4. Site 11691

Shipboard Scientific Party2

PRINCIPAL RESULTS

Site 1169 is located in deep water (3568 m) in a flat plain on the western part of the South Tasman Rise (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 55S compared to its present latitude of 47S. 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 advanced hydraulic piston corer/extended core barrel (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 in the "Leg 189 Summary" chapter). 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. Ostracodes 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.

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-104

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