Implications of soils on mid-Miocene-aged drifts in the McMurdo Dry Valleys for ice sheet history and paleoclimate reconstruction

Strongly developed soils on unconsolidated deposits of mid-Miocene age occur in the uplands above 1500 m elevation in the Asgard Range and Quartermain Mountains in the McMurdo Dry Valleys. Inferred from the preservation and distribution of their surfaces, these deposits have been largely unmodified since a widespread glacial erosional event prior to 10 Ma. Composition of the till plays a key role in soil development on these ancient surfaces. Soils derived from dolerite-rich till have significantly greater depths of staining and ghosts, color-development equivalents of the Bw horizon, and extractable Fe than sandstone-rich tills. However, no significant differences were found in depth of visible salts, salt stage, electrical conductivity of the horizon of salt enrichment, and profile quantities of salts, which implies a similar age for the soils. The soils on the Miocene-aged deposits are less developed than soils of early Quaternary and Pliocene age at lower elevation in the Dry Valleys, inconsistent with the conclusion that these are relict Miocene surfaces. We suggest that the lower stage of soil development on the surfaces of Miocene age deposits primarily reflects higher erosion (deflation) rates than soils on Pliocene age deposits at lower elevation. If so, several meters of material may have been removed on 10-million-year timescales and that many of the erosion features ascribed to ancient glacial erosion could simply be the result of subaerial erosion under cold desert conditions. In this case, the soils would only reflect the climate of the last few million years. Although the soils are classified primarily as Typic Anhyorthels and Typic Anhyturbels, some of the soils on doleritic till have saltpans enriched in NaNO3 or Na2SO4 and are classified in petronitric or petrogypsic subgroups.