Pliocene switch in orbital‐scale carbon cycle/climate dynamics

The high‐frequency (periods of ~10 5  years) relationship between carbon and oxygen isotopes in benthic foraminifera—the two proxies most extensively used to reconstruct past changes in Earth's carbon cycle and climate—shows two distinct patterns across the Cenozoic. The first, “glacial‐style,” pattern associates negative excursions in δ 13 C with positive excursions in δ 18 O indicative of relatively cold temperatures and greater ice volume. The second, “hyperthermal‐style,” pattern associates negative excursions in δ 13 C with negative excursions in δ 18 O indicative of warming. Here I assess the coherence and phasing of these high‐frequency, orbital‐scale cycles (in particular, the ~100 kyr eccentricity period) in δ 13 C and δ 18 O from multiple high‐resolution benthic foraminiferal records spanning the last ~65 million years of Earth history in order to identify which of these patterns is most persistent across the Cenozoic and when the switch between these patterns occurred. I find that the glacial‐style δ 13 C‐δ 18 O pattern is a feature restricted to the Plio‐Pleistocene, suggesting a fundamental change in the interplay between the carbon cycle and climate associated with the onset of Northern Hemisphere glaciation. This relative stability of the high‐frequency relationship between δ 13 C and δ 18 O across most of the Cenozoic persists despite significant secular changes in climate and may suggest a dichotomous response of terrestrial carbon cycle dynamics to orbital forcing with a switch occurring in the last ~5 Myr.