Changes in Phytoplankton Productivity and Sea Surface Temperature Across the Subtropical Convergence East of New Zealand Since the Last Glaciation Based on
Biomarker Compositions

John K. Volkman CSIRO Division of Oceanography, G.P.O. Box 1538, Hobart, Tasmania 7001, Australia

and Cooperative Research Centre for Antarctic and Southern Ocean Environment, Hobart Tasmania 7001, Australia

Elisabeth L. Sikes Cooperative Research Centre for Antarctic and Southern Ocean Environment, G.P.O. Box 252C, Hobart, Tasmania 7001, Australia

Lisette Robertson Cooperative Research Centre for Antarctic and Southern Ocean Environment, G.P.O. Box 252C, Hobart, Tasmania 7001, Australia

Helen L. Neil Department of Earth Sciences, University of Waikato, Hamilton, New Zealand

Sediment cores were obtained from both north and south of the subtropical convergence (STC) east of New Zealand in the vicinity of the Chatham Rise. The organic matter isolated from these samples was analysed by capillary gas
chromatography-mass spectrometry for a variety of biomarker compounds including alkenones, long-chain alcohols, long-chain hydrocarbons, alkyl keto-ols, 4-desmethyl sterols and 4-methyl sterols as part of studies designed to reconstruct the changes in sea surface temperature (SST) and phytoplankton productivity in the region over the past 24000 years with a particular emphasis on changes since the last deglaciation (ca. 14500-9000 years bp).

Previous paleoceanographic studies have shown that
the STC moves north coincident with the change to glacial climate in many locations in the Southern Ocean. The position of the STC over the Chatham Rise is unusually far north relative to its position in other portions of the Southern Ocean where it is a well defined front. It has been suggested that presently the STC is trapped bathymetrically on the Chatham Rise, and remained trapped there during the last glacial maximum. We have calculated paleo-SSTs based on ratios of long-chain C37 alkenones (Uk'37) which were synthesized primarily by the alga Emiliania huxleyi based on the distribution of C37-C39 compounds observed. Our sea surface temperature (SST) estimates downcore indicate significant cooling of surface waters at the last glacial maximum (by 4-6°C) both south and north of the Chatham Rise.

The neutral lipid distributions showed significant differences down-core and along the north to south transect. Lipids characteristic of terrestrial higher plants (long-chain n-alkanols with strong even chain-length predominance, long-chain n-alkanes with strong odd chain-length predominance) were surprisingly abundant particularly in cores south of the Chatham Rise presumably reflecting terrestrial inputs of organic matter from rivers in the South Island of New Zealand. These vascular plant markers increase steadily in concentration down core with highest concentrations at the last glacial maximum. Note that carbonate abundance is low in these particular sediments because of increased transport and deposition of terrigenous clays and lithological material.

The flux of marine-derived organic matter to the sediments fluctuated with age as shown by changes in sterol and alkenone abundances. Sterols showed complex distributions typical of the diversity of marine sources found in offshore environments. 24-Methylcholesta-5,22E-dien-3b-ol and 24-methylenecholesterol, which are both common in diatoms, plus dinosterol from dinoflagellates were major components in most distributions. These were most abundant in sediments deposited during the deglaciation, particularly in cores taken south of the Chatham Rise, and dropped to low values in sediments deposited in more recent times. Several unusual compounds, presumed to be of marine origin (as yet unknown), were also major constituents in many samples. Alkenone accumulation rates suggest increased productivity of prymnesiophyte microalgae in waters around the STC during the last glacial maximum.

In summary, our work supports the suggestion that the STC remained fixed over the Chatham Rise in the last glacial maximum. However, the SST and productivity proxies have a more complicated profile during the deglaciation. These results suggest either a weakening or movement of the front during the transition between the two climatic extremes.