Microscale trace-element distribution across the Cretaceous/Palaeogene ejecta layer at the Agost section: Constraining the recovery of pre-impact conditions

Abstract

A high-resolution, sub-mm scale analysis of the distribution of major and trace elements across the ejecta layer marking the Cretaceous/Palaeogene boundary (KPgB) at the Agost section (SE Spain) was performed using Laser Ablation-Inductivity Coupled Plasma-Mass Spectrometry (LA-ICP-MS). A KPgB interval ≈17 mm thick, has been selected for this study. It includes gray calcareous marl from the uppermost Maastrichtian (Cretaceous), 2.02-mm-thick red clay (the ejecta layer), and blackish-gray clay (boundary clay layer) from the lowermost Danian (Palaeogene). The unconsolidated sediments were resin-embedded under O2-free conditions and analyzed by LA-ICP-MS line continuous scan measurements at 20 μm increments and a laser-beam of 120 μm. These micron-scale analyses show that the anomalous contents of trace and major elements in this boundary are restricted to the ejecta layer, which displays a relatively uniform distribution over its ≈2 mm thickness. Trace and major elemental ratios, such as Ca/Al, Ti/Al, Cr/Al, Fe/Al, Ni/Al, Cu/Al, Zn/Al, As/Al, Sb/Al and Pb/Al have similar values below and above the ejecta layer, consistent with similar palaeoenvironmental conditions prior to and after the impact event. Such element distribution points to anomalies exclusively restricted to the ejecta, thus a nearly instantaneous re-establishment of pre-impact conditions right after the impact event is observed.