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Southern Ocean Development and Paleoceanography
Drilling during Leg 189 was conducted to test the hypothesis formulated during DSDP Leg 29 (Kennett, et al., 1974; Kennett, Houtz, et al., 1975; Kennett, 1977) that initial development and evolution of Antarctica cryosphere during the middle and late Cenozoic resulted from thermal isolation of the Antarctic by the development of the Antarctic Circumpolar Current and the Southern Ocean. The Cenozoic expansion of the Southern Ocean resulted from plate tectonic movements that caused northward movement of Australia and its southern continental extension--Tasmania and the STR--and the opening of Drake Passage (Weissel and Hayes, 1972; Barker and Burrell, 1977; Lawver et al., 1992; Cande et al., 2000). The opening of the Tasmanian Gateway created initial thermal isolation and resulted in the major cooling of Antarctica that also produced, through feedback mechanisms, the first ice sheets on at least some sectors of the continent. The four major sites drilled during Leg 189 penetrated to middle Eocene or even Upper Cretaceous sediments, thus providing a climatic and paleoceanographic record for the last 40 to 70 m.y., depending on the site. The sediment sequences reveal a remarkably coherent regional picture reflecting major paleoceanographic changes, especially during the mid- to late Paleogene and continuing into the Neogene, resulting from critical plate tectonic changes related to final separation of the STR from East Antarctica.

Changes in the sediment records drilled during Leg 189 are consistent with plate tectonic reconstructions (Royer and Rollet, 1997; Cande et al., 2000). These reconstructions suggest that before the late Eocene, the Tasmanian land bridge effectively blocked even shallow water connections between the Australo-Antarctic Gulf and the southwest Pacific Ocean (Fig. 23). Initial opening of the Tasmanian Gateway to shallow waters occurred some time in the late Eocene after ~37 Ma, and deeper waters by 33 Ma (earliest Oligocene) (Fig. 24). The two crucial effects of plate tectonics were therefore the initial development of a gateway to shallow waters, followed by subsidence of the STR and establishment of deep-interocean communication. The sediment sequences recovered during Leg 189 suggest that major subsidence of the margins cored was very rapid, beginning near the Eocene/Oligocene boundary and becoming well advanced by the earliest Oligocene. The gateway was then fully open (Fig. 24), and the Tasmanian Seaway, involving both shallow to abyssal water flow, continued to expand during the remainder of the Cenozoic (Fig. 25) as it does today.

Major reorganization of Southern Ocean circulation occurred during the Eocene-Oligocene transition, as proposed by Kennett, Houtz, et al. (1975), Kennett (1977), Murphy and Kennett (1986), as circum-Antarctic circulation commenced through the developing Tasmanian Seaway. Until the early late Eocene (Fig. 23), no Antarctic Circumpolar Current existed to interfere with the influence of the southward-flowing warm-subtropical arm of the South Pacific gyre (East Australian Current). These warm waters were transported close to Antarctica, warming this region and contributing toward the well-known relative warmth of the continent before the late Eocene, a trend supported by Leg 189 data. The opening of the Tasmanian Gateway and the development of circum-Antarctic circulation closely coincides with major cooling and cryospheric development of Antarctica during the Eocene-Oligocene transition (Shackleton and Kennett, 1975; Miller, 1991; Zachos et al., 1994). This allowed relatively cooler waters from the Australo-Antarctic Gulf into the South Pacific as the Antarctic Circumpolar Current, which led to initial decoupling of the warm-subtropical gyre from the Southern Ocean (Fig. 24) and cooling of South Pacific waters at high latitudes (Kennett, 1977; Kennett, 1978; Murphy and Kennett, 1986). The decoupling strengthened as the Tasmanian Seaway continued to open during the Oligocene (Fig. 25) and throughout the remainder of the Cenozoic, with the expansion of the Southern Ocean. The ever increasing strength and width of the Antarctic Circumpolar Current led to increased Cenozoic thermal isolation of Antarctica that, in turn, led to further positive feedbacks reinforcing Antarctic cooling and cryosphere expansion. Although the late Oligocene through Neogene record of this climatic evolution was not immediately apparent during shipboard investigations, we expect that this will be revealed as a result of postcruise isotopic and quantitative microfossil studies.

Major changes in the sediments from Leg 189 are consistent with the hypothesis that major Antarctic cooling occurred during the Eocene-Oligocene transition at the time of the Antarctic Circumpolar Current development through the Tasmanian Gateway. The most conspicuous sedimentological change for the entire last 65 m.y. was the Eocene-Oligocene transition from rapidly deposited siliciclastic sediments to pelagic carbonates. In our sequences, the transition is associated with an increase in bottom currents, while the margins were still relatively shallow, in neritic to upper bathyal depths, leading to a conspicuous reduction in sedimentation rates and deposition of glauconitic silts and sands. Widespread hiatuses in the lower to mid-Oligocene indicate increased and pervasive current activity down to abyssal depths. Increased current strength in the narrow Gateway resulted in increased thermohaline circulation synchronous with psychrospheric development. This, in turn, was linked to enhanced bottom-water production around the now seasonally freezing Antarctic margin (Kennett and Shackleton, 1976).

As discussed above, the relatively sudden decrease in deposition of siliciclastic sediments along the margins of the Tasmanian land bridge is inferred to have resulted largely from sediment starvation caused by a dramatic decrease in precipitation, humidity, chemical weathering, and, hence, run-off and sediment supply from Antarctica associated with the major cooling. This change corresponds to the large positive shift in oxygen isotopic values detected globally in the deep ocean, which reflects both cooling of the deep ocean upon development of the psychrosphere (deep-ocean cool waters) and significant ice accumulation on Antarctica, initiating the cryosphere. The change closely coincides with rapid subsidence of the STR and the Tasmanian margin, which led to open-ocean conditions and encouraged upwelling and increased biogenic sedimentation. Increased upwelling elsewhere, over broad areas of the Antarctic margin, lead to the onset of biosiliceous sedimentation, unlike the pelagic carbonate sedimentation of the Tasmanian margin.

Major glacial development of the Antarctic is inferred to have begun in the earliest Oligocene in a number of areas from the evidence of diamictites (Barron et al., 1991b; Larsen et al., 1991; Cape Roberts Science Team, 2000) and ice-rafted sediments (Hayes, Frakes, et al., 1975; Barker, Kennett, et al., 1988; Zachos et al., 1992). Shipboard investigations during Leg 189 revealed no evidence for Oligocene glacial activity on the Antarctic margin in the offshore Tasmanian region. This is consistent with the observations made during DSDP Leg 29 (Kennett, Houtz, et al., 1975). Furthermore, evidence is lacking for early Oligocene glacial activity at DSDP Site 274 in an equivalent sequence on the conjugate Antarctic margin near Cape Adare, where, even in the late Oligocene, glacial activity is extremely rare (Piper and Brisco, 1975). Our combined results strongly suggest that this sector of the Antarctic margin was relatively warm compared with others, especially in the Kerguelen sector in the southern Indian Ocean. We present the hypothesis that relative warmth of the Tasmanian region resulted from long-term transport of heat toward the Southern Ocean by the warm East Australian Current (Fig. 23, Fig. 24). However, the influence of this warm current apparently did not extend into the southern Ross Sea in the early Oligocene, when marine diamictites were deposited at 77°S at Cape Roberts (Cape Roberts Scientific Team, 2000). Thus, a strong meridional climatic gradient existed in the Ross Sea sector of Antarctica. Indeed, a corollary of Leg 189 interpretations is that Antarctica was clearly marked by strong regional differences in climate during the Oligocene.

Earlier ideas of continent-wide ice sheets of present-day proportions in the Oligocene are unlikely based on descriptions of several margin sediment sequences drilled during the DSDP and ODP programs (e.g., Kennett and Barker, 1990; Hayes, Frakes, et al., 1975), including results from Leg 189. This probably includes the earliest Oligocene interval of inferred major ice growth (Zachos et al., 1992), evidence for which is also apparently lacking in the Leg 189 sequences. This conclusion appears reasonable considering that the Antarctic Circumpolar Current was still developing in the Oligocene, so its unifying circumpolar influence probably had not developed until the Neogene. Until then, regional climatic differences probably existed around the margin. Indeed, if it is correct that deep circum-Antarctic circulation formed in the earliest Miocene (Barker and Burrell, 1977), fundamental paleoceanographic changes associated with this development may have been the basis for differentiation by earth scientists of the Paleogene and Neogene in the dawn of the scientific era.

Neogene sedimentation at Leg 189 sites on the STR and the Tasmanian margin was completely dominated by nannofossil oozes with a significant foraminiferal component. Sedimentation of these pelagic carbonates was largely continuous, except during the late Miocene and earliest Pliocene, at a number of sites. The Miocene-Pliocene transition is missing in a hiatus at Sites 1169 and 1171, and the lower-upper Miocene is missing at Site 1168. Otherwise, the lower and upper Miocene and the Pliocene to Quaternary appear to be largely complete in the sequences. The uppermost Miocene hiatus appears to have resulted from increased thermohaline circulation associated with Antarctic cryospere expansion at that time (Hodell et al., 1986). Altogether, the Neogene sequences cored during Leg 189 provide a fine suite of sequences in present-day temperate (cool subtropical) and subantarctic water masses of the Southern Ocean. These represent a treasure chest for high-resolution Neogene paleoclimatic and biostratigraphic investigations of the Southern Ocean. The Neogene carbonates exhibit changes that record changing environmental conditions in response to the northward movement of the STR, Tasmania, and the ETP from Antarctica and shifting positions of the Subtropical Convergence and the Subantarctic Front.

The pelagic carbonates accumulated at relatively low rates (~2-4 cm/k.y.) typical of the open ocean. Relatively low-diversity benthic foraminiferal assemblages indicate deposition in abyssal depths under generally well-ventilated conditions characteristic of the Antarctic Circumpolar Current region. Other than a small, pervasive clay fraction, siliciclastic sediments are absent throughout the Neogene, except in the early Neogene of Site 1168, which is the site closest to a present land mass. Nannofossil oozes are conspicuously pure white on the STR in the lower Neogene, which corresponds to when the STR was well clear of the siliciclastic influences of Antarctica and yet had not come under the late Neogene influence of increasing aridification and associated dustiness of Australia. Diatoms are consistently present throughout the Neogene carbonates but exhibit a distinct increase in abundance and diversity after the middle Miocene. This almost certainly reflects an increase in upwelling within the Southern Ocean at that time in response to the well-known expansion of the Antarctic cryosphere. A marked increase in carbonate ooze deposition during the early Pliocene at Sites 1169 and 1170, on the southeastern STR, is not observed at other sites, suggesting local concentrations of calcareous nannofossils rather than any regional trend like that in the southwest Pacific (Kennett and von der Borch, 1986). During the latest Neogene, planktonic foraminifers become much more important relative to calcareous nannofossils. This may reflect increased winnowing by deep currents and/or a decrease in relative importance of calcareous nannofossils compared to planktonic foraminifers during the late Neogene.

Postcruise investigations of Leg 189 Neogene sequences will lead to a significant increase in understanding of paleoclimatic and paleoceanographic history of the Southern Ocean. Upper Neogene sections have been satisfactorily spliced from multiple cores in four of the sites (Sites 1168 and 1170-1172) to provide essentially continuous paleoclimatic records. Pervasive sedimentary cycles are apparent throughout the entire Neogene, based on observations of the sediment record and changes in the physical properties of the sediments. Shipboard investigations of clay assemblages suggest relatively warmer conditions during the early Neogene until ~15 Ma. After that, clay assemblages suggest general regional cooling. During the late Neogene, clays became increasingly important in the pelagic carbonates, in part because of increasing dust transport from Australia. A distinct influx of kaolinite in several sites, including the southern STR, during the late Pliocene and Quaternary probably reflects increasing southeastward wind transport of relict clays from an increasingly arid Australia. The uppermost Neogene sediments at Site 1172, which is downwind from Tasmania, exhibit distinct cycles in clay abundance in the biogenic oozes, almost certainly in response to glacial-interglacial oscillations in Australian continental aridity.

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