Accumulation mechanisms of micrometeorites in an ancient supra-glacial moraine at Larkman Nunatak, Antarctica

Abstract

We report the discovery of a large accumulation of micrometeorites in a supraglacial moraine at Larkman Nunatak in the Grosvenor Mountains of the Transantarctic Range in Antarctica. The micrometeorites are present in abundances of ~600 particles Kg-1 of moraine sediment and include a near complete collection of micrometeorite types similar to those observed in Antarctic blue ice and within bare-rock traps in the Antarctic. The size distribution of the observed particles is consistent with those collected from snow collections suggesting the moraine has captured a representative collection of cosmic spherules with significant loss of only the smallest particles (<100 m) by wind. The presence of microtektites with compositions similar to those of the Australasian strewn field suggests the moraine has been accumulating for 780 ka with dust-sized debris. On the basis of this age estimate it is suggested that accumulation occurs principally through ice sublimation. Direct in-fall of fines is suggested to be limited by snow layers that act as barriers to accumulation and can be removed by wind erosion. Micrometeorite accumulation in many areas in Antarctica, therefore, may not be continuous over long periods and can be subject to climatic controls. On the basis of the interpretation of microtektites as Australasian, Larkman Nunatak deposit is the oldest known supraglacial moraine and its survival through several glacial maxima and interglacial periods is surprising. We suggest that stationary ice produced by the specific ice flow conditions at Larkman Nunatak explains its longevity and provides a new type of record of the East Antarctic ice sheet.