Chemical weathering signatures from Mt. Achernar Moraine, Central Transantarctic Mountains I: Subglacial sediments compared with underlying rock

Abstract

In order to determine chemical weathering rates on the subglacial land surface of Antarctica, we compare the composition and mineralogy of freshly emerging fine sediments to that of the underlying bedrock, as represented by glacially derived cobble-sized clasts. Samples were collected from Mt.Achernar Moraine, a large blue ice moraine, where subglacial material naturally emerges through sublimation of the surrounding ice. Both rocks and sediments were analyzed for total elemental composition, mineral abundance by X-ray diffraction, and by sequential extractions targeting chemical weathering products. The fine sediment fraction is significantly enriched in chemical weathering products and depleted in primary minerals compared with the cobble clasts. The alteration pathways consist primarily of the development of smectite, kaolinite, carbonate minerals, and amorphous material. Extensive Fe oxidation is evidenced by a decline in magnetic susceptibility and by increases in extractable Fe. If we assume the only input into the subglacial system is the water and ice-trapped gas supplied by basal melt, the net chemical alteration is explained through oxidation of organic matter equal to ~0.7% of the bedrock mass and subsequent carbonation weathering. The underlying sedimentary rock is sufficiently rich in organic matter for this pathway to be plausible. For the O2 that is oxidizing organic matter to be supplied by basal meltwater, water fluxes would need to be three orders of magnitude larger than sediment fluxes. Independent models of basal melt and sediment transport at our field site confirm that such a difference between water and sediment flux is likely at the study site.The rate of subglacial carbonation weathering inferred from the Mt. Achernar Moraine site may be *Manuscript 2 comparable to that found in high latitude subaerial environments. If Mt. Achernar Moraine is typical of other Antarctic sites, the subglacial land surface of Antarctica does play a role in global geochemical cycling.

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

“…Highly weathered, freshly exposed samples of subglacial origin at the Mt. Achernar moraine (Graly et al, 2020) were subject to sequential chemical extractions designed to isolate the three defined phases of 10 Be. A detectable 10 Be signal in these sediments would indicate the accumulation of the nuclide into those sediments from the ice due to their relatively short surface exposure of < 10 × 10 3 years (Graly et al, 2018a).…”

“…The upward flow trajectories of ice at blue ice areas cause the eventual emergence of entrained sediments at the ice surface (Bintanja, 1999;Whillans & Cassidy, 1983). Blue ice moraines are therefore important repositories of material representative of the otherwise ice-covered subglacial environment (Bader et al, 2017;Graly et al, 2020;Kassab et al, 2019). Rockfall from the surrounding topography can also contribute to sediment accumulation at blue ice moraines.…”

“…Table 5 data. Summary of correlation coefficients and significance of correlation for total 10 Subglacial chemical weathering on continental-scales in Antarctica can potentially be a major contributor to the mass balance of geochemical cycles (Graly et al, 2020;Petsch, 2014;Wadham et al, 2012).…”

“…In the presence of meltwater released from subglacial ice and the atmospheric gases therein, sediments that originate from the comminution of rock at the ice bed can be chemically altered in the subglacial environment (Graly et al, 2020). These sediments become accessible for sampling once they reach the surface of the ice sheet.…”

“…Achernar Moraine. The sediments of the moraine bear strong evidence for subglacial chemical weathering at the site, namely abundant weathering products and a decline in magnetic susceptibility compared to the source rock (Graly et al, 2020). Beryllium is known to adsorb to fine-grained soil particles in aqueous solution, to precipitate with oxides and oxyhydroxides, and be incorporated into authigenic clay structures during chemical weathering (Barg et al, 1997;You et al, 1989).…”