Site 1168 is located in middle bathyal water depths (2463 m) on the 4° slope of the western margin of Tasmania (70 km from coast) in a 25-km-wide strike-slip basin between upthrown northwest-trending ridges of Cretaceous rocks. The western Tasmania onshore margin was uplifted during the late Paleocene and early Eocene (O'Sullivan and Kohn, 1997), and this uplift and the strike-slip motion were probably coeval. 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. F12). 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 T2). 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 carbonaceous mudstone and sandstone (Units III-V) of late Eocene age.
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, radiolarians are uncommon, and diatoms rarely present. Microfossil ages (late Oligocene to early middle Eocene) show that sedimentation rates were 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 suggests the presence of gas hydrates, but well-log results are inconclusive. 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. Dinocysts and benthic foraminifers are persistent and radiolarians are uncommon. Microfossil ages indicate very low sedimentation rates in the early Oligocene. 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 an upper 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%. Dinocysts are rare but spores and pollen are abundant, and the abundant dispersed organic matter is dominantly from land plants. Geochemical and micropaleontological evidence suggests periodic brackish conditions, 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 (i.e., 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 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 low sedimentation rates during deposition of glauconite layers because of intensified bottom-water activity leading to winnowing. A condensed section with possible brief 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 reflects the three major steps in Cenozoic climate state determined by earlier researchers: "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 temporarily increased bottom-water activity in the basin. The timing of this episode is consistent with the hypothesis linking the initial opening of the Tasmanian Gateway, 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, and dinocysts 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.
Site 1169 is located in deep water (3568 m) in a flat plain on the western part of the STR, 400 km south of Tasmania. It lies 30 km east of the ridge of the Tasman Fracture Zone (TFZ) that rises 400 m above the plain. The site is ~100 km south of the Subtropical Front (Subtropical Convergence). At Site 1169 we planned to penetrate open-ocean carbonate oozes deposited from the Miocene onward as Australia moved northward from Antarctica. In the early Miocene (20 Ma), the site was at 55°S compared to its present latitude of 47°S. The primary objective was to core a complete upper Neogene sequence with high sedimentation rates in northern subantarctic waters for high-resolution biostratigraphic and paleoclimate investigations.
Seismic profiles indicate that the site is in a westerly thickening wedge of transparent young Neogene ooze, ~200 m thick at the site, that apparently onlaps a prominent reflector and unconformity below, which is more transparent ooze or chalk. This wedge of ooze appears to have been deposited in the lee of the western ridge (TFZ), which provided protection from scouring by the easterly flowing Antarctic Circumpolar Current. The results from Site 1170, where a comparable section was drilled in shallower water to the east, show that the transparent wedge results from facies change rather than younging westward.
We had planned to core three APC/XCB holes, but poor weather conditions and large heaves greatly degraded the quality of the cores, and only Hole 1169A was cored to 246.3 mbsf with 91.4% recovery (Table T2). Although recovery was high in the APC cores, flow-in and other disturbances meant that both core structure and age reliability were severely compromised. This will preclude future high-resolution paleoclimatic investigations.
The drilled sequence consists of 246.3 m of nannofossil ooze with a total age range from the late Miocene (12.2 Ma) to the late Quaternary, although two disconformities removed much of the record. The upper ~200 mbsf of the sequence represents the last ~4 m.y. and disconformably overlies a thin (~200 to 220 mbsf) sequence of late Miocene age (6.5 to 6.8 Ma). This, in turn, is underlain by sediments of middle Miocene age (~12.5 Ma), although strong sediment disturbance makes for difficult dating in this part of the sequence. This time break is correlated with a seismic unconformity. Sediments are dominated throughout by nannofossil ooze with rare to common foraminifers and siliceous microfossils that include diatoms and radiolarians. Siliciclastic sediment components are largely absent in this open-ocean location. One lithostratigraphic unit is recognized, which is subdivided into two subunits: Subunit IA (0-170.1 mbsf) is a nannofossil ooze with common to abundant siliceous microfossils. Subunit IIB (170.1-246.3 mbsf) is a nannofossil ooze with rare to few siliceous microfossils. Sedimentation rates were low (1.6 cm/k.y.) during the Quaternary through late Pliocene, very high (20 cm/k.y.) during the early Pliocene, and moderately high (10.9 cm/k.y.) during the brief late Miocene interval represented. The nannofossil oozes were deposited in upper abyssal water depths under well-oxygenated bottom-water conditions.
Although the primary objective, high-resolution climatic history, could not be met, Site 1169 provides a number of highlights. We were able to develop a useful, although relatively broad, integrated subantarctic biostratigraphy for the Pliocene and Quaternary involving planktonic foraminifers, calcareous nannofossils, diatoms, radiolarians, and organic dinocysts. Ostracods are also persistently present throughout. Few previous sites from the subantarctic region have allowed the development of such an integrated stratigraphy, particularly from the Australian sector of the Southern Ocean. This site also contains the southernmost late Neogene dinocyst record ever found. A conspicuous level of microtektites was discovered in association with the latest Miocene/earliest Pliocene disconformity, the first of this age to be reported from the Southern Ocean. Conspicuous late Miocene unconformities suggest intensification of bottom-water circulation during that time and associated carbonate dissolution on the STR at depths close to 3.5 km.
Planktonic microfossil assemblages reflect the influence of both subantarctic and temperate water masses in this northern subantarctic location. These mixed assemblages may indicate shifts in position of the Subtropical Convergence over the region. Antarctic elements are also present in some planktonic microfossil groups, reflecting influence of more highly productive Antarctic surface waters to the south. The very high sedimentation rates of the early Pliocene at this site have previously been observed over broad areas of the South Pacific and elsewhere (Kennett, von der Borch, et al., 1986). These high rates were considered to represent a significant increase in calcareous biogenic productivity associated with fundamental paleoceanographic changes affecting surface waters during early Pliocene warmth. Rates of early Pliocene biogenic sedimentation at Site 1169 may have been further amplified by winnowing of calcareous nannofossils from the STR into the local catchment basin in which Site 1169 is located. Site 1169 extends observations for the first time to the subantarctic region of remarkably high early Pliocene biogenic productivity.
Site 1170 is located in deep water (2704 m) on the flat western part of the STR, 400 km south of Tasmania and 40 km east of Site 1169. It is 10 km west of a fault scarp, ~500 m high and trending north-south, that separates the lower western and higher central blocks of the STR. The site lies within present-day northern subantarctic surface waters, ~150 km south of the Subtropical Front and well north of the Subantarctic Front. The primary objectives of Site 1170 were to core and log (1) an Eocene detrital section deposited during early rifting between the STR and Antarctica to ascertain marine paleoenvironmental conditions before and leading into the initial marine connection that developed between the southern Indian and Pacific Oceans as the Tasmanian gateway opened during the mid-Paleogene, (2) an Oligocene to Holocene pelagic carbonate sequence to document the paleoceanographic and paleoclimatic responses to the opening of the Tasmanian gateway and subsequent expansion of the Southern Ocean, and (3) an upper Neogene sequence to construct a high-resolution subantarctic biostratigraphy and a high-resolution record of paleoclimatic change.
Plate tectonic reconstructions show the site as being in the broad northwest-southeast Tasmanian-Antarctic Shear Zone during the Cretaceous and moving south with Antarctica until the latest Cretaceous, when it became welded to the remainder of the STR as part of the Australian plate. From the earliest Paleogene, the site was close to the active rift. A shallow sea associated with Paleogene rifting and east-west spreading between Australia and Antarctica placed the site in the far southeastern corner of the restricted Australo-Antarctic Gulf, on the Indian Ocean side of the Tasmanian land bridge. The ridge of the TFZ, 80 km west of the site, formed soon after fast spreading began in the middle Eocene and must have provided east-flowing debris. Marine magnetic lineations show that in the late Oligocene (26-27 Ma) the east-west spreading axis was just west of the TFZ at Chron 8. The passing of the axis probably caused nearby uplift followed by subsidence.
Seismic profiles and regional correlations suggest that the site was subject to steady deposition of prograded siliciclastic deltaic sediments from the Cretaceous into the Eocene, and hemipelagic sedimentation grading to pelagic sedimentation thereafter (Fig. F13). Much of the Cenozoic siliciclastic detritus must have come from the higher central block 10 km to the east, believed to consist largely of continental basement and Cretaceous to Eocene sedimentary rocks. Parts of the central block, which was initially the Tasmanian land bridge, may have remained subaerial and, hence, a source of siliciclastic sediments well into the Oligocene. Seismic profiles suggest that there was a period of current erosion against the fault scarp of the central block, probably during the Miocene. A wedge of sediments was deposited in the depression.
At Site 1170 we cored one APC/XCB hole, two more with the APC, and a rotary-cored hole (Table T2). Because suboptimal weather conditions affected the APC coring, construction of a composite section of the triple-cored portion of the sedimentary sequence was possible only to 70 mbsf (early late Pliocene). Beyond that, there are limited gaps, but overall core recovery averaged 90.4%. Hole 1170A reached 464.3 mbsf with 81.8% recovery. Hole 1170B was APC cored to 175.8 mbsf with 102.2% recovery, and Hole 1170C reached 180.1 mbsf with 99.7% recovery. Hole 1170D was rotary cored from 425 to 779.8 mbsf with 81.1% recovery. Wireline logging was conducted over ~540-770 mbsf in Hole 1170D with the triple-combo string, the GHMT-sonic tool string, and the FMS-sonic tool. Logging was terminated when the drill pipe became stuck in the hole, and the bottom-hole assembly had to be severed with explosives.
Site 1170, with a total sediment thickness of 780 m, ranges in age from middle Eocene (43 Ma) to Quaternary. The older sequence consists broadly of ~282 m of rapidly deposited shallow-water silty claystones of middle and late Eocene age (lithostratigraphic Unit V, see below), overlain by 25 m of shallow-water glauconite-rich clayey siltstone deposited slowly during the latest Eocene to earliest Oligocene (Unit IV). Unit IV is overlain by 472 m of slowly deposited deep-water pelagic nannofossil chalk and ooze of early Oligocene through Quaternary age (Units III-I); limestone and siliceous limestone beds are low in the Oligocene section. There is a hiatus of ~4 m.y. in the mid-Oligocene between Units IV and III. The Neogene is almost completely continuous except for a hiatus of ~4 m.y. in the upper Miocene.
The lithostratigraphic sequence has been divided into five units and a number of subunits.
Lithostratigraphic Unit I (0-93 mbsf), of early Pliocene to Pleistocene age, is a nannofossil ooze with abundant siliceous microfossils. It is generally white with some darker laminations and bioturbation. Carbonate content averages 80 wt%, and organic carbon content is <1 wt%. Average sedimentation rates are low. Deposition was in an open, well-oxygenated ocean in lower bathyal water depths. The considerable kaolinite in the clay fraction may be ancient material derived by increased wind erosion from a more arid Australia.
Lithostratigraphic Unit II (93-373 mbsf) of early Miocene to early Pliocene age has three subunits: Subunit IIA to 181 mbsf, Subunit IIB to 290 mbsf, and Subunit IIC to 373 mbsf. The unit generally consists of white nannofossil ooze or chalk, with more calcium carbonate (average 95 wt%) than Unit I. Organic carbon content is generally very low (<0.5 wt%) between 220 and 270 mbsf. Average sedimentation rates are low. Deposition was in lower bathyal water depths in open-ocean conditions.
Subunit IIA is late early Pliocene to late middle Miocene in age. It is uniform white nannofossil ooze with laminations that are light bluish to greenish gray. Subunit IIB is an upper to lower middle Miocene white nannofossil ooze that lacks laminations. Subunit IIC is white nannofossil ooze to chalk, with some laminations that are light bluish to greenish gray. The presence of quartz grains in the lower middle Miocene supports the evidence from the seismic profiles of a period of increased current activity and scouring (removing all the Oligocene) against the scarp 10 km to the east.
Lithostratigraphic Unit III (373-472 mbsf) is a light greenish gray nannofossil chalk of early Miocene to earliest Oligocene age. The lower part of the unit (below 446.6 mbsf), which is more clay rich, also contains pale gray clay-bearing limestone with evidence of pressure solution and thin hard siliceous limestone layers. Calcium carbonate percentages are lower (78 to 93 wt%) than in Unit II. Both calcareous (foraminifers and nannofossils) and siliceous (diatoms and radiolarians) microfossils are abundant throughout the unit. Organic carbon content is very low, except in the lower part where it reaches ~0.5 wt%. Sedimentation rates are moderate. Paleoenvironmental indicators suggest increasing water depths and more oxygenation from outermost shelf or upper bathyal depths in the lower part of the unit to perhaps lower bathyal depths in the upper part. Although the contact between the limestone and underlying siltstone is very sharp, the sediment character in the lowermost part of the limestone suggests a continued shallow-water influence.
Lithostratigraphic Unit IV (472-497 mbsf) is a dark greenish gray, glauconitic-rich, sandy to clayey siltstone of earliest Oligocene to latest Eocene age. Crystalline quartz, diatoms, and glauconite are very abundant in the upper part of the unit, but decrease downward as it becomes more clayey. About 1.5 m below the top of the unit, there is a break between sandier and harder sediments above and muddier sediments below. Calcium carbonate content is very low (5 wt% average, but as much as 10 wt%) and calcareous fossils are rare, whereas organic carbon content increases to <1 wt%. Carbonaceous fragments and bioturbation are ubiquitous. Sedimentation rates are low. Abundant palynomorphs (dinocysts, spores, and pollen) suggest a cool climate, and temperate forest was on the adjacent land. The clay minerals (illite/smectite) tend to support the evidence of cool climate. The lithologic transition to the underlying sequence is gradational.
Lithostratigraphic Unit V (497-779.8 mbsf) is a bioturbated, dark gray, glauconite-bearing silty claystone to clayey siltstone of late to middle Eocene age that has two subdivisions: Subunit VA to 534.9 mbsf and Subunit VB to 780 mbsf (total depth). Calcium carbonate content is low (<5 wt% on average) and calcareous microfossils are rare. Organic carbon exhibits a steady downward increase from ~0.5 wt% in the upper part of Unit V to <3.5 wt% toward the base. Sedimentation rates are high. Palynolomorphs and clay minerals (mainly smectite) both suggest that conditions were warm, and rainforests cloaked the nearby land. Dinocysts are present in massive concentrations.
Subunit VA is late Eocene in age. It consists of clayey quartzose siltstone with glauconite-rich intervals and some carbonate. Subunit VB is an upper middle Eocene silty claystone. Some horizons contain abundant small (1 mm diameter) white siliceous tubes. There are occasional occurrences of volcanic glass, solitary corals, bivalves, and pyrite nodules. There are also some decimeter-thick beds of grayish or brownish limestone in the lower part.
From a generalized biostratigraphic perspective, calcareous nannofossils at Site 1170 are abundant except in the lowermost Oligocene and the Eocene. Planktonic foraminifers and diatoms are abundant down to the middle Miocene but generally decline in older sediments. Benthic foraminifers are present, except in the upper Eocene, and suggest that water depths were 50-100 m during the middle and late Eocene and deepened rapidly during the early Oligocene. Dinoflagellate cysts are common down to the upper Pliocene, are abundant in the lowermost Oligocene and upper Eocene strata, and reach massive concentrations in the Eocene. In the middle Eocene, dinoflagellate cysts, diatoms, and nannoplankton show intriguing cycles thought to be related to variations in nutrient levels (degree of eutrophication), perhaps related to fluctuations in sea level and/or ventilation. Calcareous nannofossils suggest the possibility of two long hiatuses, one in Unit IV (Eocene/Oligocene boundary) and the other in Subunit VB (middle/late Eocene boundary). However, the existence of such hiatuses is refuted by sedimentologic, and paleontologic (palynomorphs + diatoms) information.
Sedimentation rates determined from the fossil record were rapid (10 cm/k.y.) during the early rifting phase of the middle Eocene, followed by slow sedimentation and condensed sequences during the late Eocene, slow sedimentation during the early Oligocene (1 cm/k.y.), moderate sedimentation for a brief period during the late early Oligocene (5 cm/k.y.), slow sedimentation from the mid-Oligocene to the early middle Miocene (1 cm/k.y.), rapid sedimentation during the late middle Miocene (4 cm/k.y.), and slow sedimentation to the present day (2 cm/k.y.). Intervals of minimal sedimentation or erosion mark the late Oligocene and late Miocene sequences.
The geochemistry data show a very sharp change at the base of the carbonates at the Eocene/Oligocene boundary. This sharp change is associated with a diffusion barrier for pore waters and dissolved gases (e.g., methane is abundant below the barrier but absent above). Organic carbon below the barrier averages 0.5 wt% and is dominantly marine in origin. However organic carbon peaks up to 2 wt% in the lower part of the Eocene and appears to have been caused by increased nonmarine input. A variety of evidence suggests that, despite an only slightly higher than normal present-day thermal gradient, the organic matter is nearing thermal maturity. Gases deep in the hole may have been produced thermogenically, and bitumen traces appear to be present. As at Site 1168, pore waters become fresher with depth. Determination of the source of the fresher (low Cl-) waters awaits further work.
The wireline logs covered only Subunit VB in the bottom of Hole 1170D because of hole stability problems. However, they show a very clear cyclicity of 4.1 m in the Th log, which awaits more paleontologic control before it can be converted into a time series. Magnetostratigraphy provided better results than at Site 1168, but these were convincing only in the Pliocene-Pleistocene, the middle and upper lower Miocene, and the uppermost Oligocene intervals.
The sedimentary succession of Site 1170 records three major phases of paleoenvironmental development:
A question that is being addressed by this and the other nearby sites is why there was 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 siliciclastic sediment for tens of millions of years, and, even though rifting, subsidence, and compaction had begun early in the Cretaceous, sedimentation kept up, and shallow marine sediments were deposited. In the Tasmanian-STR area, 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 during the middle Eocene (43 Ma). This separation could be expected to increase the rate of subsidence, after a time lag, as the thermal anomaly under the margin dissipated. At Site 1170, siliciclastic sedimentation kept up until the Eocene/Oligocene boundary (33 Ma), some 10 m.y. after the onset of fast spreading, even though the local sedimentation rate had declined in the late Eocene. A variety of information suggests that the ridge of the TFZ formed during the middle Eocene fast spreading and was probably a major source of clayey detritus until the late Eocene. Then, the climate changed quickly, the supply of siliciclastics dropped off, slow deposition of pelagic carbonate was established, and the sea deepened rapidly. The most likely explanation is that climatic cooling led to greatly reduced rainfall, weathering, and erosion, and hence to greatly reduced siliciclastic supply. Such changes, from siliciclastic to biogenic sedimentation, appear to be apparent and synchronous wherever ODP drilling has taken place on the Antarctic margin.
In summary, the Eocene siliciclastic sedimentary interval contains a remarkable sequence of abundant organic dinocysts, pollen, and spores in addition to sufficiently persistent calcareous microfossils to assist with age control. The microfossils will provide an integrated record of terrestrial and shallow-marine paleoclimatic history of the Antarctic continental margin in the middle Eocene through early Oligocene. The Oligocene pelagic biogenic sediments provide a sequence of calcareous and siliceous microfossils for integrated studies of the early development of the Southern Ocean, as the STR both subsided and migrated toward the north. The younger Neogene succession generally contains a sequence of calcareous and siliceous microfossils that are abundant and well preserved throughout and will provide excellent paleoceanographic records.
Site 1171 is located in lower bathyal water depths of ~2150 m on a gentle southwesterly slope on the southernmost STR, ~550 km south of Tasmania and 270 km southeast of Site 1170. At 48°30´S, Site 1171 lies in subantarctic waters between the Subtropical Convergence and the Subantarctic Front. In this area, very strong surface and bottom currents are associated with the Antarctic Circumpolar Current. The major objectives were (1) to core and log an Oligocene to Holocene pelagic carbonate section to evaluate expected major paleoceanographic and paleoclimatic effects resulting from the opening of the Tasmanian Gateway near the time of the Eocene/Oligocene boundary and later development of deep Antarctic Circumpolar Current flow, (2) to core and log an expected underlying detrital sedimentary Eocene sequence to evaluate paleoenvironmental conditions during rifting of the STR from Antarctica, and (3) to obtain high-resolution sedimentary records from critical subantarctic latitudes to better understand the role of the Southern Ocean in climate changes during the Neogene.
Site 1171 is located on thinned continental crust, just west of the strike-slip boundary between the central and eastern STR blocks that moved with Antarctica until 66 Ma. The boundary is the Balleny Fracture Zone, which extends southward to Antarctica. Seismic and other data indicate that during the Late Cretaceous to Paleocene, the blocks themselves were cut by strike-slip faults that developed as Australia moved northwestward, and later northward, past Antarctica. Basins that formed in association with this tectonism are filled with ~1000 to 2000 m of Cretaceous through Eocene rift sediments deposited during steady subsidence.
Site 1171 is in a small north-south oriented rift basin, bounded to the east by the Balleny Fracture Zone. The middle Eocene fast seafloor spreading and opening to the south strengthened the basin's connection to the Pacific Ocean and its difference in setting to that of Site 1170 in the Australo-Antarctic Gulf. Seismic profiles and regional correlations suggest that the site was subject to steady deposition of prograded siliciclastic deltaic sediments through the Cretaceous into the Eocene and hemipelagic sedimentation grading to pelagic sedimentation thereafter (Fig. F14). Much of the siliciclastic detritus must have come from the high, subaerial bounding blocks of continental crust and also along the basin from the higher northern areas. The southwestern tip of the STR cleared Antarctica during the early Oligocene and deep Antarctic circumpolar circulation became established. Site 1171 was selected because of its extreme southern location on the STR, in sufficiently shallow water to provide a carbonate sequence unaffected by dissolution. Thus, the site was designed to provide critical data about STR subsidence and on the timing of the initial surface water, and later deep-water flow, through the opening of the Tasmanian Gateway between Australia and Antarctica.
At Site 1171 we cored two APC holes, one APC/XCB hole, and a rotary cored hole (Table T2). Because weather conditions were good during the APC drilling, construction of a composite section of the total triple-cored portion of the sedimentary sequence was possible to 118 mbsf (upper Miocene). Beyond that, there are limited gaps, but core recovery averaged 81.8%. Hole 1171A was APC cored to 124 mbsf with 94.5% recovery, Hole 1171B was APC cored to 109 mbsf with 98.1% recovery, and Hole 1171C was APC/XCB cored to 275 mbsf with 89.4% recovery. Hole 1171D was rotary cored from 248 to 959 mbsf with 73.9% recovery. The interbedded hard and soft beds from 265 to 440 mbsf greatly reduced recovery of both XCB and RCB and stopped XCB coring earlier than desired. Because of operational problems, wireline logging was conducted only with the triple-combo string over most of Hole 1171D.
Site 1171, with a total sediment thickness of 959 m, ranges in age from late Paleocene (58 Ma) to Quaternary. The Neogene section is largely complete except for a hiatus in the uppermost Miocene. The Paleogene record from the early middle Eocene to the latest Oligocene is cut by five hiatuses, and the Oligocene is poorly represented. The older sequence consists broadly of ~616 m of rapidly deposited, shallow-water silty claystone of late Paleocene to late Eocene age (lithostratigraphic Units V and VI) overlain by 67 m of diatom-bearing claystone of late Eocene age (lithostratigraphic Unit IV) and 6 m of shallow water, glauconitic siltstone, deposited slowly during the latest Eocene (Unit III). Unit III is overlain by 67 m of slowly deposited, deep-water nannofossil chalk and ooze of early Oligocene to early Miocene age (Unit II); limestone and siliceous limestone beds are in the base of the Oligocene section. Unit I consists of 234 m of deep-water foraminifer-bearing nannofossil ooze and chalk of early Miocene to Holocene age.
The lithostratigraphic sequence has been divided into six units and a number of subunits.
Lithostratigraphic Unit I (0-253 mbsf), of early Miocene to Pleistocene age, has been divided into two subunits: Subunit IA to 41 mbsf and Subunit IB to 253 mbsf. Subunit IA is a white to light gray foraminiferal nannofossil ooze and foraminifer-bearing nannofossil ooze, whereas Subunit IB is a nannofossil ooze and chalk, which is distinguished from Unit IA by decreasing foraminiferal content. Carbonate content averages 93 wt% and organic carbon is very low (<0.2 wt%) in Unit I. Average sedimentation rates were low.
Lithostratigraphic Unit II (253-270 mbsf), of late Oligocene age, is a white to light greenish gray nannofossil chalk characterized by a downsection increase in the detrital components (e.g., glauconite, quartz, and mica) and a decrease in the biogenic fraction. Organic carbon content is low, and carbonate content decreases from 95 wt% at the top to 75 wt% at the base. Sedimentation rates were very low.
Lithostratigraphic Unit III (270-276 mbsf) is ~6 m of dark greenish gray to blackish green glauconitic sandy to clayey glauconitic siltstone of late Eocene age. Carbonate content decreases from 77 wt% at the top of the unit to 0.4 wt% at the base. Organic carbon content is extremely low and approaches zero.
Lithostratigraphic Unit IV (276-343.5 mbsf) is a middle to upper Eocene nannofossil-bearing, diatomaceous silty claystone that darkens downsection from olive gray to dark gray and bottoms in a black chert. Carbonate content is low (5 wt%) and organic carbon averages 0.4 wt%.
Lithostratigraphic Unit V (343.5-692.5 mbsf) is composed of claystones and silty claystones of middle Eocene age and is divided into three subunits. Subunit VA consists of dark greenish gray or dark olive-gray claystone and nannofossil-bearing claystone. Carbonate content averages 14 wt%, and organic carbon content is 0.5 wt%. Subunit VB is a dark gray claystone, occasionally organic matter-bearing, and is distinguished from Subunit VA by a lower nannofossil abundance, darker color, and higher organic carbon content (average = 1 wt%). Carbonate content is very low (1 wt%). Subunit VC is olive gray to dark olive-gray silty claystone with higher nannofossil abundance and lower organic carbon (0.5 wt%) than the overlying subunit. Carbonate content averages 8 wt%. Sedimentation rates fluctuated between 4-12 cm/k.y.
Lithostratigraphic Unit VI (692.4-958.8 mbsf) is early Eocene to late Paleocene in age and has been divided into two subunits. Subunit VIA is lower Eocene greenish gray nannofossil-bearing silty claystone in the upper part and silty claystone in the lower part. Carbonate nodules and pressure solution seams are sporadic through the subunit. Carbonate content is low, averaging 2 wt%, and organic carbon is 0.5 wt%. Subunit VIB is lowermost Eocene to upper Paleocene silty claystones that give way to dark grayish brown, organic matter-bearing clayey siltstones in the lower part. The bottom ~40 m of the subunit is pervasively laminated. Carbonate content is <1 wt% and organic carbon content (0.9 wt%) is higher than in Subunit VIA. Sedimentation rates averaged 4 cm/k.y.
In general, calcareous nannofossils at Site 1171 are abundant in the Neogene and Oligocene, highly variable in abundance in the Eocene (where they are also absent in many intervals), and rare in the Paleocene. The Neogene and Oligocene are marked by highly abundant and well-preserved calcareous nannofossils and planktonic foraminifers and relatively abundant radiolarians and diatoms. In contrast, the Eocene has many intervals barren of calcareous microfossils, especially planktonic foraminifers. Radiolarians and diatoms are rare to absent throughout much of the Eocene, although neritic planktonic and benthic diatoms are common in the upper Eocene. The shallow-water Eocene siliciclastics are distinguished by a continuous record of abundant organic dinoflagellate cysts and pervasive pollen and spores, which are critical for biostratigraphic subdivision of this interval and provide a rich paleoenvironmental record. The Paleocene sediments also contain abundant assemblages of organic palynomorphs, but only rare to few calcareous nannofossils. Planktonic foraminifers, radiolarians, and diatoms are absent. Benthic foraminifers, which have provided critical information on benthic environments, are largely present throughout the entire sequence and are noticeably more abundant in the Eocene.
Sedimentation rates determined from the fossil record were rapid (4-12 cm/k.y) during the Paleocene to middle Eocene. Biostratigraphic datums indicate four brief hiatuses (~2 m.y.) interspersed with brief periods of slow sedimentation (<1 cm/k.y.) through the late Oligocene to the middle Eocene. Sedimentation rates were low, fluctuating between 0.7-2.0 cm/k.y. in the early and middle Miocene, increased to 3.8 cm/k.y across the middle/late Miocene boundary, and decreased again to a very low 0.5 cm/k.y. in the late Miocene. The Miocene/Pliocene boundary is marked by a hiatus of at least 1.6 m.y., followed by slow sedimentation (1.3 cm/k.y.) in the Pliocene-Pleistocene.
A major result of the coring was the discovery that the unconformity separating flat-lying strata from gently dipping strata in seismic profiles corresponds to the Paleocene/Eocene boundary. This means that tectonism in this small basin, bounded by the major strike-slip fault system of the Balleny Fracture Zone, ended at ~55 Ma. This strongly suggests that the driving force for the strike-slip motion, the separation of Australia and Antarctica, no longer affected this part of the STR from that time, accurately defining the age of the onset of seafloor spreading to the south as 55 Ma. It is surely no coincidence that uplift of the western and eastern onshore margins of Tasmania occurred in the late Paleocene to early Eocene (O'Sullivan and Kohn, 1997).
Similar to Site 1168 on the west Tasmania margin, and Site 1170 on the eastern STR, pore-water freshening (13% decrease in Cl- relative to seawater) is also observed at Site 1171 below ~320 mbsf, which is coincident with the onset of methanogenesis but unexpectedly below the interpreted bottom-simulating reflector. Organic matter is immature through most of the cored interval, with maturity increasing with depth. However, organic matter is mature toward the base of the cored interval, and gases have a thermogenic signature, although total gas quantities are low. Characterization of the organic matter (hydrogen index) indicates three intervals of upwardly increasing marine influence in the early to middle Eocene.
The wireline logs were confined to a single complete run of the triple-combo tool in Hole 1171D because of technical and hole stability problems. Logging data display a strong cyclicity, especially the Th spectrum of the natural gamma-ray log in the middle Eocene section. Variability in log data also may record alternating marine and terrestrial influences. Distinct spikes in resistivity and density are observed in middle Eocene sediments, which likely correlate with indurated carbonates and/or glauconite and tend to be directly above Th and K peaks, indicating increased input of terrestrial clays.
The Paleogene (late Paleocene through Oligocene) depositional history is one of increasing ventilation and a major, rapid increase in water depths that began to occur near the Eocene/Oligocene boundary. This deepening causes transformation from shallow (neritic) to deep open-ocean conditions. Late Paleocene sediments were deposited in near anoxic conditions in nearshore highly sheltered environments, with resulting high organic carbon content. Early to middle Eocene neritic sediments show evidence of being less restricted as reflected by pervasive, well-developed sediment bioturbation and increasing abundance of calcareous nannofossils.
The Eocene-Oligocene transition at Site 1171 is marked by a series of distinct stepwise environmental changes, reflecting increasingly cool conditions and coeval rapid deepening of the basin. By the earliest Eocene, a change had occurred from inner neritic environments with freshwater influences and sluggish circulation, to outer neritic environments with increased ventilation and bottom-current activity. Concomitant cooling is indicated by episodic increases of endemic Antarctic dinocyst taxa, a trend that continued through the late Eocene to earliest Oligocene (~34.0-33.3 Ma). Sediments and biota indicate increasing bottom-water ventilation and more productive surface waters at slightly deeper depths (outer neritic to upper bathyal depths), with increasingly cold conditions. This trend culminated in the early Oligocene (33-30 Ma) with a distinct increase in open-ocean upwelling and rigorous ventilation that precluded accumulation of organic matter, despite the overall higher surface-water productivity. At this time, slow deposition of silica-rich calcareous sediments commenced in lower bathyal depths.
As at Site 1170, the sedimentary succession of Site 1171 records three major phases of paleoenvironmental development that are consistent with the hypothesis that initial development and evolution of the middle and late Cenozoic cryosphere resulted from thermal isolation of the Antarctic by the development of the Antarctic Circumpolar Current and the Southern Ocean:
Although the history of sedimentation at Sites 1171 and 1170 exhibits the same broad regional trends, strong evidence exists that, up until the late Eocene, sediments accumulated in separate basins isolated by the Tasmanian land bridge. This is most clearly shown by the poorly ventilated depositional environment at Site 1170 vs. the relatively more ventilated environment at Site 1171. This is consistent with the interpretation of highly restricted marine conditions at the easternmost end of the Australo-Antarctic Gulf, as is also suggested by the Eocene sediment record at Site 1168 off western Tasmania.
Climatic implications resulting from interpretations of data from Site 1170 and other locations on the STR include the following:
Site 1172 is located in a water depth of ~2620 m on the flat western side of the ETP, ~170 km southeast of Tasmania. At 44°S, the site lies in cool subtropical waters just north of the Subtropical Front in an area where both the Subtropical Front and the East Australian Current have had variable influence through time. The primary objectives of coring and logging at Site 1172 were to obtain in the far southwest Pacific (1) an Oligocene to Holocene pelagic carbonate section under long-term influence of the East Australian Current to construct moderate- to high-resolution paleoceanographic and biostratigraphic records, (2) an Eocene siliciclastic sediment sequence for better understanding of paleoceanographic and paleoclimatic conditions before Antarctic Circumpolar Current development, (3) an Eocene-Oligocene transitional sequence to determine the effects of the initial opening of the Tasmanian Gateway on the paleoceanography of the Pacific Tasmanian margin, and (4) to compare and contrast changing paleoenvironmental and paleoceanographic conditions on each side of Tasmania as the Tasmanian Seaway opened and the Antarctic Circumpolar Current developed. This site was also expected to provide valuable information about the tectonic history of the ETP, including evolution of an inferred volcanic hot spot in the Eocene.
Site 1172 is on thinned continental crust on the western side of the ETP. The plateau is roughly circular, 200 km across, lies in water depths of 2200-2800 m, has Upper Cretaceous oceanic crust to the east and probably to the southwest and south, and is attached to Tasmania to the northwest. During the middle Eocene, the ETP was at ~65°S when its fast northward movement (55 km/m.y.) with Australia commenced. Continental basement rocks form its margins, and seismic profiles and other evidence suggest that at Site 1172 basement is overlain by gently dipping Cretaceous sediments and flat-lying Cenozoic sediments. The late Eocene Cascade Seamount is a guyot in the middle of the plateau consisting of basaltic volcanics and volcaniclastics.
At Site 1172 we cored two APC holes, one APC/XCB hole, and a rotary cored hole. Because weather conditions were good during the APC drilling, construction of a composite section of the total triple-cored portion of the sedimentary sequence was possible to 146 m composite depth (mcd) (late Miocene) (Table T2). Beyond that, there are limited gaps, but core recovery averaged 92%. Hole 1172A was APC/XCB cored to 522.6 mbsf with 92.6% recovery. Hole 1172B was APC cored to 206.7 mbsf with 102.1% recovery. Hole 1171C was APC cored to 171 mbsf with 100.9% recovery. Hole 1172D was rotary cored from 344 to 373 mbsf, drilled to 497 mbsf, and cored to 766 mbsf with 80% recovery. Despite heave of up to 10 m, wireline logging was conducted over most of Hole 1172D with successful runs of the triple-combo tool string and the GHMT-sonic tool string. However, the heave was too great to run the FMS tool string.
The results significantly changed our precruise understanding of the history of the ETP, with much older sequences being cored at the site than expected. Site 1172 penetrated ~65 m of black shallow-marine mudstones of latest Cretaceous (Maastrichtian) age (Fig. F15). This was overlain by 335 m of Paleocene and Eocene brown, green, and gray shallow-marine mudstones and 364 m of Oligocene and Neogene pelagic carbonates. The pelagic carbonates were deposited in ever-increasing depths after rapid Oligocene subsidence, and much of the Oligocene and early Miocene sections are missing because of current action. A series of volcanic ash horizons of late Eocene to Oligocene age suggest that volcanism on Cascade Seamount continued for at least 5 m.y. The lithostratigraphic sequence has been divided into four units, with three subunits in Unit I and two subunits in Units III and IV.
Lithostratigraphic Unit I (0-355.8 mbsf), of early Miocene to Pleistocene age, is divided into three subunits: Subunit IA to 70 mbsf, Subunit IB to 271.2 mbsf, and Subunit IC to 355.8 mbsf. Subunit IA is a white foraminifer nannofossil ooze and foraminifer-bearing nannofossil ooze, whereas Subunit IB is a white and light greenish gray nannofossil ooze. The two subunits are distinguished mainly by a decrease in foraminiferal content in Subunit IB. Subunit IC is a white, pale yellow, and light gray foraminifer-bearing nannofossil chalk marked by an increase in the foraminiferal content and increasing minor components of clay and volcanic glass. Calcium carbonate content increases from 80 wt% in Subunit IA to 97 wt% in Subunit IB and decreases in Subunit IC to 90 wt%.
Lithostratigraphic Unit II (355.8-361.12 mbsf) is a thin uppermost Eocene to Oligocene transitional unit. The sediments are mainly characterized by increased glauconite and a decrease in nannofossil content and consist of variations of greenish gray glauconite-bearing silty diatomaceous claystone and dark greenish gray glauconitic diatomaceous clayey siltstone. A distinct surface at 357.27 mbsf is marked by abundant glauconite and rip-up clasts above and by angular clasts below. This transition may be a highly condensed section or a hiatus. Carbonate content decreases from 69 wt% at the top to 0.3 wt% at the base.
Lithostratigraphic Unit III (361.12-503.4 mbsf), of late to middle Eocene age, has been divided into two subunits: Subunit IIIA to 433.89 mbsf and Subunit IIIB to 503.4 mbsf. Subunit IIIA is a greenish gray and dark brownish gray diatom- and nannofossil-bearing claystone and a very dark grayish brown diatomaceous claystone. Subunit IIIB is a dark gray to dark olive-gray diatomaceous silty claystone. The two subunits are distinguished by calcium carbonate content averaging 10 wt% in Subunit IIIA and very low values, approaching zero, in Subunit IIIB.
Lithostratigraphic Unit IV (503.4-766.5 mbsf) is Late Cretaceous to early Eocene in age and is divided into two subunits: Subunit IVA to 695.99 mbsf and Subunit IVB to 766.5 mbsf. Subunit IVA is a middle Eocene to Paleocene olive-gray claystone with minor amounts of silty claystone, nannofossil-bearing claystone, and clayey siltstone. The subunit is distinguished from Unit III above by a lack of siliceous microfossils and an increase in opaque and accessory minerals, which reach a maximum of 15% at 542 mbsf. Subunit IVB is a Cretaceous (Maastrichtian) very dark olive-gray, very dark gray, and black claystone and silty claystone. It is distinguished from Subunit IVA by its darker color, lesser bioturbation, lesser glauconite content, and greater organic matter content. Sedimentological studies suggest that the Cretaceous/Tertiary (K/T) boundary is at ~696.1 mbsf, where a distinct lithologic change occurs at the subunit boundary, from brown and highly bioturbated silty claystone above to black massive claystone below. Detailed biostratigraphic postcruise studies indicate the K/T boundary is at ~696.4 mbsf. Carbonate is generally very low with a maximum of 6.5 wt% at 762.9 mbsf.
Microfossils are present throughout the entire Cenozoic and upper Cretaceous sequence at Site 1172 with dominance of different groups drastically changing with depositional environments. Siliceous microfossils are rare to absent in the Quaternary to Pliocene interval but are common to abundant and well preserved in the Miocene. The thin Oligocene succession yielded few radiolarians, whereas diatoms remained abundant downhole. Both groups are common to locally abundant and well preserved in the Eocene. The upper Paleocene to upper Maastrichtian interval is virtually barren of siliceous microfossils, although pyritized biogenic silica is present. Planktonic foraminifers and calcareous nannofossils are generally abundant in the Neogene and Oligocene, with preservation ranging from moderate to good. Although less abundant, calcareous nannofossils remain consistently present until the middle Eocene, when abundance and preservation decrease dramatically. Below the middle Eocene, the Cenozoic succession is barren of calcareous nannofossils. Planktonic foraminifers are virtually absent below the middle/upper Eocene boundary.
Well-preserved and reasonably diversified calcareous microfossils are present in the upper Maastrichtian. Calcareous benthic foraminifers are consistently present throughout the Neogene-Oligocene carbonate succession. The middle Eocene sequence yields only rare agglutinated species. However, calcareous and agglutinated taxa are present in the Paleocene to upper Maastrichtian succession. Well-preserved organic walled dinoflagellate cysts and few sporomorphs are present in the Quaternary. The remaining Neogene to lower Oligocene strata are devoid of acid-resistant organic matter. Moderate to well-preserved dinocysts are the dominant constituent of upper Paleocene to lowermost Oligocene palynological associations and are persistent below this interval. Well-preserved terrestrial palynomorphs dominate upper Maastrichtian to middle Paleocene sediments.
Sedimentation rates at Site 1172 form three distinct phases. In contrast to other Leg 189 sites, sedimentation rates were relatively low (between 2.6 and 1.04 cm/k.y.) in the Maastrichtian through late Eocene. From the late Eocene through the middle Miocene (15 Ma) sedimentation rates decreased (0.16 to 3.2 cm/k.y) and then have increased again until the present day. These three intervals coincide with, and are probably related to, the succession in global climate change from "Greenhouse" to "Doubthouse" to "Icehouse" states. Site 1172, like Site 1168, appears to have been strategically located to sensitively record these overall shifts in global climate associated with development of the Antarctic cryosphere. The higher early Paleogene and late Neogene sedimentation rates resulted from more stable climatic conditions. These were associated with the lack of any significant early Paleogene Antarctic cryosphere in the "Greenhouse" world and a late Neogene "Icehouse" world marked by a permanent Antarctic ice sheet. Reduced sedimentation rates during the middle Cenozoic at Site 1172 were associated with more highly variable climatic conditions leading to higher rates of deep-sea erosion. The lower-than-normal regional rates of sedimentation during most of the Paleogene at Site 1172 may have resulted from pervasive, but gentle shallow-water sediment winnowing by the East Australian Current. Relatively higher rates of late Neogene sedimentation probably resulted from higher marine productivity caused by stimulation of surface-water circulation upon middle Miocene expansion of the Antarctic cryosphere.
The geothermal gradient is lower at this site than in the other Leg 189 sites. Despite TOC contents that are similar to those of other Paleogene sequences at the other Leg 189 sites (0.5-1.0 wt%), complete sulfate reduction is not observed and only traces of methane are present. Organic matter is less mature thermally and more labile; however, there is evidence of bitumen in the older siliciclastic sediments, which may indicate the migration of hydrocarbons from below the drilled section. As at other sites, the presence of fresher pore waters was observed on the ETP, which indicates the regional extent of these low chloride fluids.
The sedimentary succession of Site 1172 is similar to that in the other Leg 189 sites in recording three major phases of paleoenvironmental development:
The sediment succession at Site 1172 generally reflects an upward increase in ocean ventilation. Like the other sites drilled during Leg 189, increased ventilation resulted from a fundamental change in paleogeography associated with increasing dispersal of the southern continents and the opening of the ocean basins at high latitudes in the Southern Hemisphere. Thus, the sluggish ocean circulation and restricted environments of sedimentation of the Late Cretaceous and early Paleogene were eventually replaced by well-ventilated open-ocean conditions of the later Cenozoic.
The Paleocene to middle Eocene was relatively warm based on the character of dinocyst assemblages similar to the middle Eocene record at Site 1171. Terrestrial palynomorphs, also indicative of warm conditions, are especially abundant in the lower Paleogene (Paleocene-early Eocene) sediments and suggest very shallow water restricted conditions with marked runoff at this time. An absence of foraminifers, and even of nannofossils, in most parts of the Paleocene to middle Eocene confirms the marginal marine interpretation. Maastrichtian sediments were deposited in more open ocean conditions based on higher abundances of calcareous microfossils, more offshore dinocyst assemblages, and few pyritized diatoms.
The middle/late Eocene boundary is marked by a change from an inner neritic setting with marked freshwater influence and sluggish circulation to more offshore, deeper marine environments with increased ventilation and bottom-water current activity. Concomitant cooling is indicated by the increased numbers of endemic Antarctic dinocyst species, whereas warmer episodes are also recognized. The Eocene-Oligocene transition (~34.0-33.3 Ma) is marked by a series of distinct stepwise environmental changes reflecting cooling and coeval rapid deepening of the basin. Sediments and biota indicate increasing bottom-water ventilation and the appearance of highly productive surface waters, in outer neritic to bathyal depositional settings, associated with the cooling. This trend culminated in the early Oligocene (33-30 Ma) when rigorous ventilation, and generally oxygen-rich waters, precluded sedimentation of organic matter despite overall high surface-water productivity. The condensed calcareous sequence contains abundant siliceous microfossils and was deposited in an oceanic bathyal environment. Oligocene to present-day pelagic carbonates were deposited in well-ventilated open-ocean conditions.
Although Site 1172 reflects the broad patterns of Cenozoic sedimentation for the Tasmanian region, differences from other sites are almost certainly related to the site's position astride the East Australian Current, and relatively isolated on the ETP, away from major sources of detrital sediments. A distinct increase in neritic diatoms during much of the middle Eocene appears to reflect higher productivity in this current than elsewhere on the Tasmanian margin. Increased productivity may have resulted from increased nutrient input swept into neritic environments by the East Australian Current as it moved south adjacent to Australia. A distinct upward increase in kaolinite (and illite) following the middle Miocene (~15 Ma) may reflect the increasing aridity of Australia and the transport of clays into the East Australian Current as it swept southward along the Australian margin.
A complete composite-core record was successfully obtained for the last ~8 m.y. The successful drilling of Site 1172 capped off a highly productive and satisfying coring campaign in the Southern Ocean. Much was learned at sea, and postcruise research is expected to further contribute significantly toward understanding of Southern Ocean and Antarctic environmental development and its role in Cenozoic global climate change.