meters below seafloor (mbsf). Over this interval, Sr concentration increases, whereas Ca decreases (Zone 1). Below 100 mbsf, 87Sr/86Sr values increase to a max of ~0.7100 at ~224 mbsf, and Sr concentrations decrease (Zone 2). Isotopic values then gradually decrease to ~0.7085, with an increase in Sr concentrations (Zone 3). Site U1457 displays the same general trends in pore fluid 87Sr/86Sr composition; however, there are distinct differences. First, Zone 1 occurs over a shorter interval (~0–54 mbsf) due to a lower sedimentation rate, and it also has lower Sr concentrations compared to Site U1456. Additionally, 87Sr/86Sr reaches a higher peak value in Zone 2 than at Site U1456. Finally, in Zone 3 the maximum Sr concentration reached is significantly lower than that at Site U1456. 

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4. Expedition 397T Preliminary Report: Return to Walvis Ridge Hotspot

    Sager, W. ; Blum, P. ( December 2022 , Preliminary report)

   International Ocean Discovery Program Expedition 397T sought to address the shortage of drilling time caused by COVID-19 mitigation during Expedition 391 (Walvis Ridge Hotspot) by drilling at two sites omitted from the earlier cruise. A week of coring time was added to a transit of JOIDES Resolution from Cape Town to Lisbon, which would cross Walvis Ridge on its way north. These two sites were located on two of the three seamount trails that emerge from the split in Walvis Ridge morphology into several seamount chains at 2°E. Site U1584 (proposed Site GT-6A) sampled the Gough track on the east, and Site U1585 (proposed Site TT-4A) sampled the Tristan track on the west. Together with Site U1578, drilled on the Center track during Expedition 391, they form a transect across the northern Walvis Ridge Guyot Province. The goal was to core seamount basalts and associated volcanic material for geochemical and isotopic, geochronologic, paleomagnetic, and volcanologic study. Scientifically, one emphasis was to better understand the split in geochemical and isotopic signatures that occurs at the morphologic split. Geochronology would add to the established age progression but also give another dimension to understanding Walvis Ridge seamount formation by giving multiple ages at the same sites. The paleomagnetic study seeks to establish paleolatitudes for Walvis Ridge sites for comparison with those published from hotspot seamount chains in the Pacific, in particular to test whether a component of true polar wander affects hotspot paleolatitude. Hole U1584A cored a 66.4 m thick sedimentary and volcaniclastic section with two lithostratigraphic units. Unit I is a 23 m thick sequence of bioturbated clay and nannofossil chalk with increasing volcaniclastic content downhole. Unit II is a >43 m thick sequence of lapillistone with basalt fragments. Because the seismic section crossing the site shows no evidence as to the depth of the volcaniclastic cover, coring was terminated early. Because there were no other shallow nearby sites with different character on existing seismic lines, the unused operations time from Site U1584 was shifted to the next site. The seismic reflector interpreted as the top of igneous rock at Site U1585 once again resulted from volcaniclastic deposits. Hole U1585A coring began at 144.1 mbsf and penetrated a 273.5 m thick sedimentary and volcaniclastic section atop a 81.2 m thick series of massive basalt flows. The hole was terminated at 498.8 mbsf because allotted operational time expired. The sedimentary section contains four main units. Unit I (144.1–157.02 mbsf) is a bioturbated nannofossil chalk with foraminifera, similar to the shallowest sediments recovered at Site U1584. Unit II (157.02–249.20 mbsf), which is divided into two subunits, is a 92.2 m thick succession of massive and bedded pumice and scoria lapillistone with increased reworking, clast alteration, and tuffaceous chalk intercalations downhole. Unit III (249.20–397.76 mbsf) is 148.6 m thick and consists of a complex succession of pink to greenish gray tuffaceous chalk containing multiple thin, graded ash turbidites and tuffaceous ash layers; intercalated tuffaceous chalk slumps; and several thick coarse lapilli and block-dominated volcaniclastic layers. Befitting the complexity, it is divided into eight subunits (IIIA–IIIH). Three of these subunits (IIIA, IIID, and IIIG) are mainly basalt breccias. Unit IV (397.76–417.60 mbsf) is a volcanic breccia, 19.8 m thick, containing mostly juvenile volcaniclasts. The igneous section, Unit V (417.60–498.80 mbsf) is composed of a small number of massive basaltic lava flows. It is divided into three lithologic units, with Unit 2 represented by a single 3 cm piece of quenched basalt with olivine phenocrysts in a microcrystalline groundmass. This piece may represent a poorly recovered set of pillow lavas. Unit 1 is sparsely to highly olivine-clinopyroxene ± plagioclase phyric massive basalt and is divided into Subunits 1a and 1b based on textural and mineralogical differences, which suggests that they are two different flows. Unit 3 also consists of two massive lava flows with no clear boundary features. Subunit 3a is a 10.3 m thick highly clinopyroxene-plagioclase phyric massive basalt flow with a fine-grained groundmass. Subunit 3b is a featureless massive basalt flow that is moderately to highly clinopyroxene-olivine-plagioclase phyric and >43.7 m thick. Alteration of the lava flows is patchy and moderate to low in grade, with two stages, one at a higher temperature and one at a low temperature, both focused around fractures. The Site U1585 chronologic succession from basalt flows to pelagic sediment indicates volcanic construction and subsidence. Lava eruptions were followed by inundation and shallow-water volcaniclastic sediment deposition, which deepened over time to deepwater conditions. Although the massive flows were probably erupted in a short time and have little variability, volcaniclasts in the sediments may provide geochemical and geochronologic data from a range of time and sources. Chemical analyses indicate that Site U1585 basalt samples are mostly alkalic basalt, with a few trachybasalt flow and clast samples and one basaltic trachyandesite clast. Ti/V ratios lie mostly within the oceanic island basalt (OIB) field but overlap the mid-ocean-ridge basalt (MORB) field. Only a handful of clasts from Site U1584 were analyzed, but geochemical data are similar. Paleomagnetic data from Site U1585 indicate that the sediments and basalt units are strongly magnetic and mostly give coherent inclination data, which indicates that the basaltic section and ~133 m of overlying volcaniclastic sediment is reversely polarized and that this reversal is preserved in a core. Above this, the rest of the sediment section records two normal and two reversed zones. Although there are not enough basalt flows to give a reliable paleolatitude, it may be possible to attain such a result from the sediments. 

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5. Data report: electrical resistivity of sediments from Site U1480, IODP Expedition 362, Sumatra subduction zone

   https://doi.org/10.14379/iodp.proc.362.205.2021  

   Hamahashi, M. ( March 2021 , Proceedings of the International Ocean Discovery Program)

   null (Ed.)

   Electrical resistivity of sediments was analyzed using samples recovered during International Ocean Discovery Program (IODP) Expedition 362, during which the input materials of the north Sumatran subduction zone were drilled to investigate the material properties linked to shallow seismogenic slip. Electrical resistivity is a valuable indicator for sediment consolidation, pore/grain structures, and distribution of fluid, which can affect the mechanical properties of the forearc wedge. Sediments were recovered from the seafloor to 1415.35 meters below seafloor (mbsf) at Site U1480 and from 1149.7 to 1500 mbsf at Site U1481. They consist of thick sequences of the Bengal-Nicobar Fan (Lithologic Units I–II) underlain by a thin pelagic/igneous sequence (Units III–V). In this study, electrical resistivity was measured on 35 sediment samples from Site U1480 with an Agilent 4294A component analyzer using the bridge method with a two-terminal circuit. Measured resistivity values range from 0.20 to 7.45 Ωm and generally increase with depth. Sample measurements are consistent with the downhole resistivity logs acquired during Expedition 362. Formation factor was calculated from sediment and seawater resistivities, and Archie’s coefficients (cementation [m] and tortuosity [b]) were examined from the relationship between formation factor and porosity. When plotting the sample resistivity in this study together with resistivity logs and shipboard porosity from Sites U1480 and U1481, a contrast in Archie’s coefficients are inferred between the Bengal-Nicobar Fan and pelagic sediments, where the former (m = 3.4–3.8) is characterized by higher m values compared to the latter (m = 2.2). These coefficients show differences in consolidation trend in the input sediments, providing improved equations to estimate porosity from resistivity logs. 

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