A ~ 9 myr cycle in Cenozoic δ13C record and long-term orbital eccentricity modulation: Is there a link?

The ~ 65-myr-long Cenozoic carbon isotope record (δ13C) of Zachos et al. (2001, 2008) documents a strong long-term cycle with a mean pseudoperiodicity close to ~ 9 myr. This cyclicity modulates the ~ 2.4 myr eccentricity cycle amplitude, hinting at a possible link between long-term astronomical and geological variations. Some phase shifts between ~ 9-myr δ13C and astronomical cycles suggest that additional processes (e.g., tectonics) contribute to these long-term carbon-cycle variations.The strong response of δ13C to long-term eccentricity periods (~ 9 myr, ~ 2.4 myr, ~ 400 kyr) supports the hypothesis that the long time-residence of carbon in the oceans amplifies lower frequency or dampens higher frequency orbital variations. Additionally, the strong expression of low-amplitude ~ 9 myr eccentricity cycle in the δ13C record could be explained by energy-transfer process from higher to lower frequency cycles, and all eccentricity components modulate the carrier climatic precession cycles.Finally, the Paleocene–Eocene Thermal Maximum (PETM, 55.9 Ma) event, which corresponds to a pronounced δ13C negative excursion, is situated within a strong decrease in the most prominent ~ 9 myr δ13C cycle, hinting at a link between accelerated rates in δ13C variations and the PETM. This specific ~ 9 myr δ13C cycle seems to be amplified by non-orbital mechanisms in atmosphere–continent–ocean system, such as previously suggested methane release from gas hydrate and volcanism.

► Cenozoic benthic foraminiferal δ13C data documents a prominent 9 myr cyclicity.
► The 9 myr cyclicity modulates the 2.4 myr eccentricity cycles.
► The 9 myr δ13C cyclicity correlates to long-term eccentricity variations.
► The earliest Cenozoic 9 myr δ13C cycle that includes the PETM is amplified.
► Non-orbital processes may contribute to the the onset of the PETM.