CARBONATE CEMENTATION RELATED TO CRYOGENIC BRINE FORMATION DURING CENOZOIC GLACIATION, MCMURDO SOUND, ANTARCTICA

Cryogenically formed brine, present in the subsurface of the McMurdo Sound region since at least the mid-Miocene, has been shown to be responsible for extensive ¹⁸O-depleted carbonate cements in several sediment cores encompassing the Cenozoic section. The CIROS-1 core (c. 36–22 Ma), acquired proximal to a glacier outlet, is examined to evaluate the spatiotemporal distribution of ancient brine in this area since the beginning of the Cenozoic Icehouse. This work uses standard and cathodoluminescence petrography, electron microprobe analysis, and stable isotope determinations for systematic characterization of carbonate cement phases through the core. A pre-compactional iron carbonate phase is overlain by abundant non-ferroan, low-Mg CaCO₃ (LMC). The iron carbonate cement is generally characterized by zoned equant crystals with variable isotopic compositions (d¹³C = -16 to 3‰, d¹⁸O = -19 to -8‰). The abundant LMC cement is petrographically uniform through the core, displaying mainly coarse-crystalline to poikilotopic texture. Two phases of LMC cement are distinguished by their isotopic signatures. LMC cement in compacted sandstones below a major unconformity in the core show low d¹³C (-12 to -3‰) and uniform d¹⁸O (-11±0.4‰) values. By contrast, LMC cements above the unconformity are characterized by high d¹³C (3 to 7‰) values, and relatively low and highly variable d¹⁸O (-30 to -9‰) values. Preliminary Raman spectroscopy indicates that aragonite is associated with some LMC cement, which is unexpected in this cold setting. While iron carbonate cement is interpreted to precipitate from marine pore water modified by alteration of volcaniclastic materials, subsequent coarse-crystalline cement phases are attributed to two major phases of cryogenic brine infiltration. One phase of brine formation is related to glacial expansion after uplift and erosion of a significant thickness of the lower succession (c. 33–24 Ma). The second phase of brine infiltration is linked to late Miocene glaciation (after c. 13 Ma). The unexpected presence of aragonite demonstrates the potential for the development of very high carbonate saturation states in cryogenic brine. This study highlights the extensive distribution of cryogenic brine and its impact on diagenesis in McMurdo Sound and potentially other glaciated basins.