Comment on esd-2022-46

Abstract

Along with 400 kyr periodicities, multi-million year cycles have been found in δ¹³C records over different time periods. A ~8–9 Myr periodicity is found throughout the Cenozoic, and part of the Mesozoic. The robust presence of this periodicity in δ¹³C records suggests an astronomical origin. However, this periodicity is barely visible in the astronomical forcing. Due to the large fractionation factor of organic matter, its burial or oxidation produces large δ¹³C variations for moderate carbon variations. Therefore, astronomical forcing of organic matter fluxes is a plausible candidate to explain the oscillations observed in the δ¹³C records. So far, modelling studies forcing astronomically the organic matter burial have been able to produce 400 kyr and 2.4 Myr cycles in δ¹³C, but were not able to produce longer cycles, such as 8–9 Myr cycles. Here, we propose a mechanism that could explain the presence of multi-million year cycles in the δ¹³C records, and their stability over time, as a result of preferential periodicity locking to multiples of astronomical forcing periods. With a simple non linear conceptual model for the carbon cycle that has multiple equilibria, we are able to extract longer periods than with a simple linear model, and more specifically, multi-million year periods.