Pliocene CO2 rise due to sea-level fall as a mechanism for the delayed ice age.

Global sea level fell progressively during the Pliocene in response to the growth of ice sheets at high northern latitudes. However, not until ∼2.7 Ma did the climate system actually generate and maintain major ice sheets in the Northern Hemisphere. A decline in atmospheric CO 2 levels (p CO 2) is thought to be responsible for the Pleistocene ice ages. However, the link between p CO 2 and Pliocene glaciations, or their delay, is not well understood, mainly because the magnitude of p CO 2 remains uncertain beyond the maximum age of ice cores (∼0.8 Ma). Here, we develop a carbonate clumped isotope (∆ 47) geothermometer specifically for pedogenic carbonate in Chinese loess, which we use to reconstruct temperatures in northern China, and p CO 2 , for the past 7.24 Ma. The palaeotemperature results show an oscillatory cooling trend over time in the East Asian monsoon region, in good agreement with coeval tropical sea surface temperature records. The p CO 2 reconstruction using the pedogenic carbonate palaeobarometer reveals low values (150–325 ppmv) during the intervals of 7.24–4.3 Ma and 2.6–0 Ma. In contrast, there is a three-step rise in p CO 2 up to 535 ppmv from 4.3 to 2.6 Ma, coincident with an oscillatory decrease in sea level and in benthic δ13C. We suggest that the Pliocene CO 2 rise was causally related to the sea-level fall and associated organic carbon oxidation on exposed continental shelves, thereby delaying the onset of the ice age. This implies a negative feedback mechanism induced by long-term cooling since the Late Miocene, which may have delayed the onset of the ice age but did not prevent it. • A carbonate clumped isotope (∆ 47) geothermometer was developed for Chinese loess. • Palaeotemperature reconstruction shows an oscillatory cooling trend since 7.24 Ma. • Low p CO 2 during 7.24–4.3 and 2.6–0 Ma estimated from calcic paleosols. • Sea-level falls during 4.3–2.6 Ma led to CO 2 release from continental shelves. • Cooling-induced p CO 2 rise as a negative feedback mechanism for the delayed ice age. [ABSTRACT FROM AUTHOR]