Combining meteoric 10Be and U-series isotopes to decode weathering intensity in East Antarctica’s subglacial environment

We investigated chemical weathering in the subglacial environment of East Antarctica through studies of two isotope systems: meteoric ¹⁰Be and (²³⁴U/²³⁸U). We sampled 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.

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. 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.

The use of (²³⁴U/²³⁸U) as a tracer relies on the loss of ²³⁴U due to alpha recoil. As a result, (²³⁴U/²³⁸U) in residual detrital silt and clay particles drops below equilibrium and progresses towards a low steady state value. By contrast, the surrounding solutions and authigenic minerals that precipitate from them display (²³⁴U/²³⁸U) ratios higher than equilibrium.

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 higher than those of silt, suggesting a mix of detrital and authigenic clay. Adsorbed species, carbonates, and oxyhydroxides display (²³⁴U/²³⁸U) higher than equilibrium, reflecting their precipitation from ²³⁴U-enriched solutions.

The results in total are very promising for both isotope systems. The U series system can constrain the time frame of chemical alteration to within a glacial-interglacial cycle.  The ¹⁰Be system trace 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.