Stable carbon isotope ratios of bulk organic matter have been long used to evaluate sources of organic matter in sediments and carbon flow pathways in food webs. Interpretation of bulk isotope ratios are complicated by the fact that bulk material represents mixtures of carbon from several sources, and thus isotope ratios are weighted averages.

The recent advent of compound specific isotope ratio monitoring mass spectrometry (or compound specific isotope analysis - csia - for short) has brought together the source-specificity of biomarkers with the information in isotope ratios. At SkIO we have taken advantage of this powerful new tool to help characterize the sources and cycling of lipid biomarkers in several marine environments, obtaining some of the first compound specific isotope data for oceanic particulate matter. Carbon isotope signatures have helped elucidate the sources - whether bacterial, algal, or terrigenous higher plant - sources in particulate matter and coastal and deep-sea sediments.

Among paleooceanographers, there is great interest in using isotopic data for sedimentary biomarkers to evaluate past environmetal conditions. To do so requires a strong understanding of the factors - dissolved carbon dioxide concentration, growth rate, species composition, cell geometry - involved in isotope fractionation during fixation of organic carbon and how food-web processes and diagenesis may alter the isotopic signal. Our group has been also examining the controls on isotopic fractionation in field studies in collaboration with Dr. Katherine Freeman and Drs. Brian Popp and Robert Bidigare . A series of controlled experiments in chemostats have also been conducted at Hawaii to provide a theoretical basis for interpreting field results.

Selected References:

K.H. Freeman and S.G. Wakeham. (1992) Variations in the distributions and isotopic compositions of alkenones in Black Sea particles and sediments. In: Advances in Organic Geochemistry (C.B. Eckardt, J.R. Maxwell, S.R. Larter and D.A.C. Manning, eds.), Pergamon Press, Org. Geochem. 19: 277-285.

S.G. Wakeham, K.H. Freeman, T.K. Pease and J.M. Hayes. (1993) A photoautotrophic source for lycopane in marine water columns and sediments. Geochim. Cosmochim. Acta 57: 159-165.

K.H. Freeman, S.G. Wakeham and J.M. Hayes. (1994) Predictive isotopic biogeochemistry: hydrocarbons from anoxic marine basins. In: Compound Specific Isotope Analysis in Biogeochemistry and Petroleum Geochemistry (M. Schoell and J.M. Hayes, eds.); Org. Geochem. 21: 629-644.

R.R. Bidigare, A. Flugge, K.H. Freeman, K.L. Hansen, J.M. Hayes, D.J. Hollander, J.P. Jasper, L.L. King, E.A. Laws, J. Milder, F.J. Millero, R. Pancost, B.N. Popp, P.A. Steinberg, and S.G. Wakeham. (1997) Consistent fractionization of 13C in nature and in the laboratory: growth-rate effects in some haptophyte algae. Global Biogeochem. Cycles. 11: 279-292.

R.D. Pancost, K.H. Freeman, S.G. Wakeham and C.Y. Robertson. (1997) Controls on carbon isotope fractionation by diatoms in the Peru Upwelling Region. Geochim. Cosmochim. Acta. 61: 4983-4991.  

E.A. Canuel, K.H. Freeman, and S.G. Wakeham. (1997) Isotopic composition of lipid biomarker compounds in estuarine plants and surface sediments. Limnol. Oceanogr. 42: 1570-1583. 

B.N. Popp, F. Kenig, S.G. Wakeham, E.A. Lewis, and R.R. Bidigare. (1998) Does growth rate affect ketone unsaturation and intracellular carbon isotopic variability in Emilinia huxleyi ? Paleoceanography 13: 35-41.

B.N. Popp, E.A. Laws, R.R. Bidigare, J.E. Dore, K.L. Hanson, and S.G. Wakeham. (1998) The effect of algal cell geometry on carbon isotopic fractionation. Geochim. Cosmochim. Acta. 62: 69-77.