Discussions and Conclusions-Paleocene-Eocene Paleoenvironments: Before the Gateway Opened | Table of Contents

DISCUSSION AND CONCLUSIONS (continued)

By the latest Paleocene, the areas of continental crust in the land bridge that had been thinned during the Cretaceous--the future Bass Strait, the South Tasman Saddle between Tasmania and the STR, and parts of the TASZ between Antarctica and STR--had subsided and were near sea level. From then on, limited interchange of shallow-marine waters may have occurred between the Australo-Antarctic Gulf and the Pacific Ocean. The plate tectonic reconstructions of Royer and Rollet (1997) and Cande et al. (in press) suggest, and a variety of other tectonic and sedimentary evidence (including that from Leg 189) support, the idea of a shallow-marine connection during the middle Eocene (Fig. 7). However, the very different character of the shallow-marine Eocene sediments recovered at Sites 1168 and 1170 in the restricted waters of the Gulf, and at Sites 1171 and 1172 in the open waters of the Pacific Ocean, indicates that this interchange was not significant.

However, the same lines of evidence suggest that Antarctica and the STR separated during the Eocene-Oligocene transition (Fig. 6). The ODP sites clearly show a rapid change in the earliest Oligocene to similar open-ocean conditions on both sides of the former Tasmanian Land Bridge, and a shallow-water Antarctic Circumpolar Current was established at that time. By the late Oligocene, the Tasmanian Seaway was hundreds of kilometers wide and at abyssal water depths south of the STR, and a shallow to deep Antarctic Circumpolar Current from the west was eroding older sediments. Nearly all of the former land bridge south of Tasmania had by then submerged, and the expanding current was scouring the ocean floor in places. The Tasmanian Gateway was fully open at all depths by the time Drake Passage, south of South America, is inferred to have opened to deep water during the early Miocene (Barker and Burrell, 1977) and could accommodate an ever-increasing circumpolar flow. This increasing strength continued through the remainder of the Cenozoic as the Australian mainland and Tasmania continued to move northward and Drake Passage opened further. The expanding influence of circumpolar circulation led to bottom-water erosion and winnowing over certain areas and was particularly strong during the late Neogene. The expansion of the Antarctic cryosphere during the middle and late Cenozoic, and its effect of strengthening thermohaline circulation at deep and intermediate water depths, contributed to the deep-ocean erosion and formation of hiatuses in the sequences. Examples of this include the middle Miocene scouring east of Site 1170 that removed the Oligocene and the scouring during the Miocene-Pliocene transition in the southern part of the STR.