CRYOGENIC ORIGIN FOR BRINE IN NEOGENE SEDIMENTS OF SOUTHERN MCMURDO SOUND, ANTARCTICA

Pore water sampling during deep coring of Neogene strata in Southern McMurdo Sound, Antarctica by ANDRILL revealed the presence of hypersaline brine at depths >200 m below the sea floor. Patterns in major element concentrations and oxygen isotope values are consistent with a concentration mechanism that involves the removal of water as ice and precipitation of mirabilite (Na₂SO₄·10H₂O) during progressive freezing of seawater. The brine occurs in subglacial, glacimarine, and marine facies that record the advance and retreat of glaciers through the Ross Sea embayment. The model proposed here reconciles cryogenic brine formation with sedimentological and regional stratigraphic data. During glacial advance, buildup of ice depresses the underlying lithosphere, leading to the development of a flexural trough that lies between the ice sheet margin and a basinward forebulge. When invaded by the sea, such troughs may serve as semi-isolated marine basins that are ideal sites of cryogenic brine formation. As sea ice crystallizes on the surface, the dense residual brine sinks and saturates the effective porosity of the sediment. Near the glacier margin, groundwater flow is directed downward and, at depth, in a basinward direction. During glacial retreat, accumulated salts are dissolved and washed back into the sea, resulting in the removal of any trace of the cryogenic process from the sedimentary record. Regional stratigraphic information indicates that in the Ross Sea, the requisite conditions for cryogenic brine formation have been in place since the Oligocene, with each advance-retreat cycle providing an opportunity to transform a batch of seawater into brine. In demonstrating the viability of models that propose seawater freezing as a mechanism for generating significant volumes of subsurface brine, results highlight the potential for the same process operating in similar settings during ice ages of the Quaternary and deep geologic past.