High resolution alkenone palaeobarometry indicates 1 relatively stable p CO 2 during the Pliocene ( 3 . 3 to 2 . 8

14 Temperature reconstructions indicate that the Pliocene was ~ 3 °C warmer globally 15 than today, and several recent reconstructions of Pliocene atmospheric CO2 indicate 16 that it was above pre-industrial levels and similar to those likely to be seen this 17 century. However, many of these reconstructions have been of relatively low temporal 18 resolution, meaning that these records may have failed to capture variations associated 19 with the 41 Kyr glacial-interglacial cycles thought to operate in the Pliocene. Here 20 we present a new, high temporal resolution alkenone carbon isotope based record of 21 pCO2 spanning 2.8 to 3.3 million years ago from ODP Site 999. Our record is of high 22 enough resolution (~19 Kyrs) to resolve glacial-interglacial changes beyond the 23 intrinsic uncertainty of the proxy method. The record suggests that Pliocene CO2 24 levels were relatively stable, exhibiting variation less than 55 ppm. We perform 25 sensitivity studies to investigate the possible effect of changing sea surface 26 temperature, which highlights the importance of accurate and precise SST 27 reconstructions for alkenone palaeobarometry, but demonstrate that these 28 uncertainties do not affect our conclusions of relatively stable pCO2 levels during this 29 interval. 30