Co-precipitation of calcite and (Al)-Si-OH phases in Pleistocene subglacial environments of the East Antarctic Ice Sheet

Frisia, S. and Dietzel, M. and Borsato, A. and Németh, P. and Pettauer, M. and Hellstrom, J.C. and Demény, A. and Pekker, P. and Rinyu, L. and Pécz, Béla and Augustinus, P.C. (2025) Co-precipitation of calcite and (Al)-Si-OH phases in Pleistocene subglacial environments of the East Antarctic Ice Sheet. GEOCHIMICA ET COSMOCHIMICA ACTA. ISSN 0016-7037 (In Press)

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Abstract

Mineral crusts formed under glaciers and ice sheets record the basal hydrology in their isotopic and elemental composition as well as their petrology. However, many of these crusts have been reworked, and in Antarctica few locations preserve in-situ subglacial deposits in presently deglaciated areas and consist of calcite or aragonite. In the interior of Antarctica, subglacially-precipitated minerals have been encountered as clasts in moraines, implying transport from an unknown, but presumed basal ice sheet source. Clasts consisting of opal and calcite associations were retrieved from moraines in the Transantarctic Mountains region (TAM, Antarctica) of the East Antarctic Ice Sheet (EAIS), with previous research interpreting opal precipitates as a marker of almost complete freezing of reducing basal waters associated with Late Pleistocene (ca. 129,000–11,7000 years ago) cold oceanic and atmospheric conditions. In contrast, calcite was related to basal EAIS water flow and oxic conditions associated with warm mid to Late Pleistocene climate cycles. Here, novel high-resolution transmission electron microscopy (HRTEM) observations demonstrate that opal and calcite co-precipitated in the mid to Late Pleistocene subglacial environment. In addition, HRTEM observations revealed that calcite and opal coexist with allophane, which was not detected previously. The occurrence of nanotwins in the calcite crystals and absence of porosity at the boundary between calcite and amorphous silica strongly suggest that crystallization pathways in the subglacial environment included non-classical particle attachment. Geochemical simulations based on solubility data of silicates (Ca-plagioclase) present in the bedrock and kinetic rate laws are proposed to explain coprecipitation of opal-calcite-allophane. These indicate that differing degrees of regelation at basal ice/water interface would have increased both pH and supersaturation of the residual basal fluid, thereby enabling crystallization of (Al)-Si-OH phases or calcium carbonate at near-freezing temperatures in the same aqueous environment. Considering that nano-scale observations hint at non-classical crystallization pathways, it is proposed that low temperature and the presence of dissolved organic carbon may have significantly inhibited or delayed nucleation of calcite/allophane/opal, and, therefore, supersaturation thresholds for amorphous CaCO3 formation may have been reached. Co-precipitation of calcite and opal questions a previous basal hydrology reconstruction based on the hypothesis of their separate occurrence. Furthermore, opal and calcite may have incorporated chemical species inherited from precursors, whereby an interpretation of their chemistry in terms of past environmental variations may be inaccurate if not supported by a reconstruction of crystallization pathways.

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