Late pleistocene palaeoceanography of the Tasman Sea

Three ma1or palaeoceanographic aspects of the Tasman Sea: foraminiferal dissolution patterns, oxygen-isotope stratigraphy and sea-level changes, and the hydrographic structure of the upper-water column during the Late Pleistocene (late Brunhes Chron) are discussed in this thesis. Quantitative analyses of planktonic foraminifera from 47 core-top samples (spatially distributed through the Tasman Sea) and cores RC12-113, DSDP 588, 591, 592, 593, ODP 828 and E36-23 (from a north-south transect), supplemented with stable-isotope analyses indicate that: (1) The foraminiferal lysocline, west and east of the Lord Howe Rise (west and east Tasman Sea), occurs at 3,600 m and 3,100 m respectively. For the east Tasman Sea, the foraminiferal lysocline occurs -500 m above the carbonate saturation horizon in the water column. Foraminiferal dissolution patterns parallel the o13C planktonic record (in cores E36-23 and ODP 828) whereas inconsistent fragmentation patterns between cores E36-23 and E39-75 collected in the Tasman Basin were found. Inconsistency was also determined between published foraminiferal dissolution records that are supposed to be typical of two major dissolution regimes, viz. belonging to the Atlantic-Southern Ocean and the Pacific-Indian Ocean. Global variations in o13C and/ or local productivity levels are suggested to explain the previous observations; a connection with the "switch-on" and '-off" of the North Atlantic conveyor belt is recognized. Nonetheless, oxidation of organic matter in pore waters must imprint a signature on foraminiferal dissolution patterns. (2) The high-resolution foraminiferal fragmentation record of core ODP 828 (offshore Vanuatu) lags behind the o18Q record by 8 ky during glacial termination I, and closely resembles the atmospheric record of CO2 and the 8180 stacked pattern. The lysocline level (and the carbonate saturation horizon) was deeper than 3,100 m during intervals of dissolution minima and ranged from -2,550 to 3,000 m during intervals of dissolution maxima for the last 250 ky. (3) The o 180 record of Globigerinoides sacculifer in core ODP 828 exhibits glacial/interglacial amplitudes as high as 1.96%0 that exceed any other previously recorded Pacific planktonic signal (<-1.5%0). The ODP 828 8180 pattern for the last 140 ky, when compared to the glacio-:eustatic sealevel curve (based on the Papua New Guinea record), shows a good correspondence with minimum or maximum ice models, except for isotopic stages 3 and 4. A reduced drop in sea-surface temperature ( <2°C) is suggested for the last glacial maximum in accord to CLIMAP's project estimates. The planktonic ODP 828 8180 record covaries with the benthonic record of core Vl 9-30 except for stages Sa to Sd. Because a simultaneous change in temperature between the sea-surface and the sea-bottom is unlikely, most of the 8180 record of cores ODP 828 and V19-30 is attributed to ice-volume effects. This suggestion is further supported by a sensitivity experiment applying Birchfield's (1987) approach. A conservative value of -1.7%0 is attributed to the ice-volume effect. The comparison between the ODP 828 8180 record and the DSDP 594 8180 record suggests drops in temperature for the latter core during stages 2, 4, Sb and Sd. Drops in temperature for stages 4 to Sd appear to support a previous suggestion of additional modes of operation of the ocean-atmosphere system. (4) Three planktonic foraminiferal assemblages (defined by factor analysis on core-top material) were found to correlate with three hydrographic modes defined on the basis of, sea-surface temperatures, temperature profiles, seasonal changes in temperature and mixed-layer depth. Similar assemblages were found in down-core material along the Tasman Sea transect that combined (1) with semi-quantitative analyses of foraminiferal assemblages (provinces), and (2) the coiling ratio of Globorotalia truncatulinoides, permitted reconstructing the position of the Tasman Front during the late Brunhes Chron. The Tasman Front migrated from its present position at -30°S to -26°S during the last glacial maximum. The northward migration of the Tasman Front may indicate a reduction in volume of the equatorial warm-pool and/ or an increase in the strength of cool subtropical water masses. The distribution of G. truncatulinoides leftcoiling form appears to be related to the formation of Subantarctic Mode Water thermostads. This relationship may explain the evolutionary origin of this species in the south-west Pacific at 2.4 my, simultaneously with the onset of northern hemisphere glaciations. The past position of the Subtropical Convergence is reconstructed by means of coiling directions in Neogloboquadrina pachyderma. The Subtropical Convergence apparently migrated slightly north of 43°S (core E36-23) during glacial stages 6, 10, and 12.