SINGLE-GRAIN OSL AND RADIOCARBON CHRONOLOGIES OF PERENNIALLY-FROZEN SEDIMENTS: IMPLICATIONS FOR SEDIMENTARY ANCIENT DNA STUDIES

‘Sedimentary’ ancient DNA (sedaDNA) techniques provide the opportunity to search for genetic traces of extinct fauna in contexts in which in situ macrofossils are exceedingly rare or absent. Use of sedaDNA techniques for this purpose, however, is reliant on the provision of reliable numerical age control directly on the bulk sediments from which DNA is extracted for analysis. An implicit assumption of the sedaDNA approach is that the extracted DNA is autochthonous with the host deposit and that it has not been physically transported from older source deposits or reworked within the sedimentary profile by post-depositional mixing. We investigate whether these conditions are upheld for a range of perennially-frozen wetland sites across north-central Siberia, and a loess river bluff sequence at the Stevens Village site, interior Alaska. Single-grain OSL and ¹⁴C dating are combined to constrain the ages of both the inorganic and organic fractions of perennially-frozen deposits from which sedaDNA of extinct and extant species have been recovered. OSL and ¹⁴C age/depth profiles, as well as single-grain equivalent dose (D_e) distribution characteristics, are used to assess the stratigraphic integrity of these sedaDNA sequences by (i) identifying the presence of primary or reworked organic and inorganic material, and (ii) examining the types of depositional and post-depositional processes that have affected specific sedimentary facies.

The combined OSL and ¹⁴C chronologies reveal that careful site selection is paramount to ensuring the accuracy of any sedaDNA study in perennially-frozen settings. High-energy fluvial contexts should be approached with caution, since periodic erosion of older deposits from upstream can release ‘old’ genetic material that is not readily degraded during transportation. Low-energy ponding environments, such as small thermokarst lakes, may offer more favourable settings for contemporaneous deposition of sedaDNA and dateable materials. However, even these types of settings must be treated with caution when the sedimentary infill is directly sourced from, or underlain by, much older, primary clastics and organic deposits that contain ancient DNA. Wind-blown deposits hold promise if the pre-existing DNA can be shown to have been erased during the last episode of sediment transport.