Carbonate clumped isotopes (∆47) have become a widely applied method for paleothermometry, with applications spanning many environmental settings over hundreds of millions of years. However, ∆47-based paleothermometry can be complicated by closure temperature-like behavior whereby C–O bonds are reset at elevated diagenetic or metamorphic temperatures, sometimes without obvious mineral alteration. Laboratory studies have constrained this phenomenon by heating well-characterized materials at various temperatures, observing temporal ∆47 evolution, and fitting results to kinetic models with prescribed C–O bond reordering mechanisms. While informative, these models are inflexible regarding the nature of isotope exchange, leading to potential uncertainties when extrapolated to geologic timescales. Here, we instead propose that observed reordering rates arise naturally from random-walk 18O diffusion through the carbonate lattice, and we develop a “disordered” kinetic framework that treats C–O bond reordering as a continuum of first-order processes occurring in parallel at different rates. We show theoretically that all previous models are specific cases of disordered kinetics; thus, our approach reconciles the transient defect/equilibrium defect and paired reaction-diffusion models. We estimate the rate coefficient distributions from published heating experiment data by finding a regularized inverse solution that best fits each ∆47 timeseries without assuming a particular functional form a priori. Resulting distributions are well-approximated as lognormal for all experiments on calcite or dolomite; aragonite experiments require more complex distributions that are consistent with a change in oxygen bonding environment during the transition to calcite. Presuming lognormal rate coefficient distributions and Arrhenius-like temperature dependence yields an underlying activation energy, E, distribution that is Gaussian with a mean value of μE = 224.3 ± 27.6 kJ mol−1 and a standard deviation of σE = 17.4 ± 0.7 kJ mol−1 (±1σ uncertainty; n = 24) for calcite and μE = 230.3 ± 47.7 kJ mol−1 and σE = 14.8 ± 2.2 kJ mol−1 (n = 4) for dolomite. These model results are adaptable to other minerals and may provide a basis for future experiments whereby the nature of carbonate C–O bonds is altered (e.g., by inducing mechanical strain or cation substitution). Finally, we apply our results to geologically relevant heating/cooling histories and suggest that previous models underestimate low-temperature alteration but overestimate ∆47 blocking temperatures.

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.