Evidence from carbon isotope measurements for diverse origins of sedimentary hydrocarbons

THE organic matter found in sedimentary rocks must derive from many sources; not only from ancient primary producers but also from consumers and secondary producers. In all of these organisms, isotope effects can affect the abundance and distribution of ¹³C in metabolites. Here, by using an improved form of a previously described technique¹ in which the effluent of a gas chromatograph is continuously analysed isotopically, we report evidence of the diverse origins of sedimentary organic matter. The record of ¹³C abundances in sedimentary carbonate and total organic carbon can be interpreted in terms of variations in the global carbon cycle (see ref. 2, for example) Our results demonstrate, however, that isotope variations within sedimentary organic mixtures substantially exceed those observed between samples of total organic carbon³. Resolution of isotope variations at the molecular level offers a new and convenient means of refining views both of localized palaeoenvironments and of control mechanisms within the global carbon cycle.