Methane has a very light δC signature: biogenic methane of −60‰, thermogenic methane −40‰. The release of large amounts of methane clathrate can impact on global δC values, as at the Paleocene–Eocene Thermal Maximum.
More commonly, the ratio is affected by variations in primary productivity and organic burial.  Organisms preferentially take up light C, and have a δC signature of about −25‰, depending on their metabolic pathway.  Therefore, an increase in δC in marine fossils is indicative of an increase in the abundance of vegetation.
An increase in primary productivity causes a corresponding rise in δC values as more C is locked up in plants.  This signal is also a function of the amount of carbon burial; when organic carbon is buried, more C is locked out of the system in sediments than the background ratio.