Chemical profiles in K/T boundary section of Meghalaya, India: cometary, asteroidal or volcanic

Abstract

The Cretaceous-Tertiary boundary (KTB) exposed as a 1.5-cm-thick limonitic layer in the Um Sohryngkew River basin in Meghalaya is marked by enhanced concentrations of Ir, Co, Ni, Os, Fe, Zn, Sb (by factors of 4 to ~1200) and REE (by factors of 1.7 to ~5) compared to the Cretaceous shales. Ir concentration is generally <0.01 ng g^-1 in shales away from the boundary but gradually increases by a factor of >10 in ~2-m-thick "broad band" around the KTB, abruptly reaching a peak concentration of 12.1 ng g^-1 within the limonitic layer. The Os/Ir ratio at the KTB is ~0.37 but in the adjacent shales it is ~1.8, suggesting that the sources of these elements in the peak and the broad band may be different. Cretaceous planktonic foraminifera and dynocysts start disappearing before the Ir-rich limonitic layer was deposited but several diminutive Cretaceous foraminifera survive after this layer. The observations indicate that the extinctions are confined to a short span of time but are probably not as abrupt as anticipated by the asteroidal impact hypothesis. The chemical data are discussed in terms of cometary, asteroidal and volcanic hypotheses. The profiles of Ir and Os/Ir can be understood in the frame work of a model of cometary impact in which cometary debris falls onto the Earth preceding and succeeding the fall of the cometary nucleus.