Cryogenic formation of brine and its impact on diagenetic patterns in polar shelf sediments of McMurdo Sound, Antarctica

Understanding controls on diagenesis is important for understanding the origin and distribution of subsurface fluids, application of robust geochemical proxies, and predicting properties of sedimentary fluid reservoirs. Diagenetic processes in most terrigenous clastic depositional systems are well-documented in the literature. Poorly represented are reviews focused on glaciogenic deposits. This study examines diagenetic processes and products in Cenozoic strata recovered by ANDRILL, the Cape Roberts Project, and other drilling programs in McMurdo Sound, Antarctica. In this region, the stratigraphic succession consists of a range of lithologies, including diamictite, mudrock and sandstone with minor conglomerate and diatomite. Petrographic analysis reveals a complex diagenetic history. Whereas secondary carbonate is present throughout the succession, its abundance, distribution, and morphology is highly variable. Stable isotopic data show that these phases are characterized by lower d¹⁸O values (–10 to –18‰ VPDB) than might be expected in sediments that accumulated under freezing conditions. Some skeletal components important for paleoclimate reconstructions also have unexpectedly low ¹⁸O values. Pore water geochemical profiles generated during ANDRILL drilling show that a body of dense, carbonate-saturated brine resides in the subsurface at depths greater than c. 200 mbsf in McMurdo Sound. The brine is characterized by low d¹⁸O values (c. –10‰ VSMOW). The most likely mechanism for brine generation involves sea ice formation along ice sheet margins. The brine is subsequently distributed basinward through subsurface flow. Clumped isotope data confirm that the brine is the main agent of cementation and alteration. Although most sandstones are tightly cemented, there are significant exceptions. Some intervals of clean sand up to 25 m thick maintain porosities as high as 41%. Sequence stratigraphic relationships indicate that these sandstones are best developed in highstand delta systems that formed during ice minima, when substantial volumes of meltwater were released from glacier termini. Porosity in these sandstones was retained by discharge of dilute meltwater during deposition and subsequent isolation of sands between impermeable barriers. Results show that diagenesis in glaciomarine deposits is controlled by factors that are absent or of reduced significance in lower latitude systems. For example, cold temperatures, which limit the rate of chemical reactions, are maintained to significant burial depth. Another key factor is climate change, the effects of which are amplified in polar settings. As glacial severity increases, so does the propensity for the formation of cryogenic brine, a potentially important diagenetic fluid, in glaciomarine systems. Results of this study have the potential to change the way burial history and diagenesis are evaluated in sedimentary successions that formed on polar shelves.

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