Biosynthetic isotope fractionation negligibly impacts biomarker 14C ages

Abstract

Radiocarbon (14C) 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 14C ages accurately reflect the time that has passed since biosynthesis and are not complicated by issues such as isotope fractionation. Reported 14C ages are thus always corrected for mass-dependent fractionation using a 14C/13C mass law, b, of 2.0. However, anomalous deviations from mass dependence could theoretically lead to biases in measured 14C ages, particularly for compounds such as acetogenic lipids that undergo large 13C 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 ln(13α)→0. Combined, these results imply maximum 14C age offsets due to biosynthetic fractionation of ~20 to 40 yr. Biomarker 14C ages are therefore robust to biosynthetic isotope fractionation and can be confidently interpreted to reflect carbon-cycle turnover times.