PRINCIPAL RESULTS

Site 1168
Site 1168 is located in middle bathyal water depths (2463 m) on the 4° slope of the western margin of Tasmania (50 km from coast) in a 25-km-wide strike-slip basin between upthrown northwest-trending ridges of Cretaceous rocks. The site is 80 km southeast of DSDP Site 282, which is located in deeper water and on a structural high. It lies north of the oceanographic Subtropical Front. Site 1168 was planned to penetrate marine rift to open-margin sediments deposited from the Eocene onward as Australia moved northward from Antarctica. Initially, the site was at the far eastern end of the restricted Australo-Antarctic Gulf and separated from the Pacific Ocean by the Tasmanian land bridge. Plate movements and related margin subsidence led to its Neogene location in open water facing a broad Southern Ocean. The primary objectives were to core and log (1) a prograding detrital sequence, formed during Eocene opening of the ocean south of Australia, for its paleoceanographic, paleoclimatic, and biotic history, (2) an Oligocene to present-day pelagic carbonate sequence for better understanding of the evolution of the Southern Ocean during its expansion in the Cenozoic and for high-resolution paleoclimatic studies, and (3) a Cenozoic sequence for high-resolution biostratigraphic studies.

Seismic profiles suggest that the site was subject to downslope sediment movement to the northwest in the Paleogene but was protected from downslope movement from the east in much of the Neogene by the upslope high (Fig. 12). The Paleocene and Eocene sediments prograde to the northwest, and the drilled late Eocene is hummocky in the southwest-northeast section, with ridges and troughs 0.5-1 km across, suggesting deltaic lobes. Younger sequences are poorly parallel bedded and almost transparent seismically.

We cored two advanced hydraulic piston corer/extended core barrel corer (APC/XCB) holes, and a third hole with just the APC, at Site 1168. Hole 1168A reached 883.5 mbsf with 98% recovery (Table 2). Hole 1168B was APC cored to 108.4 mbsf with 98.3% recovery, and Hole 1168C reached 290.5 mbsf with 85.4% recovery. Wireline logging was conducted in Hole 1168A with the triple-combination (triple-combo) tool string (877 to 101 mbsf) and the geological high-sensitivity magnetic tool (GHMT)-Sonic tool string (730 to 102 mbsf). A bridge prevented running the GHMT-sonic tool string to the base of the cored interval, and we chose not to run the Formation MicroScanner (FMS) because of the poor hole conditions.

Construction of a composite section of the triple-cored portion of the sedimentary sequence (~110 mbsf) indicates that there are no stratigraphic gaps to that depth. Beyond that, there are limited gaps, but overall core recovery averaged 93%, producing an excellent record of near continuous deposition since the early late Eocene. Biostratigraphy indicates no major time breaks. Sedimentation rates were relatively low throughout (6.9-1.5 cm/k.y.) for a margin setting close to land. The drilled sequence broadly consists of 260 m of nannofossil ooze of middle Miocene and younger age (lithostratigraphic Unit I); 400 m of clayey chalk, nannofossil siltstone, and sandstone of early Miocene and Oligocene age (Unit II); and 220 m of shallow-marine late Eocene carbonaceous mudstone and sandstone (Units III-V).

Lithostratigraphic Unit I (0-260 mbsf) was subdivided into two subunits: Subunit IA to 45 mbsf and Subunit IB to 260 mbsf. Subunit IA is light greenish gray foraminifer-bearing nannofossil ooze with minor calcareous turbidite sands, and Subunit IB is white nannofossil ooze. Carbonate content averages 90 wt%, and magnetic susceptibility and organic carbon content are both very low. Calcareous microfossils are abundant and little altered, benthic foraminifers are always present, dinoflagellate cysts are absent only in the middle Miocene, and radiolarians and diatoms are common in the upper Miocene. Microfossil ages (early middle Miocene to Pleistocene) show that average sedimentation rates were low at 1.65 cm/k.y. Deposition was in middle bathyal depths in well-oxygenated bottom waters.

Lithostratigraphic Unit II (260-660 mbsf) has three subunits: Subunit IIA to 410 mbsf, Subunit IIB to 540 mbsf, and Subunit IIC to 660 mbsf. These three olive-gray subunits become darker and more clayey and silty downward. Calcareous microfossils are abundant and little altered in the Miocene and moderately preserved in the upper Oligocene. Dinoflagellates and benthic foraminifers are pervasive, and radiolarians are uncommon. Microfossil ages (late Oligocene to early middle Eocene) show that sedimentation rates are higher, averaging 4.3 cm/k.y. through the Miocene and late Oligocene. Deposition was bathyal with variation in the oxygenation of bottom waters. Visible bubbling in the cores and high methane content in headspace and vacutainer samples indicate production of biogenic gas in the upper part of the unit. An association with reduced pore-water chlorinity initially suggested the presence of gas hydrates; however, a lack of a well-log resistivity shift suggests that this may not be the case. Nevertheless, the presence of fluid escape structures (soft-sediment deformation) in Subunit IIA suggests that hydrates may indeed have been present in the past.

Subunit IIA consists of clay-bearing nannofossil chalk to nannofossil claystone. Carbonate content averages 40 wt%, magnetic susceptibility is moderate, and the organic carbon content is low. Subunit IIB consists of nannofossil claystone and nannofossil-bearing claystone. Carbonate content averages 30 wt%, magnetic susceptibility is moderate, and the organic carbon content is low. Subunit IIC consists of silty nannofossil chalk to nannofossil siltstone. Carbonate content averages 40 wt%, magnetic susceptibility is fairly low, and organic carbon content is low.

Lithostratigraphic Unit III (660-748.6 mbsf) has two subunits: Subunit IIIA to 725 mbsf and Subunit IIIB to 748.6 mbsf. These two olive-gray units become darker downward. They contain calcareous microfossils that are abundant but only moderately preserved. Dinoflagellates and benthic foraminifers are persistent and radiolarians are uncommon. Microfossil ages (early Oligocene) indicate very low sedimentation rates. The environment of deposition was bathyal marine in a relatively tranquil environment. Carbonate content is variable but averages 20-30 wt%, magnetic susceptibility is moderate, and organic carbon content is low. Subunit IIIA consists of clayey nannofossil chalk to nannofossil-bearing organic clayey siltstone. Subunit IIIB consists of organic-bearing silty claystone and organic clayey siltstone.

Lithostratigraphic Units IV and V (748.6-883.5 mbsf) form a late Eocene package of related sediments. These two units are dark gray to black. Carbonate content is low, magnetic susceptibility is moderate but variable, and the organic carbon content is as high as 5 wt%. Dinoflagellates are rare but spores and pollen are abundant, and the abundant dispersed organic matter is dominantly from land plants. Geochemical and micropaleontological evidence suggests a presence of brackish conditions in part, with normal marine salinities at other times. Characterization of the organic matter indicates that it is largely terrigenous in origin and is immature, but with increasing maturity toward the base of the hole (approaching the oil window).

Two nannofossil datums give average sedimentation rates of 6.9 cm/k.y. Calcareous microfossils are sporadic, rarer downward, and moderately to poorly preserved. Agglutinating benthic foraminifers are sporadic. The environment of deposition was reducing, shelfal to bathyal marine, and protected from currents and waves. Evidence from palynology suggests a subtropical to temperate climate, with a terrestrial plant assemblage containing abundant ferns. Calcareous nannofossils are represented by a warm-water assemblage containing warmer water elements than those previously found elsewhere at equivalent latitudes in the Southern Ocean. Plate reconstructions suggest that the Kerguelen Plateau may have been shielding the Australo-Antarctic Gulf from cold water from the west so that the only water entering the gulf came from warmer areas north of western Australia.

Lithostratigraphic Unit IV (748.6-762 mbsf) consists of dark gray glauconitic, quartzose sandstone, and clayey siltstone, with interbedded black carbonaceous silty claystone. Thin calcareous stringers contain microfossils. Both glauconite and quartz are fine to very coarse grained, and the quartz is subangular. Microfossil ages within this Eocene-Oligocene transition indicate very low sedimentation rates during deposition of glauconite layers because of intensified bottom-water activity leading to winnowing. A condensed section with possible hiatuses is indicated.

Lithostratigraphic Unit V (762-883.5 mbsf) consists of black carbonaceous silty claystone and clayey siltstone and is finely laminated in part. Pyritic replacements of burrows and fossils are common. There are rare, thin lenses of rippled fine sand, and thin calcareous stringers contain microfossils.

In summary, the sediment sequence records paleoenvironmental changes, beginning with a shallow-water, nearshore, restricted basinal setting with poor ventilation and siliciclastic sedimentation, low oxygenation, and high organic carbon deposition. Site 1168 Eocene sediments, similar to those at DSDP Site 282 to the northwest and DSDP Site 280 just south of the STR, suggest widespread late Eocene anoxic conditions in the eastern Australo-Antarctic Gulf. Following a transitional phase during the Oligocene, by the Neogene these conditions had been replaced by deposition of carbonate ooze in a well-oxygenated, open ocean on a passive margin at middle bathyal depths.

The succession of sediment, climatic, and biotic changes recorded at Site 1168 reflect three major steps in the state of Cenozoic climate: "greenhouse" in the late Eocene; "doubthouse" of intermediate mode in the Oligocene through early Miocene; and "icehouse" since the middle Miocene. Relatively rapid changes mark the boundaries at the Eocene-Oligocene transition and during the middle Miocene at ~14 Ma. The most conspicuous change in the sediment and biotic sequence occurred during the transition from the Eocene to the early Oligocene with conspicuous reduction in sedimentation rates and deposition of glauconite sands. This transition reflects a transient event associated with temporary increased bottom-water activity in the basin. The timing of this episode is consistent with the hypothesis of linkage with initial opening of the Tasmanian Gateway and/or major cooling of Antarctica and associated cryospheric development. The changes are documented in part by excellent microfossil sequences of calcareous nannofossils, planktonic and benthic foraminifers throughout, and dinoflagellates after the late Eocene. Spores and pollen are abundant in the upper Eocene, fewer in the lower Oligocene, and intermittently present in younger sequences. Major biostratigraphic achievements will be the first comprehensive Cenozoic zonations for the cool temperate region south of Australia for planktonic foraminifers, calcareous nannofossils, and dinoflagellates.