PALEOBOTANICAL EVIDENCE FOR MIOCENE SUMMER MOISTURE DEFICITS IN SOUTHERN NEW ZEALAND

The modern southern New Zealand terrestrial climate is strongly ocean-moderated and experiences little intra-annual precipitation variation. This is due to the strength and the position of the Southern Westerly Wind belt (SWW), which supplies moisture to southern New Zealand (~44–47°S) year-round. Further northward in New Zealand (35–40°S), summer weakening of the SWW causes seasonal moisture reduction. The SWW has been shown to strengthen and weaken with global temperature variations during the Holocene and is linked to sea surface temperature variation. However, it is unclear how the strength of the SWW changed during periods when global temperatures were much higher. We examine the early to mid/late Miocene (23–11 Ma) period, when average temperatures in New Zealand were 3–5°C warmer than today. We used the climate leaf analysis multivariate program (CLAMP) on ten fossil floras of early to mid/late Miocene age from southern New Zealand, deposited at paleolatitudes of 47–48°S, to investigate variation in seasonal precipitation. All floras suggested marked seasonal precipitation and the potential of summer moisture deficits of >100 mm^-mth, with the increased summer/winter contrast amplified by the higher overall temperatures in the Miocene and causing increased potential evapotranspiration. This moisture regime does not exist in modern New Zealand, but is present in southern Australia and southern South America at 30–40°S. These results suggest that during Miocene New Zealand summers, the SWW may have receded to below 48°S, thus reducing moisture supply at mid-latitudes and increasing drought potential.