FOSSIL CHARCOAL IN CHINLE GROUP STRATA (UPPER TRIASSIC), NORTHERN ARIZONA: DISTRIBUTION AND SIGNIFICANCE

Previous studies have suggested that fusain (fossil charcoal) is scarce in sedimentary rocks of Late Triassic age. This apparent lack was cited as evidence for an infrequent occurrence of wildfire and of relatively low levels (compared to modern) of atmospheric oxygen during this time interval, thereby supporting earlier geochemical models based on mass balance and isotope fractionation calculations. We report here new discoveries in northern Arizona of abundant fusain in latest Carnian to earliest Norian-age alluvial strata of the Sonsela Member (or equivalent) of the Petrified Forest Formation (Chinle Group). The fusain, identified by standard techniques of macro-, micro- and ultramicroscopic morphology, and resistance to chemical oxidation, occurs in multiple stratigraphic horizons at two locations that are approximately correlative, although separated by several hundred km. The morphology of this fossil charcoal includes large, partially charred logs, completely charred smaller limbs, charcoal fragments reworked in fluvial deposits, and particulate fusain disseminated in sediments. Carbon isotope values for fusain samples from the new Arizona sites range from δ¹³C= -25 to -22.8 ppt (PDB).

In concert with prior reports from Carnian- to Norian-age strata of the Chinle and Fundy groups (Chama and Fundy basins, respectively), these findings demonstrate a significantly greater abundance of fusain in Upper Triassic strata than previously acknowledged. We further suggest that these findings indicate that wildfire was not an unusual occurrence during the Late Triassic. This conclusion supports more recent experimental studies and geochemical models that indicate atmospheric oxygen levels at or above modern levels during the Late Triassic. Robust levels of atmospheric oxygen during the Late Triassic are consistent with the widespread deposition of coals, and may be linked with the first appearance of mammals, and of pterosaurs and dinosaurs, whose evolutionary success likely resulted from their enhanced, oxygen intensive metabolisms.