Cretaceous-Tertiary boundary event: Evidence for a short time scale

Abstract

Three non-meteoritic trace elements (Sb, As and Zn) are strongly enriched at eleven K-T boundary sites, along with mainly or partly meteoritic elements (Ir, Ni, Cr, Fe and Co). The proportions (As, Sb, Zn/Ir) are remarkably constant over a ≈ 100-fold range in concentration. This correlation persists in sub-layers of boundary clay and even extends to soot (from burned land biomass). Apparently, all the components, despite their diverse origins, became associated in a single, global component prior to deposition. No wholly satisfactory source is available for As, Sb and Zn: the trace element pattern in volcanic gases does not match that in KT boundary clay, with ratios to Ir falling short by 1 to 2 orders of magnitude, terrestrial rocks do not reach high enough concentrations and (modern) ocean water contains too little Zn, but on balance, the latter source seems preferable—perhaps augmented by volatiles from the impact crater. Apparently, Ir-bearing ejecta and soot from forest fires coagulated in the stratosphere and then fell out together, sweeping out oceanic biomass and anoxically precipiting As, Sb and Zn. Significantly, the amounts of marine and land biomass at the KT boundary correspond to about the steady-state global inventory (1 generation), and the amounts of As, Sb and Zn are also within a factor of ≈5 of the global inventory. This is expected in a catastrophic but not a gradualist scenario.