Much of our understanding of Earth’s past climate comes from the measurement of oxygen and carbon isotope variations in deep-sea benthic foraminifera. Yet, long intervals in existing records lack the temporal resolution and age control needed to thoroughly categorize climate states of the Cenozoic era and to study their dynamics. Here, we present a new, highly resolved, astronomically dated, continuous composite of benthic foraminifer isotope records developed in our laboratories. Four climate states—Hothouse, Warmhouse, Coolhouse, Icehouse—are identified on the basis of their distinctive response to astronomical forcing depending on greenhouse gas concentrations and polar ice sheet volume. Statistical analysis of the nonlinear behavior encoded in our record reveals the key role that polar ice volume plays in the predictability of Cenozoic climate dynamics.
Global warming during the Palaeocene-Eocene Thermal Maximum1,2 (PETM, ~56 Ma) is commonly interpreted as being primarily driven by the destabilization of carbon from surficial sedimentary reservoirs such as methane hydrates3. However, the source(s) of carbon remain controversial1,3–5. Resolving this is key to understanding the proximal cause, as well as quantifying the roles of triggers versus feedbacks in driving the event. Here we present new boron isotope data – a proxy for seawater pH – that demonstrate the occurrence of persistently suppressed surface ocean pH across the PETM. Our pH data, alongside a paired carbon isotope record, are assimilated in an Earth system model to reconstruct the unfolding carbon cycle dynamics across the event6,7. We find strong evidence for a much larger (>10,000 PgC) and on average isotopically heavier carbon source than considered previously8,9. This leads us to identify volcanism associated with the North Atlantic Igneous Province, rather than carbon from a surficial reservoir, as the main driver of the PETM10,11. We also find that, although amplifying organic carbon feedbacks with climate likely played only a subordinate role in driving the event, enhanced organic matter burial was important in ultimately sequestering the released carbon and accelerating the recovery of the Earth system12.
The Early Eocene Climate Optimum (EECO, which occurred about 51 to 53 million years ago) 1 , was the warmest interval of the past 65 million years, with mean annual surface air temperature over ten degrees Celsius warmer than during the pre-industrial period 2-4 . Subsequent global cooling in the middle and late Eocene epoch, especially at high latitudes, eventually led to continental ice sheet development in Antarctica in the early Oligocene epoch (about 33.6 million years ago). However, existing estimates place atmospheric carbon dioxide (CO 2 ) levels during the Eocene at 500-3,000 parts per million [5][6][7] , and in the absence of tighter constraints carbon-climate interactions over this interval remain uncertain. Here we use recent analytical and methodological developments 8-11 to generate a new high-fidelity record of CO 2 concentrations using the boron isotope (δ 11 Β) composition of well preserved planktonic foraminifera from the Tanzania Drilling Project, revising previous estimates 6 . Although species-level uncertainties make absolute values difficult to constrain, CO 2 concentrations during the EECO were around 1,400 parts per million. The relative decline in DOI: 10.1038/nature17423 Page 2 of 38 CO 2 concentration through the Eocene is more robustly constrained at about fifty per cent, with a further decline into the Oligocene 12 . Provided the latitudinal dependency of sea surface temperature change for a given climate forcing in the Eocene was similar to that of the late Quaternary period 13 , this CO 2 decline was sufficient to drive the well documented high-and low-latitude cooling that occurred through the Eocene 14 . Once the change in global temperature between the pre-industrial period and the Eocene caused by the action of all known slow feedbacks (apart from those associated with the carbon cycle) is removed 2-4 , both the EECO and the late Eocene exhibit an equilibrium climate sensitivity relative to the pre-industrial period of 2.1 to 4.6 degrees Celsius per CO 2 doubling (66 per cent confidence), which is similar to the canonical range (1.5 to 4.5 degrees Celsius 15 ), indicating that a large fraction of the warmth of the early Eocene greenhouse was driven by increased CO 2 concentrations, and that climate sensitivity was relatively constant throughout this period.Over the past 540 million years, Earth's climate has oscillated between a globally warm 'greenhouse state' and an 'icehouse state' with substantial continental glaciation 16 . The most recent of these transitions occurred between the warmest time interval of the last 65 million years-the EECO (about 14 ± 3 °C warmer than preindustrial times 2 )-and the rapid growth of ice on Antarctica in the earliest icehouse state of the Oligocene (~33.6 Myr ago 1 ). It has been suggested that variations in the concentration of the greenhouse gas CO 2 were responsible for both the overall warmth of the Eocene and the subsequent cooling 17 . Recent studies have documented the importance of CO 2 decline for the final step into the icehous...
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