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The reorganisation of Earth's climate system from the Oligocene to the Miocene was influenced by complex interactions between Tethyan tectonics, orbital parameters, oceanographic changes, and carbon cycle feedbacks, with climate modelling indicating that pCO(2) was an important factor. Oscillating episodes of climate change during the Oligocene Miocene transition (OMT) have however been difficult to reconcile with existing pCO(2) records. Here we present a new pCO(2) record from the OMT into the early Miocene, reconstructed using the stomatal proxy method with a database of fossil Lauraceae leaves from New Zealand. The leaf database derives from three relatively well-dated sites located in the South Island of New Zealand; Foulden Maar, Mataura River and Grey Lake. Atmospheric pCO(2) values were obtained based on four separate calibrations with three nearest living equivalents, using the stomatal ratio method as well as transfer functions. Our results, based on the mean values of each of the four calibrations, indicate pCO(2) ranging 582-732 ppm (average 650 ppm) at the OMT, falling precipitously to mean values of 430-538 ppm (average 492 ppm) for the earliest Miocene and similar to 454-542 ppm (average 502 ppm) in the early Miocene. The much higher values of pCO(2) at the OMT indicate that pCO played an important role in climate dynamics during this time, potentially including the abrupt termination of glaciations.