Early Pleistocene Obliquity‐Scale pCO₂ Variability at ~1.5 Million Years Ago

Abstract

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−1·W−1·m−2, as compared to the late Pleistocene S[CO2] value of ~1.75 (±0.6) °C−1·W−1·m−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.

The Magnitude of Surface Ocean Acidification and Carbon Release During Eocene Thermal Maximum 2 (ETM‐2) and the Paleocene‐Eocene Thermal Maximum (PETM)

“…To convert their δ 11 B data to δ 11 B borate (and hence pH), we use the different δ 11 B calcite versus δ 11 B borate sensitivities of two modern symbiont‐bearing species for the two extinct species based on their ecological characteristics. We apply the sensitivity of modern symbiont‐bearing, surface‐dwelling T. sacculifer to M. velascoensis (m = 0.73 ± 0.08; Dyez et al, ; Sanyal et al, ), and modern symbiont‐bearing, slightly deeper surface‐dwelling O. universa (m = 0.99 ± 0.07; Hönisch et al, , refit of Henehan et al, ) to A. soldadoensis . We calibrate the species‐specific δ 11 B foram intercept to an initial pre‐PETM pH of 7.72 (i.e., intercept c = 0.37 for A. soldadoensis and c = 4.57 for M. velascoensis ).…”

A 14.5 million-year record of East Antarctic Ice Sheet fluctuations from the central Transantarctic Mountains, constrained with cosmogenic <sup>3</sup>He, <sup>10</sup>Be, <sup>21</sup>Ne, and <sup>26</sup>Al

“…CC BY 4.0 License. (Herbert et al, 2016); (b) Boron-isotope-, paleosol-, and stomataderived CO2 records (Beerling et al, 2009;Breecker and Retallack, 2014;Da et al, 2019;Dyez et al, 2018;Ji et al, 2018;Sosdian et al, 2018;Wang et al, 2015;Zhang et al, 2013) (d) benthic oxygen isotope stack (De Vleeschouwer et al, 2017), (e) Moraine age and uncertainty at Roberts Massif, plotted against deposit elevation. Note that deposition of the Misery moraines required ice to be > 300 m thicker than today, which is not reflected in the moraine elevation.…”

Including the efficacy of land ice changes in deriving climate sensitivity from paleodata

“…Several ways have been put forward to constrain ECS, for example, through the use of paleoclimate data (e.g. Covey et al, 1996;Edwards et al, 2007), which is also the focus of this study. However, unlike results of models, temperature reconstructions based on paleoclimate proxy data always contain a mixed signal of all processes active in the climate system.…”