28 March 2017 (University of Southampton) – A team of scientists led by the University of Southampton has found that the Antarctic ice cap underwent dramatic cycles of expansion and melt-back millions of years ago when carbon dioxide levels were similar to those experienced today.
The research, led by palaeoclimatologist Dr Diederik Liebrand as part of an International Ocean Discovery Program collaboration, suggests that 20 to 30 million years ago the Antarctic periodically gained and lost huge ice caps – equivalent to the entire modern-day East Antarctic Ice Sheet.
Dr Liebrand said: “Our research shows that even slow, naturally forced climate change is capable of driving rapid large-scale changes in ice volume in Antarctica – and therefore global sea levels.
“This is of particular interest to scientists because humans are now the main agents of climate change, and the rates of change today are much faster than those that occurred millions of years ago during the interval that we studied.
“Increasingly we are understanding that the Antarctic ice cap is not some enduring monolithic block but a much more slippery ephemeral beast – and the implications of that realisation for the future of Antarctic ice sheets in a very rapidly warming world have not escaped us.”
The scientists examined oxygen isotopes in fossilised micro-organisms – found in a drill core of marine sediments taken from a water depth of 2.5km in the South Atlantic – to reach their findings, published this week in the journal Proceedings of the National Academy of Sciences of the United States of America (PNAS).
Large swings in isotopic composition suggest that the Antarctic lost and regained almost all of its ice in numerous 110,000-year cycles between the Oligocene and early Miocene epochs.
“At that time, large ice sheets had not yet developed in the northern hemisphere, so the cycles that we observe mean that Earth was switching back and forth between a unipolar glacial state and a largely deglaciated state,” said Dr Liebrand.
Professor Paul Wilson, a University of Southampton colleague also involved in the study, added: “All of this happened during an interval when atmospheric carbon dioxide levels ranged between today’s human-influenced value and those that, at current rates of fossil fuel-burning, we will experience in 50 to 100 years from now.
“The Antarctic ice cap was incredibly dynamic – it underwent repeated large-scale expansion and melt-back in the twinkling of a geological eye.”
Funding for UK researchers involved in the study was provided by the Natural Environment Research Council.
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.
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