DISCUSSION AND CONCLUSIONS (continued)Paleogene Margin Carbonates: Interplay between Tectonics and Climate
Why are carbonates generally better preserved on the STR during the Paleogene than elsewhere
on the Antarctic margin? This region may have been under the strong influence of warm low
latitude surface waters carried southward along the eastern margin of Australia by the East
Australian Current and southward around western Australia into the Australo-Antarctic Gulf.
Southward flow of warm waters along the east Australian margin would have been enhanced by
constriction in the Indonesian Seaway in the Oligocene (Hall, 1996). These warm tropical waters
would have been relatively saline and, thus, would have helped promote production of deep
waters. Hence, this sector of the margin may have operated in an antiestuarine mode (Berger et al.,
1996) marked by a downward flux of deep waters and an inward flow of surface waters, as in the
modern North Atlantic. In this case, upwelling of nutrient-rich waters is diminished and carbonate
accumulation is favored. Other sectors of the Antarctic margin, marked by strong carbonate
dissolution at shallow water depths and high biosiliceous accumulation, may have operated in
estuarine mode, marked by an upwelling of nutrient-rich deep waters and an outflow of surface
waters. There, carbonate dissolution is favored by the upwelling of old, deep, low-alkalinity, high
pCO2 waters such as in the modern north Pacific Ocean.
A major strengthening of oceanic thermohaline circulation occurred at the climatic threshold of the Eocene-Oligocene transition. This resulted largely from the major cooling and cryospheric development of Antarctica (Kennett and Shackleton, 1976). This cooling, in turn, led to increased aridity of the continent and a major reduction of freshwater flow to the surrounding continental margin, which is reflected by marked reduction in the transport of siliciclastic sediments to the Antarctic margin. Surface waters near the margin would therefore have increased in salinity. A major positive feedback almost certainly would have resulted, with further strengthening of bottom-water production and expansion of the oceanic psychrosphere (deep-ocean circulation). Thus, the delivery of high salinity surface waters to the STR sector of the Antarctic margin, caused by its unusual plate tectonic setting, may well have especially enhanced bottom-water production and, in turn, increased carbonate-biogenic accumulation on the margin.
Discussions and Conclusions-Southern Ocean Development and Paleoceanography | Table of Contents