Closing the Plio-Pleistocene 13C cycle in the 405-kyr periodicity by isotopic signatures of geological sources

To unravel which processes are responsible for changes in atmospheric CO₂ the carbon isotopes are useful helpers widely applied in the past. This study helps to better understand long-term changes in ¹³C, which has also consequences for the interpretation of atmospheric δ¹³CO₂ measured together with CO₂ in ice cores. 

The ¹³C cycle of the Plio-Pleistocene, as recorded in δ¹³C of benthic foraminifera, has power in periodicities related to the long eccentricity cycle of 405-kyr that is missing in corresponding climate records (e.g. δ¹⁸O). Using a global carbon cycle model I show that the long eccentricity cycle in δ¹³C might have been caused by variations in the isotopic signature of geological sources, namely of the weathered carbonate rock (δ¹³C_rock) or of volcanically released CO₂ (δ¹³C_v). This closure of the ¹³C cycle in these peridicities also explains the offset in atmospheric δ¹³CO₂ seen between the penultimate and the last glacial maximum. The necessary isotopic signatures in δ¹³C_rock or δ¹³C_v which align my simulations with reconstructions of the ¹³C cycle on orbital timscales have most power in the obliquity band (41-kyr) suggesting that land ice dynamics are the ultimate cause for these suggested variations. Since the Asian monsoon as reconstructed from speleothems has also an obliquity-related component it is possible that these proposed changes in weathering are indeed, at least partly, connected to the monsoon as previously suggested. Alternatively, the suggested impact of land ice or sea level on volcanic activity might also be influential for the ¹³C cycle. This indirect influence of ice sheets on the long eccentricity cycle in δ¹³C implies that these processes might not have been responsible for the 405-kyr periodicity found in ice-free times of the pre-Pliocene parts of the Cenozoic.

See preprint (https://doi.org/10.5194/cp-2024-63) for details.