Organic carbon affects isotopes

Figure 3. Idealized carbon isotope cycle in a small stratified lake. The isotopic composition of organic matter buried in sediments is determined by the proportions of different terrestrial and lacustrine organic mailer. Ihccartron isotopic composition of dissolved inorganic carbon (DlC), and the rates of primary production and respiration within the water column. Isotope enrichment factors (e), listed here as the difference between the product and the substrate, vary with the form of DlC that lake algae assimilate (e.g., C02(aq) or HCOy ). Inorganic carbonate (CaC03> typically forms in isotopic equilibrium with the dissolved inorganic carbon pool and. as such, is indirectly affected by organic matter sources and primary production and respiration rales.

Fractionation and Contamination. The ratio 14C/12C in certain materials may be affected by isotopic fractionation. For example, the uptake of carbon dioxide and its incorporation into plant tissue may be accompanied by substantial fractionation that depends on the plant species. With marine organisms, fractionation may also be important, especially when inorganic carbonate and bicarbonate are involved. Corrections for fractionation must be made for precise radiocarbon dating. 

Sr/ Sr ratios for parent waters indicate that calcite formed from fluids with isotopic compositions similar or close to present-day formation fluids. The elevated Sr/ Sr and 5 0 of calcite-forming waters resulted mainly from pervasive dissolution of detrital and diagenetic feldspar prior to calcitiza-tion, and may be in part related to the introduction of oil-accompanying basinal brines. Calcite 5 C values reflect carbon sourced by organic matter and possibly influenced by biodegradation-fermentation processes that affected petroleum emplaced in the reservoir. 

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