PRINCIPAL RESULTS (continued)
Site 1171
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. 14). 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 circum-Antarctic 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 the subsidence of the STR 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 2). 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 (late 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 the late Paleocene (58 Ma) to the Quaternary. The Neogene section is largely complete except for a hiatus in the latest 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 low in 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 (0-234 mbsf), of early Miocene to Pleistocene age, has been divided into two subunits: Subunit IA to 41 mbsf and Subunit IB to 234 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 content of foraminifers. Carbonate content averages 93% and organic carbon is very low (<0.2%) in Unit I. In general, calcareous and siliceous microfossils are abundant. Average sedimentation rates were low.
Lithostratigraphic Unit II (234-270 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. Carbonate content decreases from 95% at the top to 75% 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% at the top of the unit to 0.4% at the base. Organic carbon content is extremely low, approaching zero.
Lithostratigraphic Unit IV (276-343.5 mbsf), of middle to late Eocene age, is a 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%) 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%, and organic carbon is 0.5%. Subunit VB is a dark gray claystone, occasionally organic matter-bearing, and is distinguished from Subunit VA by the lower nannofossil abundance, darker color and higher organic matter content (average = 1%). Carbonate content is very low (1%). Subunit VC is olive gray and dark olive gray silty claystone with higher nannofossil abundance and lower organic matter (0.5%) than the overlying subunit. Carbonate content averages 8%. 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 seams are sporadic through the interval. Carbonate content is low, averaging 2%, and organic carbon is 0.5%. Subunit VIB is lowermost Eocene to upper Paleocene silty claystones that gives way to dark grayish brown, organic matter-bearing clayey siltstones in the lower part. The bottom ~40 m of the subunit contains pervasive laminations. Carbonate content is <1% and organic carbon content (0.9%) is higher than in Sububit VIA. Sedimentation rates averaged 4 cm/k.y.
In general, calcareous nannofossils 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 present in the late 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 richness of paleoenvironmental information. 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.
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 observed bottom-simulating reflector. Organic matter is immature through most of the cored interval, with maturity increasing with depth. However, toward the base of the cored interval organic matter is mature, and gases have a thermogenic signature, although 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 appear to be strongly cyclic, especially the Th spectrum of the natural gamma-ray log in the middle Eocene section. Variability in log data also may be recording 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 peaks of Th and K, indicating increased input of terrestrial clays.
The depositional history of the Paleogene (late Paleocene through Oligocene) is one of increasing ventilation and a major, rapid increase in water depths that began to occur near the Eocene/Oligocene boundary causing transformation from shallow (neritic) to deep open-ocean conditions. Late Paleocene sediments were deposited in near anoxic conditions in near-shore highly sheltered environments, with resulting high organic carbon content. Neritic environments of the early to middle Eocene 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 step-wise 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 fresh water 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 continues 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 culminates in the early Oligocene (33-30 Ma) with 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 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.
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, they accumulated sediments in separate basins isolated by the Tasmanian land bridge. This is most clearly shown by the poorly ventilated environment of deposition at Site 1170 vs. the relatively more ventilated environment at Site 1171. This is consistent with the interpretation of more restricted environmental conditions in 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: