Abrupt plant physiological changes in southern New Zealand at the termination of the Mi-1 event reflect shifts in hydroclimate and pCO₂

A rise in atmospheric CO₂ is believed to be necessary for the termination of large-scale glaciations. Although the Antarctic Ice Sheet is estimated to have melted from ∼125% to ∼50% its modern size, there is thus far no evidence for an increase in atmospheric CO₂ associated with the Mi-1 glacial termination in the earliest Miocene. Here, we present evidence from a high-resolution terrestrial record of leaf physiological change in southern New Zealand for an abrupt increase in atmospheric CO₂ coincident with the termination of the Mi-1 glaciation and lasting approximately 20 kyr. Quantitative pCO₂ estimates, made using a leaf gas exchange model, suggest that atmospheric CO₂ levels may have doubled during this period, from 516±111ppm to 1144±410ppm, and subsequently returned back to 425±53ppm. The 20-kyr interval with high pCO₂ estimates is also associated with a period of increased moisture supply to southern New Zealand, inferred from carbon and hydrogen isotopes of terrestrial leaf waxes. The results provide the first high-resolution record of terrestrial environmental change at the Oligocene/Miocene boundary, document a ∼20 kyr interval of elevated pCO₂ and increased local moisture availability, and provide insight into ecosystem response to a major orbitally driven climatic transition.