Geochemical studies of Cretaceous strata rich in organic carbon (OC) from Deep Sea Drilling Project (DSDP) sites and several land sections reveal several consistent relationships among amount of OC, hydrocarbon generating potential of kerogen (measured by pyrolysis as the hydrogen index, HI), and the isotopic composition of the OC. First, there is a positive correlation between HI and OC in strata that contain more than about 1% OC. Second, percent OC and HI often are negatively correlated with carbon isotopic composition (σ ¹³C) of kerogen. The relationship between HI and OC indicates that as the amount of organic matter increases, this organic matter tends to be more lipid rich reflecting the marine source of the organic matter.

Cretaceous samples that contain predominantly marine organic matter tend to be isotopically lighter than those that contain predominantly terrestrial organic matter. Average σ ¹³C values for organic matter from most Cretaceous sites are between −26 and −28‰, and values heavier than about −25‰ occur at very few sites. Most of the σ ¹³C values of Miocene to Holocene OC-rich strata and modern marine plankton are between −16 to −23‰. Values of σ¹³C of modern terrestrial organic matter are mostly between −23 and −33‰.

The depletion of terrestial OC in ¹³C relative to marine planktonic OC is the basis for numerous statements in the literature that isotopically light Cretaceous organic matter is of terrestrial origin, even though other organic geochemical and(or) optical indicators show that the organic matter is mainly of marine origin. A difference of about 5‰ in σ ¹³C between modern and Cretaceous OC-rich marine strata suggests either that Cretaceous marine planktonic organic matter had the same isotopic signature as modern marine plankton and that signature has been changed by diagenesis, or that OC derived from Cretaceous marine plankton was isotopically lighter by about 5‰ relative to modern plankton OC. Diagenesis does not produce a significant shift in σ ¹³C in Miocene to Holocene sediments, and therefore probably did not produce the isotopically light Cretaceous OC. This means that Cretaceous marine plankton must have had σ ¹³C values that were about 5‰ lighter than modern marine plankton, and at least several per mil lighter than Cretaceous terrestrial vegetation. The reason for these lighter values, however, is not obvious. It has been proposed that concentrations of CO₂ were higher during the middle Cretaceous, and this more available CO₂ may be responsible for the lighter σ ¹³C values of Cretaceous marine organic matter.