The Eocene–Oligocene transition: a review of marine and terrestrial proxy data, models and model–data comparisons
<p>The Eocene–Oligocene transition (EOT) was a climate shift from a largely ice-free greenhouse world to an icehouse climate, involving the first major glaciation of Antarctica and global cooling occurring <span class="inline-formula">∼34</span> million years ago (Ma) and...
| Main Authors: | , , , , , , , , , , , , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2021-01-01
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| Series: | Climate of the Past |
| Online Access: | https://cp.copernicus.org/articles/17/269/2021/cp-17-269-2021.pdf |
| Summary: | <p>The Eocene–Oligocene transition (EOT) was a climate shift from a largely ice-free greenhouse world
to an icehouse climate, involving the first major glaciation of Antarctica and global cooling
occurring <span class="inline-formula">∼34</span> million years ago (Ma) and lasting <span class="inline-formula">∼790</span> <span class="inline-formula">kyr</span>. The change is
marked by a global shift in deep-sea <span class="inline-formula"><i>δ</i><sup>18</sup>O</span> representing a combination of
deep-ocean cooling and growth in land ice volume. At the same time, multiple independent proxies
for ocean temperature indicate sea surface cooling, and major changes in global fauna and flora
record a shift toward more cold-climate-adapted species. The two principal suggested explanations
of this transition are a decline in atmospheric <span class="inline-formula">CO<sub>2</sub></span> and changes to ocean gateways,
while orbital forcing likely influenced the precise timing of the glaciation. Here we review and
synthesise proxy evidence of palaeogeography, temperature, ice sheets, ocean circulation and
<span class="inline-formula">CO<sub>2</sub></span> change from the marine and terrestrial realms. Furthermore, we quantitatively
compare proxy records of change to an ensemble of climate model simulations of temperature change
across the EOT. The simulations compare three forcing mechanisms across the EOT: <span class="inline-formula">CO<sub>2</sub></span>
decrease, palaeogeographic changes and ice sheet growth. Our model ensemble results demonstrate
the need for a global cooling mechanism beyond the imposition of an ice sheet or palaeogeographic
changes. We find that <span class="inline-formula">CO<sub>2</sub></span> forcing involving a large decrease in <span class="inline-formula">CO<sub>2</sub></span> of
ca. 40 % (<span class="inline-formula">∼325</span> <span class="inline-formula">ppm</span> drop) provides the best fit to the available proxy evidence,
with ice sheet and palaeogeographic changes playing a secondary role. While this large decrease is
consistent with some <span class="inline-formula">CO<sub>2</sub></span> proxy records (the extreme endmember of<span id="page270"/> decrease), the
positive feedback mechanisms on ice growth are so strong that a modest <span class="inline-formula">CO<sub>2</sub></span> decrease
beyond a critical threshold for ice sheet initiation is well capable of triggering rapid ice sheet
growth. Thus, the amplitude of <span class="inline-formula">CO<sub>2</sub></span> decrease signalled by our data–model comparison
should be considered an upper estimate and perhaps artificially large, not least because the
current generation of climate models do not include dynamic ice sheets and in some cases may be
under-sensitive to <span class="inline-formula">CO<sub>2</sub></span> forcing. The model ensemble also cannot exclude the possibility
that palaeogeographic changes could have triggered a reduction in <span class="inline-formula">CO<sub>2</sub></span>.</p> |
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| ISSN: | 1814-9324 1814-9332 |