(last updated October 2006)


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

E. L. Sikes W. R. Howard H. L. Neil and J. K. Volkman

Paleoceaongraphy, 17 (2), 10.1029/2001PA000640.

We present sea surface temperature (SST) estimates based on the relative abundances of long-chain C37 alkenones ( ) in four sediment cores from a transect spanning the subtropical to subantarctic waters across the subtropical front east of New Zealand. SST estimates from are compared to those derived from foraminiferal assemblages (using the modern analog technique) in 2 of these cores. Reconstructions of SST in core tops and Holocene sediments agree well with modern average summer temperatures of ~18C in subtropical waters and ~14C in subpolar waters, with a 4-5C gradient across the front. Down-core SST estimates indicate that the regional summer SST was 4-5C cooler during the last glaciation with an SST of ~10C in subpolar waters and an SST of ~14C 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 4C to ~14C. In contrast, reconstructions for subantarctic pre-Holocene waters indicate a cooling of 8C with glacial-age warm season water temperatures of ~6C. Thus, the alkenones suggest the glacial temperature gradient across the front was the same, or reduced slightly to 3.5-4C, whereas foraminiferal reconstructions suggest it doubled to 8C. 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 requires additional explanations. 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. Full Article [pdf]

Alkenone temperature records and biomarker flux at the Subtropical Front on the Chatham Rise, SW Pacific Ocean

Elisabeth L. Sikes * Teresa OLeary Scott D. Nodder John K. Volkman

Deep Sea Research, 52(5), 721-748.

Alkenones and a suite of sterol biomarkers were examined in two sediment trap arrays deployed at 300 m water depth in subtropical and subantarctic waters to the east of New Zealand from late winter to autumn in 1996-1997. The two traps were located within 200 km of one another and the main difference between the two sites are the differential physical, chemical and biological characteristics of the different water masses in which they were situated. The alkenone-based reconstructions of water temperatures ( ) were compared to the COADS monthly averaged satellite and real-time weekly temperatures for the deployment period. The records correlate well with seasonal sea surface temperatures (SST) for the 9 months of the deployment, with temperature reconstructions within 2C of regional monthly averages for most of the year. There are a few short periods of poorer agreement where alkenone-based reconstructions deviate by up to 4C in both traps. Weekly averages of satellite SST obtained during the time of the deployment indicate that these deviations were not associated with short-term changes in surface temperatures overlying the traps. These instances of poor correlation are not due to lateral advection of particles, but rather seem to reflect differences in environmental controls on alkenone-derived SSTs in the two water masses. Subantarctic traps showed deviations only to warmer than average temperatures. These occurred in early winter and late summer, during times of low lipid fluxes, suggesting that slow growth associated with light limitation may have affected unsaturation levels in the alkenones. The subtropical traps showed deviations only to cooler temperatures, which occurred in the late summer to early autumn. These biases occurred during times of highest lipid fluxes and lowest nutrients in the surface mixed-layer. Alkenone temperatures during maximum flux periods were too cool to be caused by subsurface production alone, suggesting that nutrient limitation effects may have significantly depressed alkenone unsaturation levels. Sterol biomarker associations indicate very different timing and trophic interactions between phytoplankton in the respective water masses. In subtropical waters, fluxes of dinosterol (a biomarker for dinoflagellates) peak first in the spring bloom while alkenones and diatom marker sterols respond synchronously during the main bloom event. In subantarctic waters sterol fluxes indicate a succession of phytoplankton export production in the spring bloom, whereas lipid fluxes normalized to organic flux indicate that alkenone producers were a relatively small proportion of the main spring bloom and proportionally more important when organic fluxes were lower in summer. We infer that the difference in nutrient concentrations between the water masses may drive these trophic differences and the inverse relationship between flux and the temperature response of between the two sites. Full Article [pdf]

Paleoventilation of the Southwest Pacific and Southern Ocean in the Holocene and late Quaternary

E.L. Sikes and T.P. Guilderson

The scientific purpose of this work is to construct a profile of 14C ages for several time slices in the glaciation and deglaciation in order to better constrain past deep water age of the southwest Pacific and Southern Oceans. This project is intended to resolve questions arising from the Sikes et al. [2000] study and address issues encountered in the work of Sikes and Guilderson [2002]. Doing so will better determine the effect of global CO2 cycling on the atmospheric and marine carbon pools for times in the past when ocean circulation was significantly different from today.

The scientific objective of this coring effort is to determine the temporal evolution of the vertical distribution of 14C ages in deep water masses during the glaciation and deglaciation and reconstruct the ventilation history of the deep-ocean for the South-west Pacific region. The ash layers in this study serve as stratigraphic tie points and provide coeval terrestrial (atmospheric) 14C values for our reconstructions. The analyses we will conduct in this sector of the Southern Ocean and the Southern Pacific will permit us to characterize the ventilation state of the waters that comprise North Pacific Deep Water (NPDW). NPDW represents a significant portion of the deep water masses in the ocean. Vertical profiles from this region will provide picture of paleoventilation representative of the whole ocean for the last glacial and deglacial periods. The New Zealand area is well suited for such work with submerged banks across the latitudinal range of interest that rise above the calcite compensation depth (Figure 2). This provides the opportunity to core across a continuous depth range in locations with moderate to high sedimentation rates and high calcium carbonate content.

This work will use ash layers as stratigraphic markers and takes advantage of the well developed identification, chronology, and stratigraphy of tephras in the New Zealand region. In marine cores, the ashes can be used as stratigraphic tie points to accurately date coeval layers in different locations and water depths. Full Article [pdf]

Deglacial paleoceanographic history of the Bay of Plenty, New Zealand

Catherine R. Samson, Elisabeth L. Sikes* and W. R. Howard

*Corresponding Author

Paleoceaongraphy, 20, PA4017, doi:10.1029/2004PA001088.

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 BP. Average Holocene SSTs are 15.6C for winter and 20.3C for summer whereas average glacial values were 14.2C for winter and 19.5C for summer. Compared to modern, cooling of SSTs at the last glacial maximum (LGM) were ~0.9C in winter and ~1.5C in summer. The shift from glacial to Holocene temperatures began at 14.25 14C ka BP, warming by ~2C until 12.85 14C ka BP when temperatures dipped back to glacial values at 11.65 14C ka BP. 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 BP an SST maximum in summer SSTs of up to 3C warmer than modern occurred (~24C), after which SST dropped and remaining at present day temperatures since 9.3 14C ka BP. 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. Full Article [pdf]