7. Site 11721

Shipboard Scientific Party2

PRINCIPAL RESULTS

Site 1172 is located in a water depth of ~2620 m on the flat western side of the East Tasman Plateau (ETP), ~170 km southeast of Tasmania. At 44S, 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, and (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 to compare and contrast (4) 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 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 ~65S 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 consisting of basaltic volcanics and volcaniclastics in the middle of the plateau.

At Site 1172 we cored two advanced hydraulic piston corer (APC) holes, one APC/XCB (extended core barrel) 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 in the "Leg 189 Summary" chapter). 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-combination (triple combo) tool string and the geological high-sensitivity magnetic tool (GHMT)-sonic tool string. However, the heave was too great to run the Formation MicroScanner (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. F1). This sequence 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 lower Miocene sections are missing because of current action. A series of volcanic ash horizons of late Eocene to Oligocene age suggests 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 that 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 at 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:

  1. Maastrichtian to early late Eocene deposition of shallow-water siliciclastic sediments during rifting between Antarctica and the STR, a time of minimal or no connection between the Pacific Ocean and the southern Indian Ocean.
  2. A transitional interval of slow sedimentation, with shallow-water late Eocene glauconitic siliciclastic sediments giving way suddenly to earliest Oligocene deep-water clayey pelagic carbonates representing the activation of bottom currents as the Tasmanian Gateway opened and deepened during early drifting.
  3. Oligocene through Quaternary deposition of pelagic carbonate sediments in increasingly deep waters and more open ocean conditions as the Southern Ocean developed and expanded with the northward flight of the ETP and the Australian continent.

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.

1Examples of how to reference the whole or part of this volume can be found under "Citations" in the preliminary pages of the volume.
2Shipboard Scientific Party addresses can be found under "Shipboard Scientific Party" in the preliminary pages of the volume.

Ms 189IR-107

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