Abstract:
The response of the surface ocean and terrestrial climate in the New Zealand region to interglacial Marine Isotope Stage (MIS) 11 (423-380ka) is documented, using assemblages of fossilised marine algae (dinoflagellate cysts, or dinocysts) and spores/pollen from terrestrial plants, analysed from marine sediment cores. This work is underpinned by studies on the modern distribution of dinocysts, factors that influence their accumulation in marine sediment, and the use of dinocyst assemblages to quantify past sea surface temperature (SST). In the first of the modern-process studies, a dataset of modern sea-floor dinocyst assemblages from the Southern Hemisphere is collated, including new observations from the SW Pacific. Variations in the assemblages are related to environmental gradients. Cluster analysis reveals distinct biogeographic assemblage zones, individual taxa indicative of specific water masses are identified, while ordination of the databases indicates that the assemblages vary most with changes in SST. A second modern process study reports on the dinocyst assemblages from two time-incremental sediment traps (3 years of data) moored north and south of the Subtropical Front in the ocean east of New Zealand. This study provides observations of seasonal and inter-annual variability of dinocyst flux to the deep sea, which are used to identify possible biases in the sea-floor dinocyst assemblages. Observations from these first two studies are used in a systematic analysis of the strengths and weakness of using dinocyst assemblages to quantify SST in the SW Pacific. The best transfer function performance achieved was a root mean squared error of 1.47˚C, for an artificial neural network model, and the benefits in considering a range of model results are also established. Fossil records that document the oceanographic and terrestrial response to MIS11 are developed from two areas around New Zealand; (i) dinocysts assemblages are collected from the east Tasman Sea, from giant piston cores MD06-2987, -2988, and 2989, and (ii) dinocysts and pollen assemblages are analysed from Deep Sea Drilling Project (DSDP) Site 594, from the east of New Zealand. Dinocyst assemblages confirm that SST in the east Tasman Sea was ~2-3˚C warmer than the present during late MIS11 (415-400ka), while SSTs were slightly below modern levels during an early phase (428-415ka). Two assemblage – based productivity indices suggest that the elevated SSTs during MIS11 were accompanied by lower rates of primary productivity in the east Tasman Sea study area than the present. As in the east Tasman Sea, two distinct phases of MIS11 are recognised in both the dinocyst and pollen assemblages at DSDP 594. The dinocyst assemblages of late MIS11 are similar to, but qualitatively represent warmer waters than the Holocene. The succession of pollen assemblages during MIS12-11 is very similar to that observed during the previous two interglacials at this site (MIS1 and MIS5), with two notable variations: (i) the deglacial vegetation succession during MIS11 was prolonged, and (ii) the pollen assemblage representing the warmest forest type was also present for longer (ca. 15ky) than later interglacials. Changes in the pollen record during MIS11 at DSDP 594 correlate more closely to SST variations in the east Tasman Sea than to ocean variations at DSDP 594, suggesting that the eastern ocean had only limited influence on conditions on the adjacent landmass during MIS11.
