T19 Cold Regions Weathering and Biogeochemical Cycling: Bridging Environments and Approaches

Monday, 26 October 2020
1:30 PM - 5:30 PM
Meeting Rooms

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

Paper number: T19-8
Program: Technical Sessions
Time: 3:15 PM – 3:30 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-357361

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