INVESTIGATING LANDSCAPE RESPONSE TO HUMAN ARRIVAL TO AUSTRALIA AND ANTHROPOGENIC FIRE USING COSMOGENIC ISOTOPES AND LOW-TEMPERATURE THERMOCHRONOMETERS

Advances in our knowledge of cosmogenic and low-temperature thermochronometric techniques allow for the investigation of landscape response to human land use over 10³-10⁴ yr timescales. Landscape response to ancient human land use is of particular interest in Australia because the arrival of humans and human-set fires at ~60 ka has been implicated in continent-wide ecological change. We present initial findings and interpretations of cosmogenic analyses of quartz extracted from a >100 kyr sediment core (K6E) in northern Australia and observations of zircon grains, acid-etched to reveal fission tracks, extracted from the top of a nearby core. Ten cosmogenic ²⁶Al/¹⁰Be ratios were corrected for radioactive decay and post-burial production of cosmogenic nuclides using a preliminary luminescence age-depth relationship. Only the core-top sample clearly post-dates human arrival; the other nine samples are ≥60 ka. All corrected ²⁶Al/¹⁰Be ratios are less than ratios expected from steady state erosion, indicating that sediment was deposited with an inherited burial signal. Our initial interpretation of the ²⁶Al/¹⁰Be ratios is that the core-top sample carries ~600 kyr of total prior burial; all other samples, within uncertainty, carry an average of ~350 kyr of total prior burial. We infer that sediment sources remained unchanged from ~100-60 ka after which sediment appears to originate from a different source. Whether data from the uppermost sample reflects a change in landscape dynamics following the arrival of humans and human-set fires or landscape response to modern land-use practices is unclear. We consider the possibility that ²⁶Al/¹⁰Be data is characteristic of aeolian sediment because ²⁶Al/¹⁰Be ratios from Australian dune sand are similar to those in core K6E, though analyses and interpretations for core K6E are ongoing. Additionally, ~25% of etched zircon grains from the top of a nearby core exhibit no fission tracks, which is perplexing given the tectonic inactivity in northern Australia. Zircon analyses of the K6E core are ongoing but will supplement cosmogenic data. We hypothesize that track-less zircons have been annealed by wildfires prior to late Pleistocene burial. If so, northern Australian sediment cores may constrain new archives of wildfire history and its temporal relationship to human arrival.