Chemical weathering signatures at Mt. Achernar, Central Transantarctic Mountains II: Surface exposed sediments

Graly, Joseph, Licht, Kathy J., Bader, Nicole A., Kassab, Christine M., Bish, David L. and Kaplan, Michael R. (2022) Chemical weathering signatures at Mt. Achernar, Central Transantarctic Mountains II: Surface exposed sediments. Geochimica et Cosmochimica Acta. ISSN 0016-7037 (In Press)

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Mt Achernar Moraine is a high altitude, high latitude blue ice moraine where typical conditions preclude the presence of liquid water. Cosmogenic and salt accumulation dating indicate that the moraine’s surface is progressively older away from the active ice margin, with surface exposure ages up to 1 Ma. We analyze the chemical and mineralogical transformations in the <63 µm fraction along transects across the moraine. Data include bulk chemical composition, crystalline mineralogy by X-ray diffraction (XRD), and the composition of amorphous or low abundance products of chemical weathering by sequential extraction. These data are analyzed by multiple regression as a function of exposure age and as a function of composition of the moraine’s cobble and pebble-sized clasts. Change with exposure age is defined by the development of salts and carbonate minerals along with the input of detrital material, principally from sedimentary rocks. Clay minerals and amorphous cements breakdown as detrital material in proportions far above their abundance in the rock clasts, whereas framework silicates (i.e. feldspars and quartz) break down in relatively small proportions. Both the carbonate minerals and some of the salts form from atmospheric acids (i.e. H2CO3) that in turn react with other minerals. Mass balance shows that the input of these atmospheric acids balances with gains in authigenic smectites, zeolites, and amorphous material. Many of these minerals also form in the subglacial environment, but are poorly represented in the underlying rock, suggesting a similar chemical weathering regime in both the subglacial and surface environments of this hyper cold and arid setting. The rate of CO2 drawdown into carbonate minerals increases as the moraine progressively thickens, from 3 mg·m2·a−1 in freshly emerging sediments to ∼50 mg·m2·a−1 after 500 ka of exposure. Weathering from acidic aerosols is proportional to atmospheric flux documented in ice cores and does not vary with moraine thickness. The carbonate mineral formation rates are more than an order of magnitude below those of the subglacial environment and as much as two orders of magnitude below those found in warm desert soils. Nevertheless, the drawdown of atmospheric CO2 into carbonate minerals occurs in a terrestrial setting where water exists only in vapor form.

Item Type: Article
Additional Information: Funding information: This work was supported by National Science Foundation Antarctic grants PLR-1744879, PLR-1443433, PLR-1443213, ANT-0944578 and ANT-0944475, by an Indiana University Collaborative Research Grant, and by funding from the IUPUI Office of the Vice Chancellor for Research. The US Antarctic Program, Kenn Borek Air, Ltd., and field team members from the 2010 and 2015 seasons are gratefully acknowledged for field support. T. Kennedy assisted with magnetic susceptibility analyses. H. Johnston assisted with particle size analyses. S. Cox and S. Olund assisted with the sequential extraction analyses. Reviews by J. Compton and by an anonymous reviewer, along with editing by L. Jin and J. Catalano, were helpful in improving the manuscript. We would like to acknowledge the Integrated Nanosystems Development Institute (INDI) for use of their Bruker D8 Discover X-Ray Diffraction Instrument, which was awarded through NSF grant MRI-1429241.
Uncontrolled Keywords: Antarctica; Chemical weathering; Clay minerals; Carbonate minerals; Salts; Geochemical cycles
Subjects: F600 Geology
F800 Physical and Terrestrial Geographical and Environmental Sciences
Department: Faculties > Engineering and Environment > Geography and Environmental Sciences
Depositing User: Elena Carlaw
Date Deposited: 04 Jul 2022 16:01
Last Modified: 04 Jul 2022 16:15

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