Fire across the K–T boundary: initial results from the Sugarite Coal, New Mexico, USA

Abstract

It has been hypothesized that major global fires occurred following a bolide impact at the Cretaceous–Tertiary boundary. Evidence for this has been mainly from ‘soot’ or black carbon at a number of marine boundary sites as well as the occurrence of inertinites (fusinite) in coals above the boundary. In addition, the occurrence of fossil charcoal (inertinites) in potential Tsunami deposits has been used to strengthen this idea. However, fire is known to have been widespread throughout the Cretaceous based on the distribution of fossil charcoal, and it is necessary to evaluate the claims for a global fire at the K–T boundary within the context of this more extensive record of ancient fires. The occurrence of the K–T boundary within the Sugarite coal sequence in the Raton Basin, New Mexico, offers the opportunity to assess the incidence of fire in local peat-forming vegetation, in the latest Cretaceous, across the Cretaceous–Tertiary boundary interval and in the earliest Tertiary. The distribution of fire products, i.e. fossil charcoal, is assessed using a combination of inertinite group macerals in polished blocks of coal and plant particles released by chemical maceration of coal. Inertinite group macerals (fusinite, semifusinite, inertodetrinite), which represent fossil charcoal, and particles of charcoal in maceration residues are abundant throughout the Sugarite coal sequence, both before, during and after the K–T boundary event. Samples from below the boundary yield inertinites that constitute more than 20% of the coal; three horizons are inertinite rich (>50%). Fire was obviously an important element of the terrestrial environment during the latest Cretaceous. High inertinite values and abundant charcoal particles characterize the latest Cretaceous part of the coal, the carbonaceous shale just below the boundary, the boundary interval itself and Tertiary coals and carbonaceous shales above the boundary. Charcoal in mineral-rich units of the latest Cretaceous, immediately below the boundary, in the boundary interval and in the Tertiary, is dominated by small inertodetrinite particles. These might have been wind blown from regional fires or they may reflect reworking during erosion, which led to sediment input into the mire. In either case the charcoal signature in these units is the same, irrespective of their position in the sequence. Our data, combined with that from two other sites in terrestrial sequences in North America, show that fires were an integral part of mire ecosystems through the latest Cretaceous and into the early Tertiary. Therefore, evidence for distinctive global wildfires at the Cretaceous–Tertiary boundary would have to be sought from careful consideration of other aspects of charcoal deposition, including flux rates for charcoal and soot.