THE ROLE OF PCO2 IN ORBITALLY-PACED CLIMATE AND CARBON CYCLE VARIATIONS IN THE MIDDLE MIOCENE

The pace of Earth's background climate variability is known to be driven by the Milankovitch cycles, variations in the shape of Earth's orbit and its and axial tilt. While the Milankovitch (orbital) theory of climate change is very nearly universally accepted, the climate system mechanisms and feedbacks responsible for amplifying orbital cycles preserved in the geologic record remains uncertain. For the late Pleistocene, the ice core-derived record of atmospheric carbon dioxide (pCO₂) is strongly coupled with global temperature on orbital time scales, indicating that internal feedbacks involving the carbon cycle amplify or even cause the large changes in global temperature during orbitally driven glacial-interglacial cycles. However, for earlier time periods beyond the range of ice cores (the last ~800 kyr), it is not possible to directly compare records of pCO₂ to orbital climate cycles due to a lack of high-resolution (orbitally-resolved) records of pCO₂ before the Pliocene. We address this deficiency with a high-resolution (~5-10 kyr) record of planktonic foraminiferal d¹¹B-derived surface seawater pH (as well as trace metal analyses and benthic d¹³C and d¹⁸O) over a 500 kyr time window in a sedimentary record with clear Milankovitch-scale climate and carbon cycle oscillations: the Middle Miocene (14.0 - 14.5 Ma) at ODP Site 926 (subtropical North Atlantic). The resulting pH record can be used to constrain atmospheric pCO₂ changes over orbital cycles during a background climate significantly warmer than today's. These new records of planktic d¹¹B and d¹³C will then be used to guide simulations of astronomical climate forcing in Earth System models, resulting in refined estimates of pCO₂ changes over orbital cycles and providing quantitative constraints on the mechanisms and feedbacks responsible for the Milankovitch control of climate and carbon cycling.