Evolution of the early Antarctic ice ages
- aNational Oceanography Centre Southampton, University of Southampton, Southampton SO14 3ZH, United Kingdom;
- bDepartment of Physical Geography, Faculty of Geosciences, Utrecht University, 3508 TC Utrecht, The Netherlands;
- cLittoral Environnement et Sociétés, Université de La Rochelle, 17042 La Rochelle, France;
- dDepartment of Earth Sciences, Faculty of Geosciences, Utrecht University, 3584 CD Utrecht, The Netherlands;
- eCenter for Marine Environmental Sciences, University of Bremen, 28359 Bremen, Germany;
- fInstitute of Geosciences, Goethe-University Frankfurt am Main, 60438 Frankfurt, Germany;
- gDepartment of Earth Sciences, University of Oxford, Oxford OX1 3AN, United Kingdom;
- hDepartment of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, United Kingdom;
- iSchool of GeoSciences, Grant Institute, University of Edinburgh, Edinburgh EH9 3FE, United Kingdom;
- jDipartimento di Ingegneria e Geologia, Università degli Studi “G. d’Annunzio” di Chieti–Pescara, 66013 Chieti Scalo, Italy
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Edited by Mark H. Thiemens, University of California, San Diego, La Jolla, CA, and approved February 24, 2017 (received for review September 15, 2016)

Significance
The Antarctic ice cap waxed and waned on astronomical time scales throughout the Oligo-Miocene time interval. We quantify geometries of Antarctic ice age cycles, as expressed in a new climate record from the South Atlantic Ocean, to track changing dynamics of the unipolar icehouse climate state. We document numerous ∼110-thousand-year-long oscillations between a near-fully glaciated and deglaciated Antarctica that transitioned from being symmetric in the Oligocene to asymmetric in the Miocene. We infer that distinctly asymmetric ice age cycles are not unique to the Late Pleistocene or to extremely large continental ice sheets. The patterns of long-term change in Antarctic climate interpreted from this record are not readily reconciled with existing CO2 records.
Abstract
Understanding the stability of the early Antarctic ice cap in the geological past is of societal interest because present-day atmospheric CO2 concentrations have reached values comparable to those estimated for the Oligocene and the Early Miocene epochs. Here we analyze a new high-resolution deep-sea oxygen isotope (δ18O) record from the South Atlantic Ocean spanning an interval between 30.1 My and 17.1 My ago. The record displays major oscillations in deep-sea temperature and Antarctic ice volume in response to the ∼110-ky eccentricity modulation of precession. Conservative minimum ice volume estimates show that waxing and waning of at least ∼85 to 110% of the volume of the present East Antarctic Ice Sheet is required to explain many of the ∼110-ky cycles. Antarctic ice sheets were typically largest during repeated glacial cycles of the mid-Oligocene (∼28.0 My to ∼26.3 My ago) and across the Oligocene−Miocene Transition (∼23.0 My ago). However, the high-amplitude glacial−interglacial cycles of the mid-Oligocene are highly symmetrical, indicating a more direct response to eccentricity modulation of precession than their Early Miocene counterparts, which are distinctly asymmetrical—indicative of prolonged ice buildup and delayed, but rapid, glacial terminations. We hypothesize that the long-term transition to a warmer climate state with sawtooth-shaped glacial cycles in the Early Miocene was brought about by subsidence and glacial erosion in West Antarctica during the Late Oligocene and/or a change in the variability of atmospheric CO2 levels on astronomical time scales that is not yet captured in existing proxy reconstructions.
- unipolar icehouse
- early Antarctic ice sheet
- Oligocene−Miocene
- glacial−interglacial cycle geometries
- bispectral analysis
Footnotes
- ↵1To whom correspondence should be addressed. Email: diederik.liebrand{at}noc.soton.ac.uk.
Author contributions: D.L., A.T.M.d.B., H.M.B., P.A.W., S.M.B., G.R., H.P., S.J.B., F.J.H., D.A.H., C.E.H., D.K., I.R., M.J.M.S., A.E.v.D., and L.J.L. designed research; D.L., A.T.M.d.B., H.M.B., M.J.M.S., and A.E.v.D. performed research; P.A.W., G.R., H.P., F.J.H., and L.J.L. contributed analytic tools; D.L., A.T.M.d.B., H.M.B., M.J.M.S., and A.E.v.D. analyzed data; D.L., A.T.M.d.B., P.A.W., and S.M.B. wrote the paper; and all other authors contributed to writing the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1615440114/-/DCSupplemental.
Freely available online through the PNAS open access option.
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