Stable isotope, TOC and CaCO 3 record across the cretaceous/tertiary boundary at El Kef, Tunisia

Abstract

We report the results of stable oxygen and carbon isotope analysis of carbonate fine fraction and benthic and planktic foraminifers, %CaCO 3 and TOC determinations of sediments across the Cretaceous/Tertiary (K/T) boundary at El Kef, Tunisia. We used grain-size analysis of the fine fraction carbonate to determine possible compositional effects and smearslide and scanning electron microscope examinations to evaluate possible diagenetic alteration and microfossil preservation. The data suggests the following sequence of events. Upper Maastrichtian oxygen and carbon isotope signals indicate relatively warm temperatures, a decreasing thermal gradient possibly related to a shallowing sea and relatively high surface water productivity. A sharp cooling appears to begin just below the K/T boundary. The K/T boundary is characterized by a sudden 2‰ negative shift in fine fraction δ 13C, a slight 0.6‰ enrichment in benthic δ 13C and a reduction of about 40% in CaCO 3 sedimentation. These data imply strongly reduced surface water productivity as also observed in numerous deep-sea sequences worldwide. The lower Danian (planktic foraminiferal Zones P0,P1a) is marked by generally low productivity and unstable environmental conditions as indicated by generally low but fluctuating fine fraction δ 13C values and low (5–15%) carbonate deposition. At the base of Subzone P1b, %CaCO 3, benthic and fine fraction δ 13C values increase gradually, and reach near pre-K/T boundary levels in Subzone P1c indicating initial recovery after the K/T boundary event about 300,000–400,000 years after the K/T boundary. The prolonged low productivity episode after the K/T crisis and the pre-K/T boundary cooling associated with a major reduction in planktic foraminiferal diversity are difficult to explain by a single K/T boundary bolide impact.