Early Pleistocene obliquity-scale pCO₂ variability at ~1.5 million years ago

In the early Pleistocene, global temperature cycles predominantly varied with ~41-kyr (obliquity-scale) periodicity. Atmospheric greenhouse gas concentrations likely played a role in these climate cycles; marine sediments provide an indirect geochemical means to estimate early Pleistocene CO₂. Here we present a boron isotope-based record of continuous high-resolution surface ocean pH and inferred atmospheric CO₂ changes. Our results show that, within a window of time in the early Pleistocene (1.38-1.54 Ma), pCO₂ varied with obliquity, confirming that, analogous to late Pleistocene conditions, the carbon cycle and climate covaried at ~1.5 Ma. Pairing the reconstructed early Pleistocene pCO₂ amplitude (92 ±13 μatm) with a comparably smaller global surface temperature glacial/interglacial amplitude (3.0 ±0.5 K), yields a surface temperature change to CO₂ radiative forcing ratio of S _[CO2]~0.75 (± 0.5) °C/Wm^-2, as compared to the late Pleistocene S _[CO2] value of ~1.75 (± 0.6) °C/Wm^-2. This direct comparison of pCO₂ and temperature implicitly incorporates the large ice sheet forcing as an internal feedback and is not directly applicable to future warming. We evaluate this result with a simple climate model, and show that the presumably thinner, though extensive, northern hemisphere ice sheets would increase surface temperature sensitivity to radiative forcing. Thus, the mechanism to dampen actual temperature variability in the early Pleistocene more likely lies with Southern Ocean circulation dynamics or antiphase hemispheric forcing. We also compile this new carbon dioxide record with published Plio-Pleistocene δ¹¹B records using consistent boundary conditions and explore potential reasons for the discrepancy between Pliocene pCO₂ based on different planktic foraminifera.