Abstract

Pervasive 18O-depleted carbonate cements in sediment cores acquired by the Cape Roberts Project (CRP) in McMurdo Sound, Antarctica, were previously attributed to mixing of glacial meltwater and seawater in the subsurface. However, a more recent discovery of 18O-depleted, connate brine formed by seawater freezing in a nearby sediment core (AND-2A core) called this interpretation into question. This core contains widespread carbonate cements in the glaciomarine sediments, which have been demonstrated to precipitate from the brine by conventional (δ18O) and clumped (Δ47) isotopic data. Building on findings from the AND-2A core, this study investigates the geochemical nature, origin, and distribution of diagenetic fluids, and re-evaluates their impact on diagenetic patterns in glaciomarine deposits of the composite Oligocene to lower Miocene succession from the CRP cores. Sandstones were characterized in the context of a well-established chronostratigraphic and sequence stratigraphic framework by systematic point counting and modal analysis using optical microscopy. Diagenetic carbonate phases and mineralogies were assessed by cathodoluminescence microscopy and energy-dispersive X-ray spectrometry. Low-Mg calcite cement is the most widespread diagenetic phase and was selected for analyses of stable carbon and oxygen isotopes. Similar to the AND-2A core, the cement increases in crystal size and decreases in δ18O value (–4.8 to –19.5‰ VPDB) with increasing depth to ∼ 700–800 m. These patterns indicate calcite precipitation from interstitial brine along local geothermal gradients. When considered in the context of Neogene climate, tectonic, and burial history, timing of calcite cementation is linked to a period of extensive formation of cryogenic brine during c. 13 to 10 Ma when the region transitioned into a cold, polar climate regime and McMurdo Sound was periodically glaciated by expanded, cold-based Antarctic ice sheets. Beyond McMurdo Sound, the propensity for formation of cryogenic brine exists in most glaciomarine settings. As such, the potential for diagenetic alteration by such fluids should be considered in studies of the deposits of ancient glaciated continental margins.

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