METEORIC 10BE AS A TRACER FOR SUBGLACIAL CHEMICAL WEATHERING IN EAST ANTARCTICA

Subglacial chemical weathering in Antarctica is potentially a significant contributor to global geochemical cycles, but weathering rates are currently not well known. We estimate subglacial melt rates based on meteoric ¹⁰Be concentrations in freshly-emerging tills from a moraine at Mt. Achernar in the Central Transantarctic Mountains. We infer the degree of subglacial chemical weathering by identifying which sediment phases the ¹⁰Be occupies. Meteoric ¹⁰Be is a cosmogenic nuclide produced in the atmosphere and is deposited on the surface of the Antarctic Ice Sheet. ¹⁰Be in Antarctic ice (avg. conc. 2 ×10⁵ atoms g^-1) reaches the subglacial environment by ice flux. As the ice melts, ¹⁰Be is released from the ice and adsorbs to fine-grained particles in the subglacial till. It complexes with oxides, oxyhydroxides and authigenic clays during chemical weathering. The half-life of ¹⁰Be, 1.39 Ma, makes it a useful tracer for processes on time scales of <10 Ma, thereby distinguishing chemical weathering products from the subglacial environment from those that pre-date Antarctica’s >30 Ma glaciation. The tills at Mt. Achernar Moraine are enriched in clays and oxyhydroxides compared to the local bedrock. Freshly-emerging till samples contained 10⁷ –10⁸ atoms g^-1, suggesting that accumulation in the till takes 10³ years based on modeled melt rates. Adsorbed ¹⁰Be accounts for about a third of the concentrations in all till samples, indicating a significant amount meltwater flux through the till. Clays are the primary ¹⁰Be-bearing phases in the tills, holding 40-50% of the total ¹⁰Be in the samples and with finer, authigenic clays more ¹⁰Be-rich than coarser clays. Oxides/oxyhydroxides contain the most ¹⁰Be in grains of 63-250 µm. We interpret ¹⁰Be concentrations of such magnitude and the primary complexation of ¹⁰Be with chemical weathering products as strong evidence for clay formation in the subglacial environment. Chemical activity under Antarctic ice may be a significant factor in global chemical cycles.