William R. Howard

Old radiocarbon ages in the southwest Pacific Ocean during the last glacial period and deglaciation

Marine radiocarbon (14C) dates are widely used for dating oceanic events and as tracers of ocean circulation, essential components for understanding ocean–climate interactions. Past ocean ventilation rates have been determined by the difference between radiocarbon ages of deep-water and surface-water reservoirs, but the apparent age of surface waters (currently ∼400 years in the tropics and ∼1,200 years in Antarctic waters) might not be constant through time, as has been assumed in radiocarbon chronologies and palaeoclimate studies. Here we present independent estimates of surface-water and deep-water reservoir ages in the New Zealand region since the last glacial period, using volcanic ejecta (tephras) deposited in both marine and terrestrial sediments as stratigraphic markers. Compared to present-day values, surface-reservoir ages from 11,900 14C years ago were twice as large (800 years) and during glacial times were five times as large (2,000 years), contradicting the assumption of constant surface age. Furthermore, the ages of glacial deep-water reservoirs were much older (3,000–5,000 years). The increase in surface-to-deep water age differences in the glacial Southern Ocean suggests that there was decreased ocean ventilation during this period.

Palaeoclimatology: A warm future in the past

To understand the impact of a future Earth which may be warmer than now, geologists look to records of warmer times in the past - often the last interglacial period, around 120,000 years ago. But a better match to todays conditions might be found in a period around 400,000 years ago, an earlier interglacial known as stage 11, when Earths orbital parameters were closer to those of the present day. Stage 11 was an unusual period, with severe climate change and, at its peak, exceptionally warm temperatures.

Global deep-sea burial rate of calcium carbonate during the last glacial maximum

Global databases concentrations rates in Holocene and last glacial of calcium carbonate and mass accumulation maximum sediments were used to estimate the deep-sea sedimentary calcium carbonate burial rate during these two time intervals. Sparse calcite mass accumulation rate data were extrapolated across regions of varying calcium carbonate concentration using a gridded map of calcium carbonate concentrations and the assumption that accumulation of noncarbonate material is uncorrelated with calcite concentration within some geographical region. Mean noncarbonate accumulation rates were estimated within each of nine regions, determined by the distribution and nature of the accumulation rate data. For core-top sediments data coverage 67% of the high-calcite the regions of reasonable encompass (>75%) sediments globally,and within these regions we estimate an accumulation rate of 55.9 +- 3.6x1011 mol yr-1. The same regions cover 48% of glacial high-CaCO3 sediments (the smaller fraction is due to a shift of calcite deposition to the poorly sampled South Pacific) and total 44.1 +- 6.0x1011 mol yr-1. Projecting to 100% coverage South and total 44.1 +- 6.0x1011 both estimates yields accumulation estimates of 8.3x1012 mol yr-1 today and 9.2x1012 mol yr-1 during glacial time. This is little better than a guess given the incomplete data coverage, but it suggests that glacial deep sea calcite burial rate was probably not considerably faster than today in spite of a presumed decreasein shallow water burial during glacial time.

Glacial-interglacial sea surface temperature changes across the subtropical front east of New Zealand based on alkenone unsaturation ratios and foraminiferal assemblages

0 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 U37 K 0 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. INDEX TERMS: 4267 Oceanography: General: Paleoceanography; 4850 Oceanography: Biological and Chemical: Organic marine chemistry; 1050 Geochemistry: Marine geochemistry (4835, 4850); 1055 Geochemistry: Organic geochemistry; KEYWORDS: paleoceanography, sea surface temperature, alkenones, Southern Ocean, Last Glacial Maximum

Southern Ocean seasonal temperature and Subtropical Front movement on the South Tasman Rise in the late Quaternary

[1] The Subtropical Front (STF) marking the northern boundary of the Southern Ocean has a steep gradient in sea surface temperature (SST) of approximately 4C over 0.5 of latitude. Presently, in the region south of Tasmania, the STF lies nominally at 47S in the summer and 45S 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 � 4C colder than today. Additional temperature changes greater in magnitude than 4C 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 � 2C warmer and the STF was largely positioned south of 47S. 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 47S in summer during the deglaciation but remained north of 45S 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.

Planktonic foraminiferal flux seasonality in Subantarctic sediment traps: A test for paleoclimate reconstructions

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.

Seasonality of foraminiferal flux in sediment traps at Chatham Rise, SW Pacific: implications for paleotemperature estimates

Abstract Analysis of sediment traps located either side of the Subtropical Front east of New Zealand reveals a strong association between water masses and foraminiferal assemblages. The composition and timing of foraminiferal productivity is distinct between waters north and south of the front, and these differences are also reflected in the assemblages of nearby core-tops. The sediment trap data indicate highly seasonal flux patterns in this region, so sedimentary records may represent flux during a particular season, rather than throughout the annual cycle. This pronounced seasonality has implications for our estimates of the annual temperature range based on faunal assemblages. This study shows that despite strong flux seasonality the annual sea-surface temperature (SST) range is reliably estimated from the sediment trap foraminiferal assemblages by the modern analog technique. The successful estimation of the annual SST range also indicates that the annual flux obtained from these sediment traps is representative of the longer term flux preserved in surface sediments. Core-top assemblages from this region can therefore be directly related to modern sea-surface conditions, providing an analogue for interpreting past environmental change from fossil assemblages.

Unique and exceptionally long interglacial marine isotope stage 11: window into Earth warm future climate

1 Dept. of Earth Science, Rice University, P.O. Box 1892, Houston, TX 77251, USA andre@rice.edu 2 EMS Environment Inst. and Dept. of Geosciences, Pennsylvania State Univ., University Park, PA,16802, USA 3 Antarctic CRC, University of Tasmania, GPO Box 25280, Hobart, Tasmania 7001, AUSTRALIA 4 U.S. Geological Survey, 12201 Sunrise Valley Drive Reston, VA 20192, USA 5 Lamont-Doherty Earth Observatory, Columbia University, P.O. Box 1000, Palisades, NY 10964-1000 USA

Middle Pleistocene sea-surface temperature change in the southwest Pacific Ocean on orbital and suborbital time scales

A record of estimated sea surface temperature (SST) change between 575 and 400 ka has been obtained from planktonic foraminifera at Deep Sea Drilling Project Site 594 in the southwest Pacific Ocean. The Site 594 record indicates that SSTs during marine oxygen isotope stage 11 were similar to those of the Holocene, in contrast to suggestions of warmer than Holocene SSTs during stage 11. If these SSTs reflect global conditions, then ice-sheet collapse may not require temperatures warmer than in the Holocene. Millennial-scale oscillations in SST (~3 °C) occurred within the stage 12 glacial interval, spaced every ~5-10 k.y., on time scales similar to those observed within stage 12 in the North Atlantic. The consistency between these records may require global-scale mechanisms capable of producing rapid climate change, as suggested for later Quaternary intervals.

Pliocene sea surface temperature changes in ODP Site 1125, Chatham Rise, east of New Zealand

Abstract Planktonic foraminiferal census counts were converted to sea surface temperature (SST) estimates using the modern analogue technique (MAT) for the middle–late Pliocene (4.0–2.37 Ma) in ODP Site 1125, north side of Chatham Rise, SW Pacific Ocean. MAT SSTwarm records range between 8°C and 20.5°C, and MAT SSTcold records parallel that pattern but with a temperature range of 5–15°C. The modern position of Site 1125 is just north of the Subtropical Front and has an annual temperature range of ∼14–18°C. Pliocene warmest temperatures are 1–2° warmer than modern summers, whereas cold season SST records are up to 6–10°C cooler than modern winters. Overall average temperatures at the site are 2–3°C cooler than modern temperatures during a time of sustained global warmth. Three major cold excursions centred on 3.35, 3.0, and 2.8 Ma showed warm season temperatures over 5°C colder than the last glacial maximum, experiencing temperatures typical of modern subantarctic waters. Two minor cold excursions at 2.7 Ma and 2.4 Ma experienced temperatures cooler than modern winters but not as cold as last glacial conditions. Cold season SSTs show a shift to warmer climate upward through the study interval, whereas warm season estimates remain essentially unchanged. We interpret the strong regional cooling of subtropical Southwest Pacific water through the middle–late Pliocene as having been caused by increased upwelling. It is also possible that the subtropical frontal zone moved north over the site in the Pliocene, however, this is considered the least likely interpretation. Our record of cool conditions in the Southwest Pacific corroborate evidence of cooler than modern conditions in other regions of the western Pacific through the mid-Pliocene despite overall global warming.