PRINCIPAL RESULTS (continued)
Site 1170 is located in deep water (2704 m) on the flat western part of the STR, 400 km south of Tasmania and 60 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) a late 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 a northwest-southeast rift between Antarctica and Tasmania during the Cretaceous and moving south with Antarctica until the latest Cretaceous, when it became welded to the remainder of the STR and became 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. Marine magnetic lineations show that in the late Oligocene (26-27 Ma) the east-west spreading axis was 100 km west of the site 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 through the Cretaceous into the Eocene, and hemipelagic sedimentation grading to pelagic sedimentation thereafter (Fig. 13). Much of the Cenozoic siliciclastic detritus must have come from the higher central block 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 2). Because the 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 the middle Eocene (43 Ma) to the 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 late 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%, and organic carbon content is <1%. Average sedimentation rates are low. Deposition was in an open, well-oxygenated ocean in upper abyssal 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%) than Unit I. Organic carbon content is generally very low (<0.5%) between 220 and 270 mbsf. Average sedimentation rates are low. Deposition was in upper abyssal water depths in open ocean conditions.
Subunit IIA is late early Pliocene to late middle Miocene. It is uniform white nannofossil ooze exhibiting laminations that are light bluish to greenish gray. Subunit IIB is late middle Miocene to early middle Miocene. It is also white nannofossil ooze, but 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 early 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 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 layers of hard siliceous limestone. Calcium carbonate percentages are lower (78 to 93%) than in Unit II. Both calcareous (foraminifers and calcareous 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%. 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 more sandy and harder sediments above and more muddy sediments below. Calcium carbonate content is very low (5% average, but as much as 10%) and calcareous fossils are rare, whereas organic carbon content is <1%. 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% on average) and calcareous microfossils are rare. Organic carbon exhibits a steady downward increase from ~0.5% in its upper part to <3.5% in its lower part. Sedimentation rates are high. Palynology and clay minerals (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 of late middle Eocene age and consists of silty claystone. Some levels 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.
Calcareous nannofossils are abundant except in the earliest Oligocene and the Eocene. Planktonic foraminifers and diatoms are abundant down to the middle Miocene, but generally decline below in older sediments. Benthic foraminifers are present, except in the late 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 late Pliocene, are abundant in earliest Oligocene and late Eocene, and reach massive concentrations in the Eocene. During the middle Eocene, dinoflagellate cysts, diatoms, and nannoplankton show intriguing cycles, thought to be related to differing levels of nutrients (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, diatom, and palynologic 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.). There were periods of minimal sedimentation or erosion affecting 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% and is dominantly marine in origin. However organic carbon peaks up to 2% 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 low in the hole are approaching the thermogenic range, and bitumen traces appear to be present. Similar to 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 Unit 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 certainty before it can be converted into a time series. Magnetostratigraphy gave better results than at Site 1168, but these were convincing only in the Pliocene-Pleistocene, the middle and late early Miocene, and the latest 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 siliciclastics for tens of millions of years, and, even though rifting, subsidence, and compaction had started 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 before 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. Then, the climate changed quickly, the supply of siliciclastics dropped off further, slow deposition of pelagic carbonate took over, and the sea deepened rapidly. The most likely explanation is that the 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.
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