Abstract
The New Zealand landmass occupies an area on the edge of the Southern Ocean at 35–47°S. Though situated north of the subtropical front, the New Zealand landmass experiences direct influence of the Antarctic Circumpolar Current and westerly winds. A projection of the effect of fluctuations of the Antarctic ice mass and Southern Ocean circulation on the New Zealand environment and global atmospheric circulation has to rely on the assessment of terrestrial climatic conditions during a time when the Antarctic ice sheet was reduced.
The early – middle Miocene (23–12 Ma) represents a period when the extent of the Antarctic ice sheet, as well as the Southern Ocean circulation was reduced. Two terrestrial depositional systems were active in southern New Zealand during this time: the Waipiata Volcanics and Lake Manuherikia. Quantification of floral response to climate in the Waipiata Volcanic and Manuherikia Groups is used to elucidate the effect of ocean circulation change on the New Zealand terrestrial environment.
The earliest Miocene environment, prior to the northward movement of the Subtropical and Subantarctic fronts, can be quantified using floras in the Waipiata Volcanic Group (Foulden and Hindon Maar). Floras are diverse and suggest subtropical to warm-temperate conditions prevailed. There is some evidence for seasonality in the precipitation regime, but temperatures appear to be relatively consistent between the two investigated floras. A seasonal mid-latitudinal light regime may have amplified seasonal contrast.
During deposition of the Manuherikia Group (Grey Lake, Vinegar Hill, Kawarau River and Nevis Valley) the environment was more changeable, both seasonally and between floras. Absolute time constraints on the Manuherikia Group are poor, but all floras are between 19–15 Ma old and relative superposition of the floras is well-constrained. The trend in this period was from warm-temperate conditions and seasonal temperatures in the lowermost floras, Grey Lake and Vinegar Hill, to subtropical conditions with seasonal precipitation in the youngest floras, Kawarau River and Nevis Valley. This occurs during a period when the major front systems were shifting northwards, although still not converging with the New Zealand landmass. The seasonal precipitation regime could be caused by southern migration of the westerly wind belt in response to the presence of a subtropical high-pressure cell over mid-latitudes during the early Miocene.
The middle/late Miocene boundary is represented in the Dunedin Volcanic Group (Double Hill and Kaikorai Valley), which is constituted of volcanoclastics and basalts of a multiple-vent volcanic edifice associated with the Waipiata Volcanic Field. Floras of this group are warm-temperate, but Kaikorai Valley appears to have the signature of frost-adaptation. Possibly, southern New Zealand during the middle/late Miocene boundary was subject to influence from the Southern Ocean. This period may therefore mark the northward movement of the Subantarctic Front, the interception of this front with the southernmost extent of New Zealand and initiation of the on-land climatic transition towards modern conditions.
This account of the early to middle Miocene terrestrial climate of southern New Zealand is in agreement with global climatic models and Miocene climatic reconstructions from oceanic proxies. During the Miocene the New Zealand landmass became gradually more susceptible to influences from the Antarctic and therefore cooled. Additionally, under generally warmer climatic conditions in the Miocene, such as is projected for future global scenarios under increased atmospheric carbon levels, seasonality in New Zealand appears to increase.