Origin, distribution, and significance of brine in the subsurface of Antarctica

Brine formed through the cryogenic concentration of seawater remains one of the least understood components of Earth's cryosphere. Cryogenic brine is a prominent feature of the McMurdo region of Antarctica, where it has been documented below the Taylor Glacier, in numerous ice-covered lakes in the McMurdo Dry Valleys, and in an extensive, seaward-flowing groundwater system in the Taylor Valley. A much larger and less studied body of brine exists offshore in the McMurdo Sound subsurface, where it saturates the pore space and has precipitated intergranular cement in Cenozoic glaciomarine strata deep within the Victoria Land Basin. This review examines the nature and distribution of brine in the McMurdo region to assess its volume, genesis, and significance. Constraints on subsurface fluid migration are considered in the context of available data. Geochemical data indicate that the brine is the product of seawater freezing, with water-rock interaction and evaporation, especially in the McMurdo Dry Valleys where brines stayed close to the surface, leading to further compositional modification. The volume of brine in the region is currently estimated at 27,000 km³. However, accounting for the widespread distribution of brine-precipitated carbonate cement requires that a much larger volume of brine moved through the subsurface in the past. Brine geochemistry is consistent with a genetic relationship between brine in the Taylor Valley and offshore in the subsurface of the Victoria Land Basin. Chronostratigraphic constraints point toward episodic, rather than continuous brine formation. A proposed scenario involves flooding of the McMurdo Dry Valleys during Neogene periods of relative warmth to form fjord-like systems, which were then cut off during subsequent cooling and ice sheet growth. Isolated bodies of seawater were concentrated via freezing to form brine, which percolated into the subsurface and flowed seaward. Among the enduring features that may record the presence of brine in the geologic record, brine-precipitated cement phases hold the most promise for preservation on Earth as well as on Mars.