Refining Patterns and Magnitude of Abrupt Climate Change During the Late Glacial Transition in the New Zealand Region.

The origin of Southern Hemisphere millennial-scale climate events during the Late Glacial Transition remains a central issue of paleoclimate studies. The specific details of an in or out-of-phase relationship could support either Northern Hemisphere forcing or a Southern Hemisphere driver for thermohaline switches. Southern New Zealand, in the mid-latitudes of the southwest Pacific, has become a prime location for establishing inter-hemispheric relationships and hence for testing models of abrupt climate change. Sediments from a kettle-hole pond located in the climatically sensitive eastern margin of the Southern Alps, New Zealand, provide a continuous, isotopically dated, paleo-chironomid and pollen record of deglacial climate fluctuations since 18,000 cal BP. Paleochironomid analysis utilizes our newly developed chironomid-temperature inference model for the Southern Alps of New Zealand. Principal changes in Late Glacial Transition lithostratigraphy and pollen assemblages, notably the expansion of alpine grasses and herb taxa between c. 14,300 and 13,000 cal BP, are indicative of climate reversal and cooling that corresponds in time with the Antarctic Cold Reversal (ACR). Cold temperature chironomid assemblages were more abundant prior to 13,000 cal BP. The abundance of temperate chironomid assemblages between 13,000 cal BP and the early Holocene does not show evidence of climate reversal associated with Younger Dryas (YD) cooling. These results confirm the timing of an ACR-like period of cooler climates in the southern New Zealand region and indicate that the YD chronozone was a period of warming with temperatures similar to those of the early Holocene. These sequences provide opportunities for comparing terrestrial changes in southern New Zealand with marine and Antarctic records and can help clarify the relationship between the hemispheres and hence further distinguish the coupling mechanisms.