Geochemical markers of the Cretaceous-Tertiary boundary event at Brazos River, Texas, USA

The Cretaceous-Tertiary boundary sites around the Gulf of Mexico are close to the Chixculub impact site and are relatively well studied, yet much remains to be learned about them. Therefore, the first integrated study of carbon, soot, and fullerenes in a Cretaceous-Tertiary boundary section was undertaken at the Brazos-1 site on the Brazos River in Texas at the most complete section of end Cretaceous and basal Paleocene deposits on the Texas segment of the Gulf Coast area. Up to 409 ppm of native sulfur (S degrees) were serendipitously discovered in a spherule-bearing unit of the BR-1 section, and lesser amounts were found in spherule-bearing units of nearby Brazos riverbed sections in a section on Darting Minnow Creek. The isotopic composition, delta(33)S = -12.97 parts per thousand, delta(34)S = -24.89 parts per thousand, and delta(36)S = -46.4 parts per thousand, implies that this S degrees cannot have come to Earth by the impactor that formed the Chicxulub crater, but, most likely, was produced by sulfate-reducing bacteria during a local, transient bacterial bloom for which the sulfate was provided by CaSO4-bearing spherules. Carbon and soot were determined in twelve samples representing all units of BR-1 from the Cretaceous Corsicana/Kemp Formation to the Tertiary Kincaid Formation. A significant increase of C and soot contents, up to 2.2 x 10(4) ppm and 1.4 x 10(4) ppm, respectively, occurs in a sandy bed at the top of the KT complex. Fullerenes were determined in fifty-four samples from all units of the same BR-1 section. Less than 1 ppb was reliably detected at the same sandy bed where the strongest Ir anomaly of the section is known to occur. It is suggested here that the Chicxulub impact 65 Ma ago ignited local wildfires that produced C, soot and fullerene, which settled onshore, or near-shore, whence they were transported to the Brazos site by coastal flooding and associated sediment-laden water plumes moving offshore. Copyright (C) 1998 Elsevier Science Ltd.