Inbuilt age, residence time, and inherited age from radiocarbon dates of modern fires and late Holocene deposits, Western Transverse Ranges, California

Radiocarbon dates of sedimentary deposits include the elapsed time between formation of the organic material and deposition at the sample site, known as the inherited age. Long inherited ages reduce the accuracy of estimates of the timing of depositional events used to infer paleoclimate change, fire histories, and paleoearthquake timing. An inherited age distribution combines the inbuilt age distribution, which reflects the age composition of the vegetation of the source area, and the residence time distribution, which includes transport and interim storage prior to final deposition. Differentiating residence time and inbuilt age is difficult given typical dispersion of ages in a sedimentary deposit. We address this problem by comparing charcoal dates from two modern fires in southern California, the 2020 Bobcat and the 2013 Grand Fire, with a well-dated late Holocene deposit in the Pallett Creek watershed. The modern fire deposits have negligible transport time (<1 year), and 56 radiocarbon dates indicate a median age of 25 years (300-year 95% range) provides an estimate of inbuilt age for the San Gabriel Mountains. The inherited age calculated from the paleodeposits is older with a median age of ~90 years and has a positive skew (850-year 95% range). A modeled inherited age, calculated by applying the pre-bomb radiocarbon calibration curve to the modern fire age distribution, is shorter than the paleodeposit inherited age by only 21 years, indicating samples with long residence times are not common in the deposit. Comparison of inherited ages calculated from organic-rich and clastic paleodeposits indicate a slight facies dependence that may reflect longer residence time in clastic deposits. The results provide insight into the transport of charcoal through the landscape are useful for refining estimates of past environmental and tectonic events.