Orbital Forcing, Ice Volume, and CO₂ Across the Oligocene‐Miocene Transition

Abstract

Paleoclimate records suggest that a rapid major transient Antarctic glaciation occurred across the Oligocene‐Miocene transition (OMT; ca. 23 Ma; ~50‐m sea level equivalent in 200–300 kyr). Orbital forcing has long been cited as an important factor determining the timing of the OMT glacial event. A similar orbital configuration occurred 1.2 Myr prior to the OMT, however, and was not associated with a major climate event, suggesting that additional mechanisms play an important role in ice sheet growth and decay. To improve our understanding of the OMT, we present a boron isotope‐based CO2 record between 22 and 24 Ma. This new record shows that δ11B/CO2 was comparatively stable in the million years prior to the OMT glaciation and decreased by 0.7‰ (equivalent to a CO2 increase of ~65 ppm) over ~300 kyr during the subsequent deglaciation. More data are needed, but we propose that the OMT glaciation was triggered by the same forces that initiated sustained Antarctic glaciation at the Eocene‐Oligocene transition: long‐term decline in CO2 to a critical threshold and a superimposed orbital configuration favorable to glaciation (an eccentricity minimum and low‐amplitude obliquity change). When comparing the reconstructed CO2 increase with estimates of δ18Osw during the deglaciation phase of the OMT, we find that the sensitivity of the cryosphere to CO2 forcing is consistent with recent ice sheet modeling studies that incorporate retreat into subglacial basins via ice cliff collapse with modest CO2 increase, with clear implications for future sea level rise.

Elevated CO<sub>2</sub>, increased leaf-level productivity and water-use efficiency during the early Miocene

“…2c) is independent of calibration error, based on universal gas-exchange mechanisms, and represents plant vegetative organs of multiple plant species that directly interacted with the available pool of atmospheric carbon dioxide. Previous Ca estimates from the Oligocene/Miocene boundary based on boron isotopes and paleosol carbonates are generally lower than our estimates (Ji et al, 2018;Greenop et al, 2019) (Fig. 4b), whereas Ca estimates based on stomatal index and recent alkenone-based Ca estimates are more similar to our results (Kürschner et al, 2008;Super et al, 2018).…”

“…The ESS envelope was determined using deep-sea δ 18 O of benthic foraminifera (Zachos et al, 2001) and the transform function approach from Hansen et al 2013(Supplementary Information). Proxy-based Neogene Ca reconstructions are derived from a previously published compilation (Foster et al, 2017) and are supplemented with more recently published data (Ji et al, 2019;Londoño et al, 2018;Super et al, 2018;Greenop et al, 2019;Moraweck et al, 2019, Steinthorsdottir et al, 2019. Error bars on gas-exchange based proxy estimates represent ±1σ.…”

“…Reconstructions of globally elevated temperatures of 5-6 °C in the early Miocene (Hansen et al, 2013) with a Ca of ~300 ppm (Ji et al, 2018;Greenop et al, 2019) upsets the expected ESS to Ca during this period (Henrot et al, 2010). Geochemical Ca proxy estimates consistently produce Ca estimates that are too low to satisfy ESS to Ca prior to the Pliocene (Royer, 2016) (Fig.…”

Proxy evidence for state-dependence of climate sensitivity in the Eocene greenhouse

“…We employ the δ 11 B proxy on mixed-layer species of planktonic foraminifera in all core sites to first reconstruct surface ocean pH. The majority of Paleogene foraminiferal species selected for this study were previously identified to reflect surface mixed layer conditions 4,10 , and are likely characterized by a reduction in the degree of pH modification in the micro-environment surrounding the foraminifera by physiological processes compared to observations in modern foraminifera 4,14 . When thermocline dwelling species were used, or additional species not previously analysed, we ensured that our new analyses of δ 11 B overlapped with previously studied mixed-layer planktonic foraminiferal species ("Methods" and Supplementary Data 1) in order to constrain site-specific intra-species offsets and thus provide consistency and confidence in the derived mixed-layer pH (as in ref.…”

Midlatitude Temperature Variations in the Oligocene to Early Miocene

“…In the ~1 Myr prior to and following the O/M boundary (from 24.5 to 22 Ma)—an interval encompassing both warmer temperatures and lower average benthic δ 18 O than the preceding Oligocene—the larger amplitude (5°C) variation in midlatitude SST is coupled to a 1‰ range in benthic δ 18 O, with the warmest temperatures coinciding with minima in benthic δ 18 O. The Oligocene Miocene transitionitself is marked by a two‐step decrease in North Atlantic SST, analogous to the two‐step rise in high resolution benthic δ 18 O in orbitally resolved records (Liebrand et al, ; Mawbey & Lear, ), the pacing of which has been recently suggested to be driven by both changes in atmospheric carbon dioxide and obliquity (Greenop et al, ; Levy et al, ). Across the O/M boundary, previous high‐resolution analysis of a tropical Atlantic site between 22.4 to 23.7 Ma showed that intervals of increased δ 18 O sw interpreted as recording ice growth were coupled to intervals of bottom water cooling, inferred from benthic foraminiferal Mg/Ca (Mawbey & Lear, ).…”