Diatoms of the McMurdo ice shelf, Antarctica: Implications for sediment and biotic reworking

Abstract

In the course of investigations of the origin of the McMurdo Ice Shelf (MIS), we analyzed diatoms in 59 sediment samples collected from its upper surface. Fragments of centric marine species, most of which occur in the Ross Sea today, were found in all samples, but identifiable marine taxa dominated the total diatom flora in only two samples. In contrast, non-marine diatoms are abundant and diverse in most MIS samples. These non-marine species probably live in the numerous melt ponds which are distributed widely over the shelf surface during the warmest few weeks of each austral summer. Non-marine diatoms are clustered in four assemblages, which are probably related in a complex way to geographic position on the ice shelf, sediment age, local geochemical variations, and seasonal blooms. The presence of marine diatoms in MIS sediments is consistent with isotopic evidence that the MIS consists of frozen seawater, and with Debenham's hypothesis that transport of marine macrofossils to its surface is accomplished by freezing on at the base and upward movement through the shelf caused by surface melting. The combined marine and non-marine diatom floras of the MIS suggest a new mechanism to explain mixed diatom floras and sediments, observed in antarctic cores. These composite floras and sediments may be introduced or reintroduced to the marine environment during austral summers by melt water streaming off the front of the MIS or through crevasses. Additionally, icebergs calved from the MIS release theie load of mixed biota and sediment when they melt. These observations may pertain to oher present and former antarctic ice shelves that are characterized by surface ablation and basal freezing.