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 in an inverse approach that the long eccentricity in δ13C might have been caused by variations in the isotopic signature of geological sources, namely of the weathered carbonate rock (δ13Crock) or of volcanically released CO2 (δ13Cv). This closure of the 13C cycle in these periodicities also explains the offset in atmospheric δ13CO2 seen between the Penultimate Glacial Maximum (PGM) and the Last Glacial Maximum (LGM). The necessary isotopic signatures in δ13Crock or δ13Cv, which align my simulations with reconstructions of the 13C cycle on orbital timescales, have the 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 also has an obliquity-related component and since precipitation (or runoff) is one main driver for local weathering rates, 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 times of the pre-Pliocene parts of the Cenozoic that have been largely ice-free in the Northern Hemisphere.