Weathering Under East Antarctica investigated chemical weathering in the subglacial environment through studies of two isotope systems: meteoric ¹⁰Be and ²³⁴U/²³⁸U. The study relied on Antarctic blue ice moraines as a window into Antarctica’s subglacial environments. Here vapour-starved winds ablate near-stagnant ice, allowing sediment-rich basal ice to be thrust against mountains and nunataks. These moraines form across a wide swath of the continent. Many blue ice moraine sediments are substantially altered by chemical weathering; sediment grains are often coated in a mix of clays, oxides, and amorphous material that does not resemble soil but speaks to a chemical weathering regime specially found in the subglacial environment.

We have demonstrated that the combination of meteoric ¹⁰Be and ²³⁴U/²³⁸U analyses on glacigenic sediments are very promising for interpreting subglacial geochemical processes. The U series system can be used to identify the time frame of chemical alteration to within a glacial-interglacial cycle.  The ¹⁰Be system traces the meltwater input and also confirm the presence of authigenic mineral phases. These tracers, especially in combination, allow us to define the relationship between meltwater input and weathering intensity across Antarctica and make large scale influences about the ice sheet’s influence on its substrate and on global biogeochemical cycles.

Meteoric ¹⁰Be works as a tracer in this system because its concentration in ice is relatively well known from ice cores, it is very unlikely to occur in detrital minerals, and it has a strong propensity to become incorporated in authigenic minerals, such as clays and oxyhydroxides.  The total abundance of meteoric ¹⁰Be therefore traces meltwater input over the sediment residence time and the speciation of meteoric ¹⁰Be traces the formation of authigenic minerals.

In developing the meteoric ¹⁰Be tracer, we initially focused on Mt. Achernar Moraine, a site in the central Transantarctic Mountains containing highly weathered fine sediments of subglacial origin. We tested a variety of extraction procedures to most effectively extract ¹⁰Be from minerals formed during chemical weathering. The site’s chemical / mineral mass balance and the outcome of the methods development are now published (Arnardottir et al., 2023; Graly et al., 2020, 2022). At Mt. Achernar Moraine, the total meteoric ¹⁰Be strongly correlates to the abundance of authigenic minerals (particularly smectite clay) and aligns well with mass balance calculations for meltwater input.

We also explored the ¹⁰Be concentrations in authigenic minerals at nearby blue ice moraine sites, including Lonewolf Nunataks, Mt. Howe, and Bates Nunatak. These samples test various other conditions prevalent in East Antarctica. Lonewolf Nunataks are located along the large and fast-moving Byrd Glacier. Mt. Howe is like Mt. Achernar Moraine in that it has a high abundance of chemical weathering products and relatively slow-moving ice. Bates Nunatak is a cold based ice catchment, where meltwater input and resulting chemical alteration is unlikely.

Lonewolf, like Mt. Achernar, has most of the ¹⁰Be speciated within authigenic clay species. Concentrations of ¹⁰Be and authigenic clay are considerably lower at Lonewolf compared to Achernar, probably reflecting the sorter residence time of subglacial sediment under the fast-flowing ice. Mt. Howe has very low concentrations of ¹⁰Be, despite its high abundance of authigenic clay. This suggests a lag between the formation of authigenic minerals and its exhumation on the order of millions of years or that the authigenic minerals formed from multi-million-year-old ice, which had lost  most ¹⁰Be through radioactive decay. Such a conclusion is consistent with the multi-million year old ice has been found at blue ice sites elsewhere on the continent.  Bates Nunatak shows very high concentrations of ¹⁰Be and hardly any complexation into clay, comparable to what we found elsewhere in blue ice moraines forming from cold based ice.  This suggests very long surface exposure with little chemical weathering beyond oxidation.

Analyses of several samples from Mt. Achernar Moraine, show that U series isotopes confirm recent (i.e. within 100 ka) authigenic weathering at the site. Clay mineral ²³⁴U/²³⁸U ratios are substantially higher than those of silt, confirming the presence of authigenic clay. Adsorbed species, carbonates, and oxyhydroxides also are confirmed as authigenic by the ²³⁴U/²³⁸U tracer.

Thus far, this work has resulted in three peer-reviewed publications and one MS thesis.  As part of this project, one student earned a MS degree and several undergraduates were trained in laboratory analysis.

Last Modified: 01/13/2025
Modified by: Kathy J Licht