Environmental interpretations of the Oligocene Ross Sea using palynomorphs

Abstract

Cenozoic strata around the Antarctic margin provide a record of past variations in ice cover, climate and sea level. This study contributes to the Cape Roberts Project, which cored through 1500 m of Oligocene and Miocene strata near the western margin of the Victoria Land basin. The strata are characterised by the cyclic repetition of facies in a nearshore glaciomarine setting, and are interpreted as a record of around 50 glacial-interglacial episodes involving significant changes in eustatic sea level. This study focuses on the record of vegetation, climate and environmental change provided by terrestrial and marine palynomorphs from two of these sedimentary cycles, the lower being of early Oligocene age (31 Ma), and the upper of late Oligocene (24 Ma). Sixty two samples were processed, yielding modest numbers of miospores (barren to 132 grains per sample, with an average frequency of 1.2 grains per gram) and more abundant marine palynomorphs (with an average frequency of 56 per gram). Previous studies of miospores from Ross Sea sediments had encountered problems differentiating between reworked and in situ miospores. Here, only samples containing a low relative abundance (<50%) of an Eocene marine palynomorph assemblage (the "Transantarctic Flora"), have been used to infer the contemporaneous Oligocene miospore flora. The miospore assemblage of the early Oligocene cycle is dominated by three species of Nothofagidites, and four types of Podocarpidites. The upper Oligocene cycle is also dominated by Nothofagidites and Podocarpidites, but with the addition of Tricolpites sp.a and one taxon of Marchantiaceae, and there is a slight decrease in floral diversity with time. While this study confirms that some floristic change during the Oligocene on the Victoria Land coast, the new data suggest that the change was minor. The vegetation represented in both cycles was probably scrub of low stature and diversity, consisting of Nothofagus, Podocarps, and bryophytes. This vegetation was growing in a much warmer climate (mean summer monthly temperature or MSMT 8-12°C) than is found on the present day Victoria Land coast (MSMT -5°C), although colder that the temperate climate suggested by the diverse flora found in Ross Sea sediments of Eocene age (MSMT 15°C). The marine palynomorph assemblage of the early Oligocene cycle is different from that of the late Oligocene. By reference to modern analogues, the late Oligocene assemblage implies cooler marine conditions with more prolonged sea ice cover, and less fresh water input than the assemblage of the early Oligocene times. This is generally consistent with the results of other studies of the Oligocene marine environment in the Ross Sea. Marine palynomorph variation within both cycles reflects the assemblages changing with the advance and retreat of the ice margin. However, the early Oligocene cycle is characterized by erratic variations in numbers of acritarchs, including Leiosphaeridia, whereas in the late Oligocene cycle the assemblage changes in a more orderly manner with inferred water depth and ice proximity. With the present state of knowledge, individual marine palynomorph taxa cannot be used to infer changes in high latitude Oligocene orbital scale sedimentary environments, but this may be possible when the ecology of modern high latitude palynomorphs is better understood.