…4 (15): 8001– 8005. Bibcode 1997PNAS...94.8001V doi 10.1073/pnas.94.15.8001 PMC 21545 PMID 9223303 Peters, Kenneth E.; Walters, Clifford C.; Moldowan, J. Michael (2005). The Biomarker Guide: Biomarkers and isotopes in petroleum systems and Earth history . Vol. 2. Cambridge Univer…