Linking marine silicate alteration signatures in pore water to dynamics of Western Antarctic Ice sheet since late Miocene

Western Antarctic Ice Sheet (WAIS) grounded below sea level is experiencing rapid basal melting and is known to have undergone complete ice sheet collapse in the past. We investigate Si and Sr isotopic signatures of marine porewater and sediment from an 800-meter-long sediment core (U1532) dating back to late-Miocene (5.7 Ma) from Amundsen Sea, retrieved as a part of IODP Expedition 379. The present work explores porewater and sediment geochemical and isotopic signatures to understand marine silicate alterations and to link sedimentary signatures to glacial-interglacial dynamics of WAIS.

The sediment lithology is dominated by silty clay, dispersed with biogenic silica and ice rafted debris (IRD). The depositional environment receives high supply of terrigenous sediments during glacial times, transported by advancing WAIS towards shelf edge, evident from high sedimentation rates (upto 61 cm/kyr) and occurrence of terrestrially derived kaolinite clays. The bulk ⁸⁷Sr/⁸⁶Sr of sediments shows an overall increasing trend with depth, ranging from 0.7095 to 0.7225. Interestingly, porewater shows a steady increase in Sr concentration with depth (from 90 to 320 μM), except the deepest strata between 660 to 760 meter bsf, where we observe a slight decrease. The pore water ⁸⁷Sr/⁸⁶Sr signatures fluctuate between 0.7084 to 0.7101, with two phases of low ⁸⁷Sr/⁸⁶Sr ratio and three phases of higher ⁸⁷Sr/⁸⁶Sr ratio than contemporaneous seawater. The interaction of fluids with more radiogenic Sr rich glacigenic detrital phases can explain observed increasing phases of pore water ⁸⁷Sr/⁸⁶Sr signatures. The more radiogenic ⁸⁷Sr/⁸⁶Sr source can be traced to subglacial erosion and weathering of lower Paleozoic/Mesozoic terranes of Western Antarctica, possibly transported through (paleo)ice stream systems into the Amundsen Sea. The sediments at distinct phases of low pore water ⁸⁷Sr/⁸⁶Sr coincides with Miocene-Pliocene transition and late Pliocene, likely resulting from basaltic alterations, associated with subglacial volcanic provinces of West Antarctic rift system.

The pore water dissolved silicon (DSi) and δ³⁰Si signatures results from a dynamic balance between dissolution of biogenic and reactive lithogenic phases, and formation of authigenic aluminosilicates. The DSi profile of pore water varies between 60 and 900 μM, exhibits three distinct phases of enriched DSi, associated with sediment intervals of abundant biosiliceous deposits (diatoms and radiolarians). Occurrence of IRD in these intervals points to a pelagic and hemipelagic sedimentation, associated with ice sheet retreat during warm interglacial periods. In order to distinguish possible sources of DSi to pore water, we employ a sequential leaching technique to separate major Si bearing phases from marine sediments. The biogenic silica associated with the Na₂CO₃ leach exhibits a higher Si/Al ratio (6 to 365) and a δ³⁰Si between 0.6 to 1.5‰, consistent with documented range of diatoms and radiolarians globally. The metal oxy-hydroxides and clay fractions represents lightest Si pool in marine sediments, with δ³⁰Si varying between -0.7 to -3.1‰, while lithogenic silica exhibits δ³⁰Si between -0.5 to 0.2‰. Through careful evaluation of pore water Si isotopic signatures, we will be able to link sediment supply of Si phases to past ice sheet dynamics of Western Antarctica.