Title: Identification of the diagenetic sedimentary environment and hydrothermal fluid fluxes in Southern Ocean sediments (IODP Exp 382) using B, Si and Sr isotopes in interstitial waters

Marcus Gutjahr1, Sonja Geilert1, Bridget Kenlee2, Klaus Wallmann1, Osamu Seki3, Ji-Hwan Hwan4, Michael E Weber5, Maureen Raymo6, Victoria L. Peck7, Trevor Williams8, Florian Scholz1, and Expedition 382 Scientists9

1GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany; 2Department of Earth Sciences, University of California, Riverside, USA; 3Institute of Low Temperature Science, Hokkaido University, Sapporo Hokkaido, Japan; 4Earth & Environmental Sciences, Korea Basic Science Institute, Chungbuk Cheongju, Republic of Korea; 5Steinmann-Institute, University of Bonn, Germany; 6Lamont Doherty Earth Observatory, Columbia University, USA; 7British Antarctic Survey, Cambridge, UK; 8International Ocean Discovery Program, Texas A&M University, USA; 9Expedition

Event: GeoKarlsruhe 2021

Date: 2021

DOI: 10.48380/dggv-tzy6-8s83

During IODP Expedition 382, two sites were drilled at 53.2°S at the northern edge of the Scotia Sea and three sites at 57.4°–59.4°S in the southern Scotia Sea within the Atlantic sector of the Southern Ocean. Sediments at both locations alternate between dominant terrigenous components during glacials and dominant biogenic components, carbonate at the northerly sites and opal in the southern Scotia Sea, during interglacials. Here we constrain the geochemical environment in interstitial waters using the boron (δ11B), silicon (δ30Si) and 87Sr/86Sr isotopic composition.

Interstitial water δ11B and δ30Si decrease in the uppermost tens of meters downcore, most likely due to in situ weathering processes preferentially releasing light isotopes to interstitial waters. This process is partly also reflected by strongly increasing alkalinities in this depth interval. While δ30Si at all sites increase already at shallow sediment depth where organic matter degradation is intense, δ11B remain relatively low beyond the lower boundary of elevated dissolved phosphate concentrations at every core site. Below this depth δ11B follow isotopic trends seen in δ30Si towards heavy compositions, presumably because of dominating secondary clay formation.

Interstitial waters obtained as deep as 550 and 670 mbsf from the southern Scotia Sea sites reveal an increasing importance of off-axis hydrothermal fluids within the basement underlying the sediments. This feature is detectable by lowest 87Sr/86Sr alongside lowest Mg/Sr and strongly decreasing δ11B at the lower end of the cores. Our key aim is to illustrate the dominant diagenetic process at each depth downcore, and how to identify these.

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