COMPARING THE RESULTS OF ISOTOPIC AND BIOLOGICAL PROXIES FOR RECONSTRUCTING PAST EXTREME WEATHER EVENTS

Pyramid Valley, a small (5 ha) closed basin in the northeastern South Island, New Zealand (42.972803°S, 172.597169°E) has attracted attention over the past 75 years as the deposit contains many well-preserved examples of New Zealand’s extinct avifauna. The potential for a paleoclimatic record preserved in the deposit has been recently explored using modern techniques. From this research the climate history for the area was reconstructed for the period 4500 - 1200 years BP, using multiple proxies, extracted from a 1.5 m undistorted core removed from the lake bed. The proxies indicate a 300-year period of extreme variability in the regional precipitation between 2700 and 2400 years B.P.

Two previously little-used proxies were key resources for reconstructing the climate history of the Pyramid Valley area. Shell carbonate oxygen isotope (δ¹⁸O) ratios from the freshwater gastropod Potamopyrgus antipodarum indicated precipitation variability, with more positive values indicating when evaporation exceeded fresh water input, and vice versa. While the use of isotopic ratios from gastropod carbonate is controversial, the oxygen isotope record from this study co-varied with the presence and abundance in the deposit of remains of several species of macro algae (Charophyta) that require deep, clear, still water. During the periods of highest abundance of Charophyte oospores, and when more than one species was present, the lake is estimated to have been 5 - 7 m deep. The lake was deepest at c. 2400 B.P., and this coincided with a sudden (c. 12 ‰) decrease in δ¹⁸O values. The highest density of charophyte oospores was recorded immediately after this sharp decline in δ¹⁸O values, suggesting that there had been a prolonged period of higher than average rainfall resulting in the lake achieving its maximum depth.

The period between 2700 B.P. and 2400 B.P. has also been identified as a period of rapid climate change in the Northern Hemisphere. This is the 300-year cold period known as the neoglacial or the 'Homeric Minimum'. The Pyramid Valley data indicate that this cooler, wet interval affected the mid-latitudes of the Southern Hemisphere as well and may be analogous to the later Little Ice Age (1200-1800 AD).