Speleothem master chronologies: combined Holocene 18O and 13C records from the North Island of New Zealand and their palaeoenvironmental interpretation

Abstract

The stable isotope records of four stalagmites dated by 19 TIMS uranium series ages are combined to produce master chronologies for δ ¹⁸O and δ ¹³C. The δ ¹⁸O records display good overall coherence, but considerable variation in detail. Variability in the δ ¹³C records is greater, but general trends can still be dis cerned. This implies that too fine an interpretation of the structure of individual isotopic records can be unreliable. Speleothem δ ¹⁸O values are demonstrated to show a positive relationship with temperature by comparing trends with other proxy records, but also to respond negatively to rainfall amount. Speleothem δ ¹³C is con sidered to be most influenced by rainfall. The postglacial thermal optimum occurred around 10.8 ka BP, which is similar in timing to Antarctica but up to 2000 years earlier than most Northern Hemisphere sites. Increasingly negative δ ¹⁸O values after 7.5 ka BP indicate that temperatures declined to a late mid-Holocene minimum centred around 3 ka BP, but more positive values followed to mark a warm peak about 750 years ago which coincided with the ‘Mediaeval Warm Period’ of Europe. Low δ ¹⁸O values at 325 years BP suggest cooling coincident with the ‘Little Ice Age’. A marked feature of the δ¹³C record is an asymmetric periodicity averaging c. 2250 years and amplitude of c. 1.9‰. It is concluded that this is mainly driven by waterbalance variations with negative swings representing particularly wet intervals. The δ¹⁸O record shows a higher-frequency cyclicity with a period of c. 500 years and an amplitude of c. 0.25‰. This is most likely to be temperature-driven, but some swings may have been amplified by precipitation.