Dating of Ross Ice Shelf Cores by Chemical Analysis

Abstract

Seasonal variations in sodium concentrations have been measured on surface-pit snow samples and on firn and ice core samples from the Ross Ice Shelf, Antarctica. Site locations include J-9 (82° 22’ S., 168° 40’ W.), Roosevelt Island dome (79° 22’ S, 161° 40’ W), C-7-1 (78° 30’ S., 177° 00’ W.), and C-7-3 (78° 20’S., 179° 51’ E.). The predominant source for the Na is sea salt, indicated by greater concentration levels at seaward sites. Al concentrations of the order of only 10–9g/g show that the input of continental dust is comparable to that at inland Antarctic locations, and that dust contributes only a negligibly small fraction of the Na on the shelf. Maximum Na concentrations occur in the winter or early spring, as is the case for Greenland ice. The annual accumulation-rate at J-9, determined by counting Na concentration peaks with depth, is 90 kg m–2year–1, in agreement with rates determined radiometrically. Annual cycles in Na concentration are also detectable at depth in the J-9 ice core. It is suggested that Na concentration is a useful diagnostic criterion for distinguishing between East Antarctic ice (10–8g Na/g), West Antarctic ice (30 × 10–9g Na/g), and ice that fell as snow on the shelf itself (> 30 × 10–9g Na/g). The transition between snow that is chemically characteristic of the ice-shelf regime to snow of an inland regime is expected to occur near the 500 m elevation contour. This position is up to 200 km inland of the grounding line. A model is presented for the large-scale decrease in Na concentration with distance inland within the ice-shelf regime. Since deeper ice in Ross Ice Shelf cores originated generally further from the ocean, the up-stream origin of shelf ice may be estimated from the chemical profile. The Little America V ice-core chemistry profile shows no discontinuity as would be expected if a recent surge of West Antarctic ice had occurred.

Derived Characteristics of the Ross Ice Shelf, Antarctica

“…At LA V Herron and Langway (1979) found the " glaciochemical transition " to be at a depth of 155 m, which is in very good agreement with our estimate for the depth of the " 500 m transition". At 19, Herron and Langway estimate the glaciochemical transition to be approximately 130 m beneath the surface, which is at the shallow end of our estimated range for T. However, at this station, the Herron and Langway estimate was obtained by extrapolating their observations using a particle-path calculation.…”

RISS, RISP and RIGGS: Post-IGY glaciological investigations of the Ross Ice Shelfin the U.S. programme

“…Values for the activation energy, which determines the temperature dependence of the resistivity, and the basal mass balance, which strongly influences the temperature profile in the ice column, were chosen to model the data. Indicated activation energies ranged from 0.25 to 0.15 eV (Bentley, 1979); after allowance for a decrease in ionic impurity concentration in the lower part of the ice shelf (Herron and Langway, 1979), the suggested best value was 0.25 eV, in agreement with laboratory studies. Except for one station where the temperature distribution may not have been in equilibrium, the calculations indicated a near-zero basal mass balance.…”

Distribution of sea salt components in snow cover along the traverse route from the coast to Dome Fuji station 1000 km inland at east Dronning Maud Land, Antarctica

“…and at D80 in Adélie Land (2.5 km a.s.l. ), both located about 430 km from the ice front, were 100 ng g −1 (Herron and Langway, 1979) and 20 ng g −1 (Legrand Fig. 3.…”