Contrasting palaeoenvironments of the mid/late Miocene Dunedin Volcano, southern New Zealand: Climate or topography?

- Southern New Zealand climate cooled from subtropical to temperate conditions at the mid/late Miocene boundary.
- Cooling of the climate is contemporaneous with strengthening of Antarctic Circumpolar Current.
- Slope aspect appears to have been an important factor in determining radiative flux and thereby palaeoenvironments.

At the mid/late Miocene boundary, a northward shift occurred in the Subtropical Front neighbouring the New Zealand continental landmass. The terrestrial palaeoenvironment of New Zealand concurrently underwent a period of cooling. The Dunedin Volcanic Group in southern New Zealand spans the mid/late Miocene boundary and the sedimentary deposits from Kaikorai Valley and Double Hill within this group contain diverse and well-preserved floral assemblages. We used terrestrial palaeoclimate proxies CLAMP (Climate Leaf Analysis Multivariate Program) and BA (Bioclimatic Analysis) to determine the paleoenvironment at southern mid-latitudes during the mid/late Miocene transition. Our results indicate that the mid/late Miocene climate of the Dunedin Volcano was warm-temperate to subtropical (mean annual temperature [MAT]: ~ 17-19 °C) at Double Hill, similar to modern coastal Queensland, and cool- to warm-temperate (MAT: ~ 12-14 °C) at Kaikorai Valley, similar to modern northern New Zealand. Winter temperatures at Kaikorai Valley were distinctly cooler (~ 6-8 °C). Differing aspect (i.e. north vs south facing) may have played a part in determining solar radiation, particularly in an area of high relief, such as the Dunedin Volcano. Because Kaikorai Valley was situated facing south and Double Hill facing north, the two sites would have received differing radiative fluxes, particularly in winter, thereby possibly influencing the local environment, biota, and potentially also the palaeoclimate signal produced by floral proxies. This study provides evidence for the temperature decline in terrestrial palaeoenvironments at mid-latitudes caused by the northward movement of the Subtropical Front and strengthening of the Antarctic Circumpolar Current during the mid/late Miocene.