| 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 theδ13C 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. |
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