U-TH/HE AND FISSION-TRACK DETRITAL GRAIN DOUBLE DATING AS A PALEO-WILDFIRE INDICATOR: TRIALS AND TRIBULATIONS FROM TWO WESTERN INTERIOR BASINS AND IMPLICATIONS TO THE PETM

About 55 million years ago, the planet experienced the most abrupt global climate event of the Cenozoic Era: the Paleocene-Eocene Thermal Maximum (PETM). While the origin of the PETM remains contested, globally distributed isotope records indicate rapid warming and introduction of isotopically-depleted carbon into the ocean/atmosphere system. While multiple hypotheses to resolve the PETM carbon cycle perturbation have been suggested, limited geological evidence exists for any model proposed.

Modeled data and field studies demonstrate detrital apatite and zircon (U-Th)/He (He) and fission-track (FT) age anomalies (i.e. FT ages < He ages) can be used as indicators of fires in the modern. In the first paleo-application of it kind, we address the proposed hypothesis that the PETM was caused by burning of terrestrial organic matter by analyzing detrital grains from terrestrial sediments within the Powder River and Bighorn Basins. Limited zircon recovered from Powder River's Wyodak-Anderson coals are clearly volcanic in origin, roughly consistent with Absaroka volcanism and likely deposited as atmospheric fallout. Bighorn apatite grains recovered from a series of fluvial and paleosol sediments are more intriguing, in that FT ages are nearly all ~55 Ma while He ages record more scatter and generally yield ages 55 Ma or younger. Currently, explanations for this wide distribution in He ages are equivocal. Nonetheless, the 55 Ma apatite FT ages are robust, and from a regional perspective, unlikely to have come from the Bighorn or Owl Creek Mountains. This limits us to three possible explanations for the preservation of a strong 55 Ma signal in these detrital grains; either 1) from exhumation of the Beartooth Mountains, 2) reworking of or fallout from Absaroka volcanism or, 3) resetting by wildfire at or near time of deposition. Paleo-drainage patterns indicate the Beartooth Mountains are an unlikely source for these apatites. At this point, either Absaroka volcanism or resetting by fires remains equally plausible explanations. However, if the grains source from the Absarokas we predict that both the zircon and micas ages would be 55 Ma, whereas, if fire reset the apatite FT ages, the zircon and micas ages would be older than 55 Ma.