Discussion and Conclusions-The Eocene-Oligocene Transition: The Gateway Opens | Table of Contents

DISCUSSION AND CONCLUSIONS (continued)

Highly restricted nearshore conditions in the middle to early late Eocene at all sites are indicated by a wide range of criteria including pervasive pollen and spore assemblages, abundant continuous low-diversity assemblages of organic dinocysts indicative of eutrophic and brackish surface waters, and limited representation of open-ocean planktonic microfossils. Diverse pollen and spore assemblages are particularly abundant in the Paleocene sequence at Site 1171, indicating particularly strong continental influence with abundant, diverse plant communities. In situ assemblages of planktonic foraminifers and radiolarians are largely absent, diatoms are rare, and calcareous nannofossils are uncommon. Good preservation of calcareous nannofossils suggests that they are uncommon, not because of dissolution, but because of the restricted coastal setting in conjunction with high sedimentation rates. However, the presence of foraminiferal linings suggests dissolution of foraminifers at some levels.

Ventilation on the shelves over much of the region was poor to limited throughout the middle to early late Eocene judging from high TOC, limited bioturbation, and benthic foraminiferal assemblages dominated by agglutinated forms and nodosariids. The sediments at Sites 1168 and 1170 suggest that the eastern, remote end of the narrow, restricted Australo-Antarctic Gulf was poorly ventilated. Low oxygen levels in waters of the eastern Gulf would have developed because of the restricted circulation (2000 km from the open Indian Ocean) and the proximity of inferred abundant terrestrial organic sources from the surrounding land masses, including Antarctica, warm climatic conditions, and possible high marine biotic productivity in eutrophic conditions. Ventilation was especially poor at Site 1168, off western Tasmania, as indicated by laminated sediments. Deeper water sediments at nearby Site 282 were also poorly oxygenated. Anoxic to dysoxic depositional environments would be expected within an expanded oxygen minimum zone that extended up onto the continental shelf at Site 1168. Eocene sediments at Site 1170, in the extreme southeastern corner of the Gulf, were also deposited in poorly oxygenated conditions on a tranquil shelf. Here, the absence of laminations and a slight increase in bioturbation in some intervals suggest greater shelf ventilation than at Site 1168, but circulation was almost certainly sluggish.

The poor ventilation in such shallow-water depths at Site 1170 suggests that the Tasmanian Gateway generally was closed to even shallow waters (~100 m) during the middle and early late Eocene, at least in the developing rift basin between Antarctica and the STR. This is supported by the contrasting Eocene record at Site 1171, near the developing seaway on the Pacific side of the Tasmanian Land Bridge, where the shelf sediments are well bioturbated, suggesting with other evidence more strongly ventilated conditions than in the Gulf and no oxygen minimum zone. If the Tasmanian Gateway had been open widely at shallow depths, there would have been little basin to basin fractionation and, hence, little contrast in sedimentary environments. Middle Eocene sedimentation at Site 1172 bears considerable resemblance to that at Site 1171, but it was in a more oceanic, better ventilated environment within the open Pacific Ocean. The proximity of Site 1172 to the large and newly formed subaerial Cascade Seamount may have increased the speed of bottom currents nearby, thus enhancing ventilation.

The middle through upper Eocene sediment sequence at Sites 1170 and 1171 reveals distinct cycles in physical properties, sediments, and microfossil assemblages. Alternations between dark, poorly bioturbated, nannofossil-poor sediments lacking glauconite and lighter, more nannofossil abundant, bioturbated sediments containing glauconite probably result from changes in water-mass ventilation. Quantitative changes in dinocyst assemblages are cyclic. The darker sedimentary intervals are associated with higher abundances of dinocysts (including massive monotaxic blooms) characteristic of eutrophic conditions and suggesting high nutrient supply to surface waters. In contrast, dinocysts characteristic of more oligotrophic surface waters dominate the lighter intervals in association with more abundant calcareous nannofossils. Clear cyclicity with probable Milankovitch periodicity is also evident in Th abundance in the logging data.

These middle to late Eocene shelfal cycles almost certainly will be shown to correlate with changes in sediment and biotic characteristics. The cause of the cycles is yet to be determined, but they probably resulted from minor climatic oscillations at high southern latitudes, perhaps associated with small sea-level changes, which caused changes in siliciclastic sediment supply, upwelling, and nutrient supply, and associated changes in bottom-water ventilation. The relative importance of marine vs. terrestrial sources of organic carbon varied at the different margin locations during the Eocene. At Site 1168, terrestrial carbon sources dominate, but marine organic carbon dominates at Sites 1170 and 1171, where marine productivity was high in a restricted nearshore setting. In contrast, Paleocene sediments, sampled only at Site 1171 on the southern STR, clearly exhibit a much stronger terrestrial influence with abundant terrestrial plant materials (e.g., pollen and spores). Throughout the Paleocene to middle Eocene, sedimentation kept up with subsidence and compaction.

Evidence for glaciation is completely lacking during the Paleocene and Eocene. Both marine and terrestrial microfossils indicate cool, temperate conditions throughout the middle and late Eocene. Calcareous nannofossil assemblages are of relatively high diversity and appear to be slightly warmer in this sector of the Antarctic than at comparable latitudes elsewhere (although almost completely lacking warmth-loving discoasters). Their diversity suggests an absence of seasonal sea ice over the shelf and marine conditions during much of the time. Middle Eocene clay assemblages at Site 1170 are completely dominated by smectite, suggesting warm temperatures, seasonal rainfall, and a predominance of chemical over physical weathering.

Relatively warm, cosmopolitan dinocyst assemblages of the middle Eocene were in part replaced during the late Eocene by cooler, endemic Antarctic forms. This inferred cooling is consistent with stable isotopic records that indicate progressive cooling of the Southern Ocean during the middle and late Eocene (Shackleton and Kennett, 1975; Stott et al., 1990; Kennett and Stott, 1990). Cooler continental conditions during the late Eocene are indicated by conspicuous increases in illite relative to smectite clays at Sites 1170 and 1171, suggesting a reduction in continental chemical weathering, perhaps in combination with increased tectonism near the expanding rift. Nevertheless, the character and abundance of the organic dinocyst and diatom assemblages throughout the entire middle to upper Eocene suggest that at no time was cooling sufficient to form important seasonal sea ice in this Antarctic region. Pollen and spore records suggest that the middle and late Eocene plant communities on the hinterlands were diverse and cool temperate. Floras were dominated by Nothofagus, podocarps, and other forms with an understory of ferns, similar to a floral assemblage of similar age previously documented in the Weddell Sea sector of Antarctica (Mohr, 1990). Although upper Eocene pollen assemblages appear cooler, they still indicate relative warmth along the Antarctic margin compared with the distinctly cooler Oligocene that followed. Oligocene floras, if they existed, left no record because palynomorphs were not preserved in the pervasive carbonate sediments, probably because of oxidation. Therefore, a nonglaciated, cool, temperate climate prevailed on this sector of the Antarctic margin during the middle to late Eocene. This contrasts with the Prydz Bay margin, where clear evidence exists for late Eocene glaciation (Barron et al., 1991a) and early Oligocene development of a major ice sheet (Zachos et al., 1992, 1993, 1996).

In summary, the contrast between the poorly oxygenated, shallow-marine waters of the shelf of the Australo-Antarctic Gulf and the better oxygenated waters in the Pacific Ocean suggests a general lack of interchange between southern Indian and Pacific Ocean waters. The evidence discussed above suggests that the Tasmanian Gateway remained more or less closed, even to shallow-surface waters, until the late Eocene at ~36 Ma. Sites 1168 and 1170 in the Gulf are much more poorly ventilated than Sites 1171 and 1172 in the Pacific Ocean.