Chemical Fractionation of Brine in the McMurdo Ice Shelf, Antarctica

Abstract

ABSTRACT. During the austral summers of 1976-77 and 1978-79, several ice cores were taken from the McMurdo Ice Shelf brine zone to investigate its thermal, physical, and chemical properties. This brine zone consists of a series of superimposed brine layers (waves) that originate at the seaward edge of the ice shelf and migrate at various rates, depending on their age and position in the ice shelf. The brine in these layers becomes increasingly concentrated as the waves migrate inland through the permeable ice-shelf firn. Chemical analyses of brine samples from the youngest (uppermost) brine wave show that, except for the advancing front, it contains sea saIts in normal sea-water proportions. Further inland, deeper and older brine layers, though highly saline (S > 200%0), are severely depleted in SO.2-, with the SO.2-/Na+ ratio being an order of magnitude less than that of normal sea-water. Consideration of the solubility of alternative salts, together with analyses of Na+, K+, Ca 2 +, Mg2+, SO /-, and cr concentrations, shows that the sulfate depletion is probably due to selective precipitation of mirabilite, Na 2 SO •. IOH 2 0. The location of the inland boundary of brine penetration is closely related to the depth at which the brine encounters the firn/ice transition. However, a small but measureable migration of brine is still occurring in otherwise impermeable ice; this is attributed to eutectic dissolution of the ice by concentrated brine as it moves into deeper and warmer parts of the McMurdo Ice Shelf. RESUME.

Brine, englacial structure and basal properties near the terminus of McMurdo Ice Shelf, Antarctica

“…Periodic lateral infiltration at the ice shelf front is considered the primary mechanism of brine infiltration into the MIS (Kovacs and others, 1982b). Evidence in support of this mechanism comes from the observation of large fractures within the MIS brine horizon that represent former, wave-like intrusions of periodic packets of sea water that originated at the ice front after a large calving event (Kovacs and others, 1982a; Cragin and others, 1983). In describing the termination of brine infiltration, Kovacs and others (1982b) invoke an equation for brine flow velocity u introduced by Thomas (1975): where B 0 is the specific permeability of firn, η is the dynamic viscosity of brine fluid and ∂ p /∂ x approximates the pressure gradient over the brine layer.…”

Recent survey of brine infiltration in McMurdo Ice Shelf, Antarctica

“…This brine zone and its infiltration characteristics have been studied by Stuart and Bull (1963), Risk and Hochstein (1967), Heine (1968), Kovacs and Gow (1975) and Kovacs and others (1982a) Kovacs and others (1982b). Isotopic fractionation in the brine was reported by Stewart (1975) and chemical fractionation was reported by Cragin and others (1983). Radio-echo sounding (RES) was used to map the inland limit of brine infiltration in 1967 (Clough, 1973) and again in 1977 (Kovacs and others 1982a Kovacs and others 1982b)…”

“…However, brine interior to this barrier can continue to advance. At the inland limit, the brine concentration reaches about seven times that of sea water (Cragin and others, 1983)…”

“…Increasing firn density encountered with depth in the interior slows the step-advancement rate. Analysis of this step led Kovacs and others (1982a) Kovacs and others (1982b) and Cragin and others (1983) to conclude that sea-water infiltration in McMurdo Ice Shelf occurred largely by recurrent influxes of similar steps.…”

The Application of the Aerospace Expeditionary Force Concept to Reserve Tactical Air Components

“…26 Of all of Services, the Air Force faced the most traumatic changes following the collapse of the Soviet Union. By 1989, the Air Deployment window refers to the period a unit is eligible to be deployed Force had lost two-thirds of its overseas bases resulting in a fourfold increase in overseas deployments.…”