ALBERT

Description: The Subtropical Front (STF) marking the northern boundary of the Southern Ocean has a steep gradient in sea surface temperature (SST) of approximately 4°C over 0.5° of latitude. Presently, in the region south of Tasmania, the STF lies nominally at 47°S in the summer and 45°S in the winter. We present here SST reconstructions in a latitudinal transect of cores across the South Tasman Rise, southeast of Australia, during the late Quaternary. SST reconstructions are based on two paleotemperature proxies, alkenones and faunal assemblages, which are used to assess past changes in SST in spring and summer. The north-south alignment in core locations allows reconstruction of movement of the STF over the last 100 ka. Surface water temperatures during the last glaciation in this region were ~4°C colder than today. Additional temperature changes greater in magnitude than 4°C seen in individual cores can be attributed to changes in the water mass overlying the core site caused by the movement of the front across that location. During the penultimate interglacial, SST was ~2°C warmer and the STF was largely positioned south of 47°S. Movement of the STF to the north occurred during cool climate periods such as the last marine isotope stages 3 and 4. In the last glaciation, the front was at its farthest north position, becoming pinned against the Tasmanian landmass. It moved south by 4° latitude to 47°S in summer during the deglaciation but remained north of 45°S in spring throughout the early deglaciation. After 11 ka B.P. inferred invigoration of the East Australia Current appears to have pushed the STF seasonally south of the East Tasman Plateau, until after 6 ka B.P. when it achieved its present configuration.

Description: We present sea surface temperature (SST) records with centennial-scale resolution from the Bay of Plenty, north of New Zealand. Foraminiferal assemblage-based paleo-SST estimates provide a deglacial record of SST since 16.5 14C ka. Average Holocene SSTs are 15.6°C for winter and 20.3°C for summer, whereas average glacial values were 14.2°C for winter and 19.5°C for summer. Compared to modern time, cooling of SSTs at the Last Glacial Maximum (LGM) was ~0.9°C in winter and ~1.5°C in summer. The shift from glacial to Holocene temperatures began at 14.25 14C ka, warming by ~2°C until 12.85 14C ka when temperatures dipped back to glacial values at 11.65 14C ka. The timing of this return to glacial-like SST correlates well with the Antarctic Cold Reversal (ACR) rather than the Younger Dryas and documents that the influence of the ACR extended into the subtropics of the Southern Hemisphere, at least in this region of the southwest Pacific. By 10.55 14C ka an SST maximum in summer SSTs of up to 3°C warmer than modern occurred (~24°C), after which SST dropped, remaining at present-day temperatures since 9.3 14C ka. This early Holocene climatic optimum has been widely noted in the Southern Ocean, and this record indicates that this phenomenon also extended into the subtropics to the north of New Zealand.

Description: Sediment trap moorings deployed during 1997 and 1998 in the Subantarctic to Polar Frontal regions of the Southern Ocean reveal distinct seasonality in foraminiferal flux. Foraminiferal assemblages vary between each site, yet major species exhibit very similar patterns of seasonal succession which can be associated with changes in mixed layer depth. Enhanced foraminiferal productivity is also associated with periods of high biogenic silica and particulate organic carbon flux. On a broader scale, foraminiferal assemblages are strongly delineated by temperature. Temperature estimates derived from the assemblages using the modern analog technique (MAT) are mostly within 2.5°C of the satellite advanced very high resolution radiometer temperatures observed during the deployment period. This indicates that core top sediments included in the MAT database do reflect modern observed conditions at the sea surface, providing a robust technique for estimating past temperature change from foraminiferal assemblages in Southern Ocean environments.

Description: We present sea surface temperature (SST) estimates based on the relative abundances of long-chain C37 alkenones (UK37') in four sediment cores from a transect spanning the subtropical to subantarctic waters across the subtropical front east of New Zealand. SST estimates from UK37' are compared to those derived from foraminiferal assemblages (using the modern analog technique) in two of these cores. Reconstructions of SST in core tops and Holocene sediments agree well with modern average summer temperatures of ~18°C in subtropical waters and ~14°C in subpolar waters, with a 4°-5°C gradient across the front. Down core UK37' SST estimates indicate that the regional summer SST was 4°-5°C cooler during the last glaciation with an SST of ~10°C in subpolar waters and an SST of ~14°C in subtropical waters. Temperature reconstructions from foraminiferal assemblages agree with those derived from alkenones for the Holocene. In subtropical waters, reconstructions also agree with a glacial cooling of 4° to ~14°C. In contrast, reconstructions for subantarctic pre-Holocene waters indicate a cooling of 8°C with glacial age warm season water temperatures of ~6°C. Thus the alkenones suggest the glacial temperature gradient across the front was the same or reduced slightly to 3.5°-4°C, whereas foraminiferal reconstructions suggest it doubled to 8°C. Our results support previous work indicating that the STF remained fixed over the Chatham Rise during the Last Glacial Maximum. However, the differing results from the two techniques require additional explanation. A change in euphotic zone temperature profiles, seasonality of growth, or preferred growth depth must have affected the temperatures recorded by these biologically based proxies. Regardless of the specific reason, a differential response to the environmental changes between the two climate regimes by the organisms on which the estimates are based suggests increased upwelling associated with increased winds and/or a shallowing of the thermocline associated with increased stratification of the surface layer in the last glaciation.

Description: We use sediment cores from the South Tasman Rise (STR) to reconstruct deep- water circulation in the southwest Pacific sector of the Southern Ocean. Sediment cores MD972106 (45° 09' S, 146° 17' E, 3310 m water depth) and GC34 (45° 06' S, 147° 45' E, 4002 m water depth) preserve records covering the last 160 kyr, with chronology controlled by calibrated accelerator mass spectrometry radiocarbon dates and benthic foraminiferal d18O tied to SPECMAP. The STR benthic foraminiferal d13C records provide new d13C values for Southern Ocean deep water spanning the last 160 kyr at sites unlikely to be affected by variations in productivity. The records establish that glacial benthic foraminifera (Cibicidoides spp.) d13C values are lower relative to interglacial values and are comparable to previous glacial benthic d13C records in the Indian and Pacific sectors of the Southern Ocean. Comparisons of the benthic foraminiferal d13C time series at the STR are made with the equatorial Pacific (V19-30 and Site 846) and the equatorial Atlantic (GeoB1115). The similarity of benthic d13C records at the STR to the equatorial Pacific suggest the Southern Ocean deep-water mass closely tracked those of the deep Pacific, and the presence of a d13C gradient between the STR and the equatorial Atlantic suggests there was continual production of northern source deep water over the past 160 kyr.