Abstract:
This study documents the biogenic debris that was recovered from the mostly abiotic landscape of Allan Hills, a nunatak at the edge of the East Antarctic Ice Sheet in South Victoria Land (Latitude ~76.4°S, elevation ~1625-2130m.a.s.l.) with a mean annual temperature of approximately -20°C. The debris was collected from wind traps up and downwind of the nunatak, from snow and glacial ice, regolith overlying Permian to Triassic sedimentary strata and Jurassic volcanic breccias as well as Sirius Group tills.
A wind trap placed on the margin of the Polar Plateau for 5 weeks recovered a diatom assemblage that was relatively diverse and well preserved with high diatom abundances (>107 specimens). Most were freshwater forms, but 6 relatively well-preserved marine specimens were found and no extinct forms were recorded. The wind trap, snow and ice samples recovered diatoms in lower abundances, but some had higher proportions of marine types (5 marine types from a total of 16 fragments). A total of 149 diatom fragments are recorded from 5 wind, snow and ice samples.
Marine diatoms were also observed in most regolith sites and one regolith site recovered a specimen of the fossil diatom species, Thalassiosira torokina dated at 8.6-1.8 Ma (Harwood and Maruyama, 1992). Distribution of the diatoms was patchy and seemed dependent on surface relief and how exposed to the katabatic wind the site was. A total of 232 fragments from 8 samples is recorded, of these 52 are marine taxa.
Aulacoseria (common globally as a eolian micro-particle) was the most common genera found in the sample set and dominated all of the Sirius Group diatom assemblages. 64 specimens were recorded from 8 samples (from 3 sites). Only one marine diatom Leudigera sp. was recovered from the Sirius Group tills and this was in the upper 3 cm of the till. However, an entirely freshwater assemblage was recovered at a depth of 1 metre. Because of the lack of porosity (1-2%) at the Sirius Group deposits sampled at Allan Hills airborne particles are unlikely to penetrate the tillto such a depth. It is concluded that this freshwater assemblage was emplaced into the Sirius Group during deposition.
This study indicates that biogenic debris including marine and freshwater diatoms are being transported from coastal areas, subsequently precipitated onto the Polar Plateau where they are blown by katabatic winds into the Transantarctic Mountains (TAM). In the TAM they become incorporated in regolith sediments and may have the potential to migrate along with melting snow into pore spaces of more consolidated sediments.
The results of this study underlies the rarity of Pliocene age marine diatoms in Sirius Group sediments in contrast to the widespread distribution of diatoms in the wind, snow and regolith of the TAM of South Victoria Land. Although this study does not entirely disprove the glacigenic origin of the Pliocene age marine diatoms within the Sirius Group, it adds support to a growing data-set that infers an atmospheric origin of the diatoms.
