Black carbon in Paleocene–Eocene boundary sediments: A test of biomass combustion as the PETM trigger

Several explanations have been proposed to account for the dramatic perturbation to Earth's carbon cycle that led to extreme warming, ocean acidification, mass extinction, and a major negative carbon isotope excursion (CIE) at the Paleocene–Eocene (P/E) boundary. One reason that the ultimate cause for this event remains ambiguous is the difficulty in directly testing these multiple hypotheses. We conducted a test of one proposed mechanism, the “wildfire hypothesis,” that suggests that combustion of a significant fraction of abundant Paleocene peat deposits within several thousand years could release enough CO2 to Earth's atmosphere to account for all of these observations. Soot and graphitic black carbon (hereafter collectively referred to as GBC), both combustion byproducts, are widely distributed (through smoke) by subaerial combustion, and are remarkably resistant to diagenetic degradation. Using samples provided by the Ocean Drilling Program, we measured GBC concentrations across the P/E boundary in two important Paleocene–Eocene Thermal Maximum (PETM) sections to test a prediction of the wildfire hypothesis, namely that the combustion of Paleocene organic matter at the onset of the PETM would produce a peak in sedimentary GBC. In the Bass River section (New Jersey margin), GBC concentrations are uniformly high (121–358 ppm), consistent with the core's continental margin setting, but there is no evidence of increased GBC flux at the PETM. Furthermore, carbon isotopic ratios of GBC display the negative CIE seen in PETM organic matter, indicating that this material was sourced from contemporaneous (P/E boundary) biomass, not geologically old carbon. At site 1210 (Shatsky Rise), GBC concentrations were uniformly below our method's detection limit (0.5 ppm), whereas the wildfire hypothesis would predict high GBC concentrations (∼ 3000) ppm accompanying the event. Together, these data appear to refute but cannot conclusively rule out biomass burning as the cause for the PETM event.