PRINCIPAL RESULTS (continued)
Site 1172 is located in a water depth of ~2620 m on the flat western side of the ETP, ~150 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 histories, (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 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 Late Cretaceous oceanic crust on three sides, and is attached to Tasmania to the northwest. During the middle Eocene, the ETP was at ~65°S when its fast movement (55 km/m.y.) north with Australia commenced. Continental basement rocks form its margins, and seismic profiles and other evidence suggest that at the site they are 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 2). 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. 15). 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 the volcanism that formed the 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 foraminifer content in Subunit IB. Subunit IC is a white, pale yellow, and light gray foraminifer-bearing nannofossil chalk characterized by an increase in the foraminifer content and increasing minor components of clay and volcanic glass. Calcium carbonate content increases from 80% in Subunit IA to 97% in Subunit IB and decreases in Subunit IC to 90%.
Lithostratigraphic Unit II (355.8-361.12 mbsf) is a thin transitional unit of latest Eocene to Oligocene age. The sediments are mainly characterized by increased glauconite and a decrease in nannofossils 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 characterized 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% at the top to 0.3 % 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% 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 of Cretaceous (Maastrichtian) age and is a very dark olive-gray, very dark gray, and black claystone and silty claystone. It is distinguished from Subunit IVA by its darker color, less bioturbation, less glauconite grains, and more organic matter. Sedimentological studies suggest that the Cretaceous/Tertiary boundary is at 695.99 mbsf, where a distinct lithologic change occurs at the subunit boundary, from brown and highly bioturbated silty claystone above to black massive claystone below. 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 with dominance of different groups drastically changing with sedimentary 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 in the Eocene with good preservation. The upper Paleocene to upper Maastrichtian interval is virtually barren of siliceous microfossils. 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/late 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 agglutinating species. However, calcareous and agglutinating taxa are present in the Paleocene to upper Maastrichtian succession. Well-preserved organic-walled dinoflagellate cysts (dinocysts) 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 palynological associations in the upper Paleocene to lowermost Oligocene and are persistent below this interval. Well-preserved terrestrial palynomorphs dominate upper Maastrichtian to middle Paleocene sediments.
Changes in sedimentation rates exhibit three distinct phases in the sequence. In contrast to other Leg 189 sites, sedimentation rates were relatively low (between 2.6 and 1.04 cm/k.y.) in the Paleogene interval (and Maastrichtian) through the late Eocene. From the late Eocene through the middle Miocene (15 Ma) sedimentation rates decrease (0.16 to 3.2 cm/k.y) and then increase 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." Site 1172, like Site 1168, appears to have been strategically located to sensitively record these overall shifts in global climate associated with the development of the Antarctic cryosphere. The higher rates of sedimentation during the early Paleogene and late Neogene resulted from more stable climatic conditions. These were associated with the early Paleogene "Greenhouse" world lacking any significant Antarctic cryosphere and the 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 sediment winnowing in shallow waters by the East Australian Current. Relatively higher rates of sedimentation in the late Neogene probably resulted from higher marine productivity caused by stimulation of surface-water circulation upon expansion of the Antarctic cryosphere in the middle Miocene.
The geothermal gradient is lower at this site than in the other Leg 189 sites. Despite TOC contents that are similar to other Paleogene sequences at earlier 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, this resulted from a fundamental change in paleogeography associated with increasing dispersal of the southern continents and the opening up 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. Terrestrial palynomorphs, also indicative of warm conditions, are especially abundant in the lower Paleogene (Paleocene-early Eocene) sediments. Assemblages suggest especially shallow water and restricted conditions at this time with marked runoff. An absence of foraminifers, and even nannofossils, in most parts of the Paleocene to middle Eocene confirms the attribution to marginal marine settings. Maastrichtian sediments were deposited in more open ocean conditions based on higher abundances of calcareous microfossils and more offshore dinocyst assemblages.
The middle/late Eocene boundary is marked by a change from inner neritic depositional environments 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 increasingly cool conditions and coeval rapid deepening of the basin. Sediments and biota indicate increasing bottom-water ventilation and the appearance of conditions supporting highly productive surface waters, in outer neritic to bathyal depositional settings, with increasingly cold conditions. This trend culminates 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 are almost certainly related to the site's position astride the East Australian Current. 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 into neritic environments swept 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 was 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.
Summary of Results-Lithostratigraphy | Table of Contents