Discussions and Conclusions-Southern Ocean Development and Paleoceanography | Table of Contents

DISCUSSION AND CONCLUSIONS (continued)

Paleogene Margin Carbonates: Interplay between Tectonics and Climate
The sequences at Sites 1170 and 1171 have revealed a marked difference in the Eocene-Oligocene sediment transition as compared with other parts of the Antarctic margin. As far as we can tell, no other margin sectors experienced pelagic carbonate deposition immediately following the Eocene/Oligocene boundary. Other areas experienced deposition of biosiliceous sediments or more slowly accumulating siliciclastic sediments with an increased siliceous biogenic component. The environment at the Tasmanian-Antarctic margin apparently favored biogenic carbonate preservation and relatively low biosiliceous productivity. Here, even Eocene siliciclastic sediments generally contain a better record of better preserved calcareous nannofossils and foraminifers than elsewhere. These observations suggest different climatic regimes in different sectors of the Antarctic margin during the Eocene and Oligocene. Circum-Antarctic climatic and oceanographic similarity, a hallmark of the modern Antarctic, did not exist during the Paleogene. Biogeographic evidence from calcareous nannofossils and lack of any evidence for glaciation in the Tasmanian sector indicate that conditions were slightly warmer than in other margin sectors, even during the Oligocene. The earliest Oligocene was a time of major cryosphere expansion in the southern Indian Ocean sector and in the southern Ross Sea. Thus, circum-Antarctic circulation was still insufficiently developed in the Oligocene to unify the character of paleoenvironmental change around the Antarctic margin.

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