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

  • Author(s): Chalk, Thomas B
  • Hain, Mathis P
  • Foster, Gavin L
  • Rohling, Eelco J
  • Sexton, Philip F
  • Badger, Marcus PS
  • Cherry, Soraya G
  • Hasenfratz, Adam P
  • Haug, Gerald H
  • Jaccard, Samuel L
  • Martínez-García, Alfredo
  • Pälike, Heiko
  • Pancost, Richard D
  • Wilson, Paul A
  • et al.
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.

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