EXTENT OF THE ROSS SEA ICE SHEET IN MCMURDO SOUND, ANTARCTICA, AT THE LOCAL LAST GLACIAL MAXIMUM: NEW CONSTRAINTS ON THE AGE AND ORIGIN OF A TERMINAL MORAINE

A moraine complex was deposited on Hjorth Hill overlooking McMurdo Sound, Antarctica, by the Ross Sea Ice Sheet (RIS) when it was near its local Last Glacial Maximum extent. We collected two cores from this moraine to determine its age and origin, which are controversial. The moraine has been interpreted as interlobate and 12,700 to 14,600 ¹⁴C yrs old. Core HH01 (8.5 m long) is from the crest at 333 m above sea level (m asl), whereas core HH02 (14.6 m long) is from 75 m to the west on the distal flank at 327 m asl.

The upper unit in both cores is a coarse till that includes a 2–3 m thick layer of dirty and clean ice. GPR reflection profiles show that this layer is widespread for at least several hundred meters. Within this ice, δ¹⁸O and δD variations are large (−26 ‰ to −37 ‰), feature several reversals, and strongly correlate. Covariation is commonly on the meteoric-water line but, in certain intervals, δ¹⁸O and δD exhibit a lower slope relationship indicative of regelation. Water-soluble ion concentrations are higher than in neighboring alpine glaciers and covary with the isotopes. Beneath the till in HH02 is a 7-m thick sequence of lake sediment that coarsens upward. AMS ¹⁴C dates on fossil algae from six levels within the lake sediment range between 9,810 ± 290 yrs BP (5.63 m) and 12,250 ± 60 yrs BP (11.76 m). With one exception, the dates become younger upwards in the core. The basal 1.5 m of HH02 contains ice.

Although fossil algae are reworked in the McMurdo region, the stratigraphic order of the ¹⁴C dates strongly suggests they are in place in this core. Whereas the H and O isotopic values in the upper ice layer are consistent with modern local precipitation, the significant fluctuations in isotopes and ions are more consistent with basal ice layers in an ice-edge moraine that has deformed. Therefore, we infer that a significant portion of the ice is glacial in origin and locates the RIS. We suggest that the lower ice layer is lake ice. The sequence indicates that the RIS margin was at the moraine elevation from about 12,200 to 9,800 ¹⁴C yrs BP, after which it receded.