Continuous CO2 reconstruction across the MPT from boron isotopes informs mechanisms of glacial CO2 change

The Mid-Pleistocene Transition (MPT) represents a fundamental shift in the operation of Earth’s climate system, yet the role of CO₂ in this transition is uncertain.  Prior to the MPT, the climate system was paced by the ~40 kyr obliquity cycle, with available CO₂ reconstructions, temperatures, and ice volume all coupled to orbitally-forced changes in solar energy at high latitudes.  Following the MPT, this relationship breaks down, with Northern Hemisphere ice sheets persisting through obliquity maxima in a series of “skipped terminations”, leading to longer glacial periods with larger ice sheets.  Here we examine the role of CO₂ over the MPT, using high resolution boron isotope data from 3 sediment cores, spanning the Atlantic, Pacific, and Indian Oceans.  These records show excellent agreement with the ice core record in their younger portions, and striking consistency between sites, supporting the robustness of our reconstruction of atmospheric CO₂.  We find that CO₂ and benthic oxygen isotopes remain largely coupled through the MPT, with limited CO₂ rise during the skipped terminations around MIS 36 and 34, notably low CO₂ during the deep glaciation of MIS 22 (the “900 ka event”), and notably high CO₂ during the “super-interglacial” of MIS31.  This underscores the key role of CO₂ in glacial and interglacial climate states.  In addition, it highlights that the mechanisms governing glacial-interglacial CO₂ change, which are thought to be largely centred on the Southern Ocean, are not forced by orbital changes alone, but must be linked to land ice volume, as the only feature of the climate system with the inertia to persist through orbital insolation peaks.  This implies the existence of teleconnections between Northern Hemisphere ice volume and Southern Ocean CO₂ storage, and we outline potential mechanisms by which this might be achieved.