ALBERT

Description: The southwest Pacific Ocean covers a broad range of surface-water conditions ranging from warm, salty water in the subtropical East Australian Current to fresher, cold water in the Circumpolar Current. Using a new database of planktonic foraminifera assemblages (AUSMAT-F2), we demonstrate that the modern analog technique can be used to accurately reconstruct the magnitude of sea-surfacetemperature (SST) in this region. We apply this technique to data from 29 deep-sea cores along a meridional transect of the southwest Pacific Ocean to estimate the magnitude of SST cooling during the Last Glacial Maximum. We find minimal cooling in the tropics (0°-2°C), moderate cooling in the subtropical midlatitudes (2°-6°C), and maximum cooling to the southeast of New Zealand (6°-10°C). The magnitude of cooling at the sea surface from the tropics to the temperate latitudes is found to generally be less than cooling at the surface of adjacent land masses.

Description: Quantitative analyses of planktonic foraminifera from 47 core-top samples (spatially distributed through the Tasman Sea) and cores RC12-113, E36-23, DSDP Sites 588, 592, 593, and ODP Site 828A (from a N-S transect), supplemented with stable-isotope analyses indicate that three planktonic foraminiferal assemblages (defined by factor analysis on core-top material) correlated 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 with semi-quantitative analyses of foraminiferal assemblages (provinces), and the coiling ratio of Globorotalia truncatulinoides, allows reconstruction of the position of the Tasman Front (or the Subtropical Divergence) during the late Brunhes Chron. The results indicate that 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 late Pleistocene distribution of G. truncatulinoides left-coiling from appears to be related to the formation of Subantarctic Mode Water thermostads. This relationship may explain the evolutionary origin of this species in the Southwest Pacific at 2.4 Ma simultaneously with the onset of late Pliocene glaciations. The past position of the Subtropical Convergence is reconstructed by means of coilingv directions in Neogloboquadrina pachyderma. The convergence apparently migrated slightly north of 43°S (core E36-23) during glacial stages 6, 10, and 12.

Description: Paleoceanographic archives derived from 17 marine sediment cores reconstruct the response of the Southwest Pacific Ocean to the peak interglacial, Marine Isotope Stage (MIS) 5e (ca. 125 ka). Paleo-Sea Surface Temperature (SST) estimates were obtained from the Random Forest model-an ensemble decision tree tool-applied to core-top planktonic foraminiferal faunas calibrated to modern SSTs. The reconstructed geographic pattern of the SST anomaly (maximum SST between 120 and 132 ka minus mean modern SST) seems to indicate how MIS 5e conditions were generally warmer in the Southwest Pacific, especially in the western Tasman Sea where a strengthened East Australian Current (EAC) likely extended subtropical influence to ca. 45°S off Tasmania. In contrast, the eastern Tasman Sea may have had a modest cooling except around 45°S. The observed pattern resembles that developing under the present warming trend in the region. An increase in wind stress curl over the modern South Pacific is hypothesized to have spun-up the South Pacific Subtropical Gyre, with concurrent increase in subtropical flow in the western boundary currents that include the EAC. However, warmer temperatures along the Subtropical Front and Campbell Plateau to the south suggest that the relative influence of the boundary inflows to eastern New Zealand may have differed in MIS 5e, and these currents may have followed different paths compared to today.

Description: Oxygen-isotope analyses of planktonic foraminifera from 57 western Pacific deep-sea cores are compared for the Holocene and the Last Glacial Maximum (LGM). Carbonate dissolution, sedimentation rates/bioturbation, sample density, and vital effects are assessed before the sea-surface salinity signal of these records is reconstructed. Average glacial-interglacial Delta delta18O values in the western Pacific are found to be close to those in the Atlantic Ocean (1.76 ‰ in the Pacific compared to 1.80 ‰ in the Atlantic), questioning previous suggestions of a larger salinity difference between the Atlantic and the Pacific oceans during the LGM. During the LGM, sea-surface salinity was higher in the western Pacific than today (by 〉~1 ‰), implying that evaporation minus precipitation was higher all over the region. The minimum change in sea-surface salinity occurred around the equator at the core of the Western Pacific Warm Pool. Holocene high-amplitude, high-frequency fluctuations in planktonic delta18O records north and south of the present limits of the Western Pacific Warm Pool are indicative of sea-surface temperature and/or sea-surface salinity variations related to its expansion and contraction at the scale of thousands of years. Such high-amplitude, high-frequency fluctuations at the edge of the WPWP are best documented in the delta18O signal of ODP Hole 828A offshore Vanuatu, so far the best high-resolution record for the western Pacific.