Atmospheric Carbon Dioxide Concentration Across the Mid-Pleistocene Transition

The dominant period of Pleistocene glacial cycles changed during the mid-Pleistocene from 40,000 years to 100,000 years, for as yet unknown reasons. Here we present a 2.1-million-year record of sea surface partial pressure of CO₂ (PCO₂), based on boron isotopes in planktic foraminifer shells, which suggests that the atmospheric partial pressure of CO₂ (pCO₂) was relatively stable before the mid-Pleistocene climate transition. Glacial PCO₂ was approximately 31 microatmospheres higher before the transition (more than 1 million years ago), but interglacial PCO₂ was similar to that of late Pleistocene interglacial cycles (