Early de-coupling of pCO2 and ice volume from insolation during the Mid-Pleistocene Transition

Abstract:

The Mid-Pleistocene transition (MPT) is marked by a major increase in glacial ice volume leading to changes in the shape, periodicity and amplitude of glacial-interglacial climate cycles. After the MPT, late Pleistocene glacial cycles are largely shaped by changes in ice volume coupled with atmospheric CO₂ (pCO₂), and other climate feedbacks. As such, low glacial pCO₂ values probably aid the development of extreme glacial maxima and may help glacial stages to sustain over multiple insolation peaks. Yet, the relationship of ice volume and pCO₂ during the MPT remains unknown. Here, we present new δ¹¹B-derived atmospheric pCO₂ reconstructions from IODP site U1476 (15°49.25′S; 41°46.12′E; 2166 m water depth) in the Mozambique Channel across the MPT (0.8Ma – 1.2Ma). Prior to 1.2Ma, pCO₂ and ice volume closely follow integrated summer insolation at 65°N with a strong obliquity (41kyr) pacing. However, during early MPT glacial period MIS30 (1.1Ma), we find that pCO₂ and ice volume diverge in-tandem from external 65°N insolation forcing for the first time. We see an early hint of this diversion already in published data from MIS36 (1.2Ma) and MIS34 (1.12Ma). This marks MIS30 and the early MPT as the first glacial cycles during which the pCO₂-ice sheet feedback de-couples from insolation forcing, a feature of Late Pleistocene climate cycles. The prolonged low glacial pCO₂ values occur in line with published stable isotope records from the deep Atlantic that show saltier Atlantic deep waters with a particularly low δ¹³C signature which we interpret to be of southern origin. This is likely indicative of increased deep ocean carbon storage helping to sustain low glacial pCO₂. We therefore conclude that the early MPT is characterised by a decoupling of the pCO₂-ice sheet feedback from external insolation forcing, which may have preconditioned the climate system for significant ice sheet expansion during the later stages of the MPT, and the creation of quasi-100kyr climate cycles.