Continuous atmospheric CO2 across the Mid Pleistocene Transition from boron isotopes: decoupling of CO2 from insolation and temperature

Changes in atmospheric CO₂ and global ice volume as a response to changes in insolation are one of the Earth’s most important feedback mechanisms during glacial-interglacial cycles. During the obliquity paced glacial-interglacial cycles of the 41kyr world prior to 1.2 million years ago (Ma), the response between insolation and the CO₂-ice volume feedback is relatively linear. However, during the Mid-Pleistocene transition (0.6Ma – 1.2Ma), this linear response breaks down leading to a large increase in ice volume with a relatively modest decrease in CO₂ during glacials in late Pleistocene. Here, we present atmospheric CO₂ records derived from boron isotopes measured in the planktic foraminifera G. ruber sensu stricto from 3 ocean sediment cores, each well validated against ice records of CO₂. We find two notable CO₂ features during the MPT, an early de-coupling of CO₂ and ice volume from insolation during MIS 36 (~1.05 Ma), where CO₂ stays relatively constant despite multiple (but muted) orbital cycles. Secondly, during MIS 22 (0.9Ma), CO₂ decreases step-wise, in combination with rising global ice volume, and recovers to “luke-warm style” interglacial levels in the following interglacial MIS 21. The periods of low CO₂ and high ice volume occur in line with saltier Atlantic deep waters enriched in δ¹³C which we interpret as southern origin water masses, and increased ocean carbon storage. We therefore conclude that changes in ocean circulation may have caused an increased uptake of atmospheric carbon during these periods. In contrast, global sea surface temperatures during MIS36 follow insolation and not CO₂ suggesting a de-coupling of the CO₂/ice volume feedback from insolation and temperature. This may have prepositioned the climate system for the significant CO₂ reduction and ice sheet expansion during and after 0.9Ma.