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

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.