Environmental changes across the Cretaceous/Palaeogene boundary: a high resolution approach for reconstructiong eco-sedimentary conditions

The mass extinction marking the Cretaceous/Palaeogene boundary (KPgB), ≈66.04 ± 0.02 million years ago, was one of the most devastating events in the history of life, being the most recent and the best studied of the ‘big five’ mass extinctions that occurred during the Phanerozoic eon. At present, the relationship between the Cretaceous/Palaeogene mass extinction and a meteorite impact is broadly accepted. The impact crater, discovered at the begining of the nineties on the Yucatan peninsula in Mexico (called Chicxulub structure), as well as the size of the meteorite (with a diameter of ≈10 km), are essential data to interpret the environmental consequences of this event. An ejecta layer was deposited worldwide, which at distal sites is characterized by the presence of spherules, anomalous concentrations of Iridium and other platinum group elements, the existence of spinels rich in Ni, and changes in the isotopic composition of certain elements, in particular C and O. After decades of research, one persistent challenge is to understand the effects of this impact on the global environmental system, including the re-establishement of pre-impact environmental conditions and marine biological productivity after such a major environmental crisis. This PhD Thesis focuses on characterizing palaeoenvironmental conditions (oxygenation and productivity), including the response of the macrobenthic tracemaker community, prior to, during and after the KPgB impact. With this aim, several KPgB distal sections located over 5,000 km from Chicxulub impact site were studied: the Agost and the Caravaca sections, from the Betic Cordillera (Southeast Spain), and the classical Gubbio sections (Bottaccione Gorge and Contessa Highway) located within the Umbria-Marche Basin, in the Apennines (Northeast Italy). A high-resolution sampling was carried out across the KPgB in the studied sections, along with a detailed sampling of trace fossils, differentiating between biogenic structures and host sediment. Geochemical and isotopic analyses were also performed both on the infilled material of the trace fossils and on the host sediment. The obtained results support that anoxic conditions are restricted to the deposit of the ejecta layer, and that oxygen levels similar to those of pre-impact times were re-established shortly after the impact. The rapid re-establishement of the pre-impact conditions (i.e., oxygenation and nutrient availability) is further supported by the tracemaker community recovery, which was favoured by the opportunistic behaviour of some macrobenthic organisms.