FIRES AT THE K/T BOUNDARY: CARBON AT THE SUMBAR, TURKMENIA, SITE

Abstract

We have measured carbon at the marine K/T boundary site SM-4 at the Sumbar river in Turkmenia, USSR, which has an undisturbed Ir profile and the largest known Ir anomaly (580 ng/cm2). Twenty samples, ranging from −100 to +100 cm, were analyzed to determine the concentration and δ13C of elemental carbon and kerogen, using a Cr2O7 oxidation method (Wolbach and Anders, 1989) to resolve these components. The samples were unusually complex, containing 3 kerogen components of distinctive IR spectra and half-lives in Cr2O7 (~200, ~100, and ⪡60 h), in addition to soot and charcoal from the K/T fire. The elemental C has δ13C = −25.96 ± 0.6 l%o, close to the mean for 11 K/T sites (-25.8 ± 0.6%; 11 mg/cm2). The first of the 2 kerogen components (δ13C = −22.8%), which dominates in the Cretaceous, appears to come from detrital carbonaceous shale; the second (δ13C = −27.7%o) occurs only in the boundary clay and may come from land plant material swept to sea, or phytoplankton grown in the presence of excess, light CO2 from fires on land.The great thickness of the boundary clay (10.5 cm) permits determination of the compositional profiles with unprecedented spatial resolution. Ir and shocked quartz—both representing impact ejecta—rise sharply at the boundary, peak in the basal layer, and then decline. Soot and total elemental C show a similar spike in the basal layer but then rise rather than fall, peaking at 7 cm. Apparently fires started before the basal layer had settled, implying that ignition and spreading of major fires became possible if not immediately then very soon after the impact.There is no basis for claims by Hansen et al. (1987) that K/T carbon black first occurs 3.5 m below the Danish K/T boundary and has δ13C of −27.4 to −27.8%o. Reexamination of a sample from Nye Kløv shows that these authors confused kerogen with carbon black.