USING RAMPED PYROLYSIS OXIDATION TO ASSESS BULK SEDIMENT RADIOCARBON GEOCHRONOLOGY AND CARBON CYCLING IN AN ARCTIC LAKE CATCHMENT

Ramped pyrolysis oxidation (RPO) can be used to evaluate bulk sediment radiocarbon dating as well as tracing organic carbon (OC) sources and their residence times in sedimentary basins. RPO pyrolyzes sediment OC over a slow thermal gradient, immediately oxidizing the volatile components into CO₂ for radiocarbon dating. Fractions of CO₂ are collected over multiple temperature intervals which, depending on the relative abundance of CO₂ at different temperatures and the ¹⁴C age spectrum, can constrain the source of the OC. The fraction of OC that pyrolyzes at the lowest temperatures is the most labile and can potentially stand in for the age of deposition where macrofossils are not present. Where datable plant macrofossils are rare and bulk radiocarbon dates may be indicative of admixture with pre-depositionally-aged OC, as found in many lakes in the Canadian Arctic, RPO ¹⁴C dating may improve lake sediment chronology and constrain the amounts of older, allochthonous OC that are stored in the surrounding landscape and washed into the lake. We performed RPO on Holocene sediments from Lake CF8, Baffin Island, Nunavut, Canada to dissect the chronology and sources of bulk OC. We pyrolyzed eight samples into five splits, each of equal CO₂ concentrations, for comparison to radiocarbon-dates from macrofossils at the same core depths. The macrofossil ages range from 950 ± 20 to 11,860 ± 40 cal yrs BP. In all eight samples, we find that the most labile 20% of bulk OC (pyrolyzed from 0 to >375 °C) ranges from 250 to 2,270 years older than the corresponding macrofossil age, with the smallest age offsets occurring in the warm Early Holocene. This result indicates a change in either the age or the amount of allochthonous carbon stored on the landscape and washed into the lake. In the three samples where we have dated all five CO₂ splits (macrofossil ages 8,890 ± 80, 10,210 ± 40, and 11,630 ± 60 cal yr BP), the full age spectrum ranges from 475 to 895 years. The radiocarbon age becomes progressively older for the higher temperature splits, except for the oldest sample, which was deposited shortly following deglaciation. The narrowness of the age spectra for Lake CF8 sediments could imply that there is limited allochthonous input to the system due to the limits on carbon cycling in seasonally frozen soils and sediments.