Biosynthetic isotope fractionation negligibly impacts biomarker 14C ages

Abstract

Radiocarbon ( 14 C) ages of acetogenic lipid biomarkers such as n-alkanes represent a powerful tool to track carbon-cycle turnover times in a range of environments including soils, fluvially exported sediments, and marine sediments. However, the fidelity of this approach requires that biomarker 14 C ages accurately reflect the time that has passed since biosynthesis and are not complicated by issues such as isotope fractionation. Reported 14 C ages are thus always corrected for mass-dependent fractionation using a 14 C/ 13 C mass law, b, of 2:0. However, anomalous deviations from mass dependence could theoretically lead to biases in measured 14 C ages, particularly for compounds such as acetogenic lipids that undergo large 13 C fractionations. Here, I test this possibility by estimating kinetic and equilibrium mass laws for various processes involved in acetogenic lipid biosynthesis using simple approximations and more robust computational chemistry methods. I find that kinetic b values range from 1:890 to 1:995 and that equilibrium b values for several chain elongation steps range from 1:856 to 1:880, consistent with previous results for other chemical and biological processes. In contrast, complex reaction networks may lead to large expressed b values, but only when net lnð 13 aÞ ! 0. Combined, these results imply maximum 14 C age offsets due to biosynthetic fractionation of ~20 to 40 yr. Biomarker 14 C ages are therefore robust to biosynthetic isotope fractionation and can be confidently interpreted to reflect carbon-cycle turnover times.