Causes of ice-age intensification across the Mid-Pleistocene Transition

• Published in: Proceedings of the National Academy of Sciences
• Main Authors: Chalk, Thomas, Hain, Mathis, Foster, Gavin, Rohling, Eelco, Sexton, Philip F., Badger, Marcus P.S., Cherry, Soraya, Hasenfratz, Adam, Haug, G.H., Jaccard, S.L., Martínez-García, Alfredo, Palike, Heiko, Pancost, Richard D., Wilson, Paul A.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2017
• Subjects: Southern Ocean; Kya; Ice Sheet
• Online Access: https://eprints.soton.ac.uk/415926/; https://eprints.soton.ac.uk/415926/1/REVISION2_PNAS_MS_ULTIMO_length_2.docx; https://eprints.soton.ac.uk/415926/2/REVISION2_PNAS_MS_ULTIMO_supplement_length.docx

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.