CRYPTIC AGE-MIXING WITHIN ENVIRONMENTAL DNA (EDNA) COMPLICATES ESTIMATES OF MEGAFAUNAL EXTINCTION

Environmental DNA (eDNA) collected from stratigraphic sections routinely identify megafauna DNA from sediments that are several thousand years younger than species’ last fossil occurrences. While such records have been used to propose the survival of mammoths and other species well into the Middle Holocene, a unique challenge for using eDNA records to estimate extinction timing is that organisms can contribute DNA to sediments long after their deaths. eDNA archives mix undatable DNA from both living and dead individuals, thus the durations that bones and other biological tissues persist on landscapes impart important controls on the scale of temporal mixing within eDNA samples. To characterize how the duration of bone persistence changes due to environmental settings, we aggregated data from the literature on the oldest radiocarbon-dated surface-collected bones from different ecosystems. We included bones that we are reasonably confident persisted without being completely buried (“never buried”), and bones for which exhumation at some point cannot be confidently excluded (“potentially never buried”). We supplemented these data with AMS radiocarbon dated bones from Arctic Alaska and temperate North America. Pairing bone persistence with mean annual temperatures from their sample localities, we find a strong link between local temperature and the logged duration of bone persistence (never buried bones, R² = 0.94, p < 0.01; potentially never buried bones, R² = 0.95, p < 0.01). Using published data on Arctic mammoths as a test case, we find that the gap between the most recent fossil occurrence and the most recent sediments containing mammoth DNA (~7 kyr) is within expectations of the predicted duration of bone persistence for Siberian temperatures. Particularly in cold, high-latitude systems, decay rates of bones and other tissues are extremely slow and remains on landscape surfaces can be ubiquitous. Bones in these settings are frozen for much of the year and even highly weathered specimens, including those exhumed from permafrost millennia after initial burial, can release viable DNA. Rather than millennial-scale extensions to species survival, DNA from sediments that significantly postdate last fossil occurrences likely identify temporal mixing in eDNA and highlight commingling of the dead with the living.