The exclusive use of carbonate reference materials is a robust method for the standardization of clumped isotope measurements • Measurements using different acid temperatures, designs of preparation lines, and mass spectrometers are statistically indistinguishable • We propose new consensus values for a set of 7 carbonate reference materials and updated guidelines to report clumped isotope measurements

The middle Miocene climate transition (~14 million years ago) was characterized by a dramatic increase in the volume of the Antarctic ice sheet. The driving mechanism of this transition remains under discussion, with hypotheses including circulation changes, declining carbon dioxide in the atmosphere and orbital forcing. Southern Ocean records of planktic foraminiferal Mg/Ca have previously been interpreted to indicate a cooling of 6-7°C and a decrease in salinity preceding Antarctic cryosphere expansion by up to ~300 thousand years. This interpretation has led to the hypothesis that changes in meridional heat and vapour transport along with an early thermal isolation of Antarctica from extrapolar climates played a fundamental role in triggering ice growth.Here, we revisit the middle Miocene Southern Ocean temperature evolution using