Atmospheric transport and concentration of diatoms in surficial and glacial sediments of the Allan Hills, Transantarctic Mountains

A global climate up to 3 °C warmer than present around 3 Ma is widely accepted, but the response of the Antarctic ice sheets at that time is still debated. The presence of rare Pliocene marine diatoms in the Sirius Group tillites of the Transantarctic Mountains has led some to infer that the warming caused a collapse of the ice sheet, exposing the Antarctic interior to inland seas. Here we document an alternative source for the Sirius Group diatoms through atmospheric transport onto the surface of the East Antarctic Ice Sheet to be carried downslope by katabatic winds flowing over the Allan Hills, one of many nunataks that border the East Antarctic Ice Sheet. Evidence comes from diatom remains recovered in windtraps, surficial sediments and Sirius Group deposits themselves. The abundance and preservation of the diatoms being actively transported and trapped in the Allan Hills far exceeds those observed in the Sirius Group. Melting of wind-blown snow during summer provides a mechanism to transport and concentrate these diatoms into “diatom-rich” layers. This mechanism, along with the extreme sparseness and sporadic occurrence of diatoms previously documented in the Sirius Group, suggests a more likely scenario in which diatoms from the atmosphere contaminated tillites that had been deposited beneath wet-based glaciers in much older and warmer times. In this scenario, Pliocene marine diatoms may have been injected into the troposphere by meteorite impact, or picked up from exposed land around the Ross Embayment following the disappearance of the Ross Ice Shelf in Pliocene times. We conclude that the Pliocene age assigned to the Sirius Group on the basis of rare diatoms should be regarded as unproven, and the inferred collapse of the East Antarctic Ice Sheet at that time unlikely.