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Biosynthetic isotope fractionation negligibly impacts biomarker 14C ages
  • Jordon Hemingway
Jordon Hemingway
Harvard University

Corresponding Author:jordonhemingway@gmail.com

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Radiocarbon (14C) ages of acetogenic lipid biomarkers such as n-alkanes are a powerful tool to track carbon-cycle turnover times. In sediments, biomarker ages are almost always older than the depositional age due to reservoir effects. Recently, Lane et al. [2021, Anomalously low radiocarbon content of modern n-alkanes, Organic Geochemistry 152, 104170] reported 14C ages up to ≈1500 yr for n-alkanes extracted from leaf tissue of living plants; they attributed this apparent “pre-aging” to biosynthetic fractionation against 14C. However, reported 14C ages are always corrected for mass-dependent fractionation using a 14C/13C mass law, b, of 2.0. Lane et al.’s interpretation therefore requires that lipid biosynthesis follows large, anomalous deviations from mass-dependent fractionation, with b reaching values as high as ≈ 124. Here, I test this assumption 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 1.880 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.
Aug 2021Published in Organic Geochemistry volume 158 on pages 104267. 10.1016/j.orggeochem.2021.104267