Palaeoclimatic interpretation of stable isotope data from Holocene speleothems of the Waitomo district, North Island, New Zealand

One straw stalactite and three stalagmites from the Waitomo district of North Island, New Zealand, were examined for stable isotopes of oxygen and carbon with a view to interpreting their palaeoclimate signal. Dating was by uranium series and AMS 14 C for the stalagmites and by gamma-ray spectrometry for the straw. Records were thus established for about 100 years for the straw and 3.9, 10.1 and 10.2 ka for the stalagmites. The range of variability in d 18 O c and d 13 C c this century is about two-thirds of that experienced over the entire Holocene, and is most simply explained in terms of the oceanic source area of rain. Stable isotope variations in three stalagmites show some general similarities, but have significant differences in detail, which underlines the necessity to base palaeoclimatic interpretations on more than one speleothem record. The d 18 O c of each stalagmite varies positively with temperature, indicating the dominance of the ocean source of evaporation in determining the isotopic composition of precipitation and hence speleothem calcite in the Holocene. This conclusion is contrary to that of other authors working in New Zealand, who identified a negative relationship between d 18 O c and temperature, while examining time periods extending across the Last Glacial Maximum. It is concluded here that, whereas the ice volume effect dominates the large climatic shifts of glacial-interglacial amplitude, the oceanic source effect becomes more important during the period of relatively stable sea level during the Holocene. Results also indicate a late-Holocene altitudinal effect of 0.2‰ d 18 O c per 100 m and an associated temperature relationship of about 0.26‰ per°C. The average of two records identifies the postglacial climatic optimum to lie in the interval from prior to 10 ka BP to 7.5 ka BP, when d 18 O c values were up to 0.6‰ less negative than present, implying an average annual mean temperature that was up to 2.3°C warmer. The average of three speleothem records for the last 3900 years reveals the coldest period of the Holocene to have occurred about 3 to 2 ka BP, when d 18 O c values were typically 0.4‰ more negative than present and average temperatures may have been 1.5°C cooler. Mean annual temperature variability of about 2°C was sometimes experienced in little more than 100 years.