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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. 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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3. 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)

   Abstract 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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4. 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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5. Data report: revised age models for IODP Sites U1480 and U1481, Expedition 362

   https://doi.org/10.14379/iodp.proc.362.202.2019  

   Backman, J.; Chen, W.; Kachovich, S.; Mitchison, F.; Petronotis, K.; Yang, T.; Zhao, X. (September 2019, Proceedings of the International Ocean Discovery Program)

   null (Ed.)

   A revised age model for Site U1480 was generated for the 0–67 Ma time interval using biomagnetostratigraphic data from which age-depth tie points have been selected to determine sediment accumulation rates and durations of identified hiatuses. This revised age model relies on biostratigraphic data between ~2 and 67 Ma and biomagnetostratigraphic data between 0 and 1.8 Ma and differs from the shipboard age model in terms of (1) the timing and duration of the major Cenozoic hiatus, (2) the late Miocene–early Pliocene transition, (3) the 0–1.8 Ma interval, and (4) the age of the sediment/volcanic interface at 1415 meters below seafloor (mbsf), here determined to be ≤67.4 Ma. Two intervals of igneous strata totaling 60 m occur in the Paleocene sedimentary rock sequence, giving a thickness of 1355 m for sediments and sedimentary rocks. In Hole U1481A, sedimentary rocks were recovered between 1150 and 1499 mbsf. The revised age model differs from the shipboard version mainly in more clearly acknowledging the lack of biostratigraphic data between 1411 and 1495 mbsf. 

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