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James Rae, University of St. Andrews: 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 as this interval extends beyond the current reach of continuous ice core records, 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 ice volume 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 teleconnection between Northern Hemisphere ice volume and Southern Ocean CO2 storage, and we outline two potential mechanisms by which this might be achieved.

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