Causes of ice age intensification across the Mid-Pleistocene Transition

Thomas B. Chalk, Mathis P. Hain, Gavin L. Foster, Eelco J. Rohling, Philip F. Sexton, Marcus P. S. Badger, Soraya G. Cherry, Adam P. Hasenfratz, Gerald H. Haug, Samuel L. Jaccard, Alfredo Martínez-García, Heiko Pälike, Richard D. Pancost, and Paul A. Wilson
PNAS December 12, 2017 114 (50) 13114-13119; published ahead of print November 27, 2017 https://doi.org/10.1073/pnas.1702143114

  1. Edited by Maureen E. Raymo, Lamont–Doherty Earth Observatory of Columbia University, Palisades, NY, and approved September 7, 2017 (received for review February 9, 2017)

Significance

Conflicting sets of hypotheses highlight either the role of ice sheets or atmospheric carbon dioxide (CO2) in causing the increase in duration and severity of ice age cycles ∼1 Mya during the Mid-Pleistocene Transition (MPT). We document early MPT CO2 cycles that were smaller than during recent ice age cycles. Using model simulations, we attribute this to post-MPT increase in glacial-stage dustiness and its effect on Southern Ocean productivity. Detailed analysis reveals the importance of CO2 climate forcing as a powerful positive feedback that magnified MPT climate change originally triggered by a change in ice sheet dynamics. These findings offer insights into the close coupling of climate, oceans, and ice sheets within the Earth System.

Abstract

During the Mid-Pleistocene Transition (MPT; 1,200–800 kya), Earth’s orbitally paced ice age cycles intensified, lengthened from ∼40,000 (∼40 ky) to ∼100 ky, and became distinctly asymmetrical. Testing hypotheses that implicate changing atmospheric CO2 levels as a driver of the MPT has proven difficult with available observations. Here, we use orbitally resolved, boron isotope CO2 data to show that the glacial to interglacial CO2 difference increased from ∼43 to ∼75 μatm across the MPT, mainly because of lower glacial CO2 levels. Through carbon cycle modeling, we attribute this decline primarily to the initiation of substantive dust-borne iron fertilization of the Southern Ocean during peak glacial stages. We also observe a twofold steepening of the relationship between sea level and CO2-related climate forcing that is suggestive of a change in the dynamics that govern ice sheet stability, such as that expected from the removal of subglacial regolith or interhemispheric ice sheet phase-locking. We argue that neither ice sheet dynamics nor CO2 change in isolation can explain the MPT. Instead, we infer that the MPT was initiated by a change in ice sheet dynamics and that longer and deeper post-MPT ice ages were sustained by carbon cycle feedbacks related to dust fertilization of the Southern Ocean as a consequence of larger ice sheets.

Footnotes

  • ↵1T.B.C. and M.P.H. contributed equally to this work.

  • ↵2To whom correspondence may be addressed. Email: T.chalk{at}noc.soton.ac.uk or M.P.Hain{at}soton.ac.uk.

  • Author contributions: T.B.C., M.P.H., G.L.F., E.J.R., A.P.H., G.H.H., S.L.J., A.M.-G., R.D.P., and P.A.W. designed research; T.B.C., M.P.H., M.P.S.B., and S.G.C. performed research; T.B.C., M.P.H., G.L.F., E.J.R., P.F.S., S.G.C., H.P., and P.A.W. analyzed data; and T.B.C. and M.P.H. wrote the paper.

  • The authors declare no conflict of interest.

  • This article is a PNAS Direct Submission.

  • Data deposition: The data reported in this paper have been deposited in the Pangaea database (https://doi.pangaea.de/10.1594/PANGAEA.882551).

  • This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1702143114/-/DCSupplemental.

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Causes of ice age intensification across the MPT
Thomas B. Chalk, Mathis P. Hain, Gavin L. Foster, Eelco J. Rohling, Philip F. Sexton, Marcus P. S. Badger, Soraya G. Cherry, Adam P. Hasenfratz, Gerald H. Haug, Samuel L. Jaccard, Alfredo Martínez-García, Heiko Pälike, Richard D. Pancost, Paul A. Wilson
Proceedings of the National Academy of Sciences Dec 2017, 114 (50) 13114-13119; DOI: 10.1073/pnas.1702143114
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