Carbon cycle tree rings

Freeman KH, Hayes JM (1992) Fractionation of carbon isotopes by phytoplankton and estimates of ancient CO2 levels. Global Biogeochem Cycles 6 185-198 Freeman KH, Hayes JM, Trendel JM, Albrecht P (1990) Evidence from carbon isotope measurements for diverse origins of sedimentary hydrocarbons. Nature 343 254-256 Freyer HD (1979) On the C-record in tree rings, I, C variations in northern hemisphere trees during the last 150 years, TeUus 31 124-137... 

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