Discussion and Conclusions-Paleogene Margin Carbonates: Interplay between Tectonics and Climate | Table of Contents

DISCUSSION AND CONCLUSIONS (continued)

Within the "green sands," despite the winnowing and hiatuses, the angularity of the quartz at some levels indicates periods with little reworking. During the late Eocene, diatoms and benthic foraminifers indicate deepening of the "green sand" environment from neritic to upper bathyal. With steadily increasing water depths in the early Oligocene, deposition changed to open-water pelagic carbonates. This reflects a change during the early late Eocene to early Oligocene, from siliciclastic to biogenic sedimentation, from a poorly to a well-oxygenated benthic environment, from tranquil to moderately dynamic environments, and from relatively warm to cool climatic conditions. The sequence of change over the Eocene-Oligocene transition is remarkable in its consistency over the width and breadth of the Tasmanian-Antarctic margin, as determined from our four deep cored sequences. Differences in detail are clearly related to individual setting at the time of deposition, such as latitude and proximity to the ocean and landmasses.

The glauconitic sediments are strongly bioturbated and were deposited in well-oxygenated bottom waters. The lack of carbonate microfossils in the "green sands" suggests a dissolution episode related to the expansion of the Antarctic cryosphere. This episode is recorded widely in deep-sea carbonate sequences and is associated with the well-known positive oxygen isotopic shift in the early Oligocene (33 Ma) (e.g., Shackleton and Kennett, 1975; Miller, 1991; Zachos et., 1994). Based on our biochronology, the changes in the sediments suggest the opening of the Tasmanian Gateway to some cool shallow-water flow in the late Eocene and intensifying current flow toward the Eocene/Oligocene boundary. This was followed in the earliest Oligocene by expansion of the Antarctic cryosphere and deep-water interchange between the southern Indian and Pacific Oceans. This interchange heralds the initiation of the circumpolar current in this segment of the Southern Ocean. Although planktonic microfossils indicate climatic cooling during this interval, there is no evidence of glacial activity. Indeed, the calcareous nannofossil assemblages suggest somewhat warmer conditions than in sequences at equivalent latitudes elsewhere in the Southern Ocean (Wei and Wise, 1990; Wei and Thierstein, 1991; Wei et al., 1992). The Oligocene clay assemblages at Site 1170 also suggest a transitional climate based on the co-occurrence of both smectite, indicating chemical weathering, and illite, indicating physical weathering.

Water depths throughout the region began to increase rapidly from the lowermost Oligocene upper bathyal depths. They reached upper abyssal depths during the early Neogene with a total water-depth increase averaging 2000 m. Clearly, such a depth increase must reflect rapid subsidence of the STR and the Tasmanian margin during the Oligocene along with expansion of the Tasmanian Gateway. The major and rapid reduction in siliciclastic sediment supply to the margin would also have contributed to the initial depth increase.

The late Eocene "green sands" in the sequences closest to Antarctica (Sites 1170 and 1171) are overlain, with little gradation, by ooze and chalk of early Oligocene age. Near western Tasmania (Site 1168), a similar sequence shows more gradation upward in the Oligocene. From the earliest Oligocene onward, sedimentation in these sequences was completely dominated by relatively slow deposition of nannofossil ooze and chalks, faster than in the "green sands," but much slower than during the early and middle Eocene. Although the age of the base of the carbonates requires better constraint, existing stratigraphic data suggest deposition commenced soon after the earliest Oligocene oxygen isotopic shift at 33 Ma. This isotopic shift represents major cooling and the initial major cryospheric development of East Antarctica (Shackleton and Kennett, 1975; Miller, 1991; Wei, 1991; Zachos et al., 1994) and major expansion of the psychrosphere with its deep ocean circulation (Kennett and Shackleton, 1976). Therefore, the synchronous commencement of biogenic carbonate deposition at all sites appears to reflect major climatic and oceanographic changes that affected broad regions of the STR and Tasmanian margins. This created a more dynamic, well-ventilated ocean with increased upwelling and higher surface-water biogenic productivity that increased rates of sedimentation of calcareous nannofossils and diatoms and decreased preservation of organic carbon. Open-ocean planktonic diatoms replaced neritic diatoms, which suggests coastal upwelling had begun on the STR and Tasmanian margin during the earliest Oligocene. Furthermore, associated cooling of the Antarctic continent apparently decreased weathering rates and transport of siliciclastic sediments to the margin. The environment of deposition was, thus, transformed from the late Eocene to the earliest Oligocene, from dominance of siliciclastics to dominance of calcareous nannofossils.

Sequences from other areas of the Antarctic margin show a similar drastic reduction in siliciclastics and increase in biogenic sedimentation during the Eocene-Oligocene transition. The lowermost Oligocene is often marked by an increase in biogenic sediments or of biogenic components in otherwise relatively slowly deposited siliciclastic sediments including diamictites (Diester-Haass and Zahn, 1996; Kennett and Barker, 1990; Salamy and Zachos, 1999). However, outside the Tasmanian region, the biogenic component is usually biogenic silica (diatoms) rather than calcium carbonate (calcareous nannofossils and foraminifers). On the shallow (probably neritic) northwest margin of the Weddell Sea, carbonate-free diatom ooze was deposited during the earliest Oligocene, suggesting significant cool-water upwelling (Barker, Kennett, et al., 1988). On the margins at Prydz Bay and southern Ross Sea, diatoms became an important component in diamictites (Barron et al., 1991a; Barron et al., 1991b; Exon, Kennett, Malone, in press).

Why was there such a sharp change from siliciclastic to carbonate sedimentation at the Eocene/Oligocene boundary? A very broad, shallow Australian-Antarctic shelf had been supplied with siliciclastics for tens of millions of years, and even though rifting, subsidence, and compaction had started early in the Cretaceous, sedimentation had kept up and shallow-marine sediments were rapidly deposited through until the end of the middle Eocene. In the Tasmanian region, there was also subsidence related to the Late Cretaceous opening of the Tasman Sea. Rifting between Australia and Antarctica gave way to almost complete separation of the continents and fast spreading in the middle Eocene (43 Ma), and this could be expected to increase the rate of subsidence. At Sites 1170, 1171, and 1172, glauconitic and siliciclastic upper Eocene sedimentation almost kept up with subsidence until the Eocene/Oligocene boundary (33 Ma), some 10 m.y. after the onset of fast spreading, even though the sedimentation rate was now low.

Then came the change to much faster subsidence at the Eocene/Oligocene boundary, an effect mirroring the rapid subsidence that formed the Victoria Land Basin in nearby Antarctica at the same time (Cape Roberts Scientific Team, 2000). This subsidence clearly had a regional cause and reduced the area available for erosion in the Tasmanian region. The contemporaneous climatic cooling would have led to greatly reduced rainfall and, thus, weathering and erosion, further reducing siliciclastic supply. Thereafter, the sea deepened rapidly, and slow deposition of pelagic carbonate dominated completely. This appears to have been the most significant interval of sediment starvation along the Antarctic margin.