Geochemical and sedimentary signatures of Phanerozoic events

Detta är en avhandling från Department of Geology, Lund University

Sammanfattning: Geological and biological catastrophic events have occurred repeatedly in the Earth's history, leaving traces in the global stratigraphical record in the form of sedimentary features, geochemical anomalies and biotic turnovers. This thesis focuses on the sedimentological and geochemical signatures of several key events in the Phanerozoic stratigraphical record, and aims to interpret the different signals and the causal mechanisms behind each of these events. Three intervals are investigated; the Upper Silurian (late Ludfordian), Jurassic-Cretaceous and the Cretaceous-Paleogene (K-T) boundaries. Sedimentological investigations coupled with analyses of major and trace element concentrations and of carbon and oxygen isotopic compositions were undertaken on drillcore and outcrop samples of Late Silurian age from central Scania, Sweden. Sedimentological and trace element analyses were performed on samples from the Jurassic-Cretaceous transition from Scania, Sweden and on ejecta sediments from the K-T transition along the Mexico-Belize border, close to the Chicxulub impact site. The common characteristic of these successions is that they represent special depositional environments related to exceptional short-lived events. The geochemical event in the Upper Silurian sedimentary rocks is marked by carbon and oxygen isotope anomalies in marine carbonates. The ?13C values increase from ca +1 ? to +10 ?, which represent the heaviest values recorded in this interval from Baltica. The ?18O values rise from ca -10 ? to -5 ? and show a somewhat unstable pattern. None of the conventional mechanisms for 13C enrichment can independently cause a ?13C shift in the oceanic dissolved inorganic carbon (DIC) that would answer to the observed magnitude of change. Thus, multiple causes are suggested herein. The isotopic excursions coincide with a mass occurrence of cyanobacteria, and the most extreme ?13C values may be explained by 13C enrichment in DIC through regional increases in photosynthetic activity, probably superimposed on an already 13C enriched (ca +3 ? to +5 ?) ocean. This initial anomaly still lacks a reasonable explanation. In the Scanian deposits an evaporitic environment is confirmed by the presence of e.g. cerebroid ooids, which form in highly saline waters and it is suggested that evaporation is responsible for the 18O enrichments. Degassing of CO2 may also explain the 13C enrichment in deposits that are not closely associated with cyanobacteria. It is, however, arguable whether evaporation, although prolonged and widespread, could modify the carbon isotope composition in deep marine settings. Still, it is possible that the pCO2atmos and cyanobacteria-stromatolites may be linked in some way that in turn links to the ?13C anomaly. However, the causes of the event remain enigmatic and further analyses of ?13C and ?18O from various environments (e.g. paleosols and lake sediments) and from different parts of the world are required. Sedimentary beds reflecting an atypical event were identified in the Jurassic-Cretaceous transition (within the Vitabäck Clays) at Eriksdal, Sweden. Samples were selected from an excavated ditch for geochemical, mineralogical and palynological analyses. Additional sedimentological studies where performed in field. Well-preserved assemblages of miospores and for the first time, dinoflagellates were identified in the Vitabäck Clays. The palynological assemblage corroborates an Early Cretaceous (Berriasian) age. The Vitabäck Clays consist predominantly of greenish to dark grey clays, but a coarse-grained unit is enclosed within the homogenous fine-grained sediments and these beds are also recognized by anomalous sedimentary structures and fossil content. The relatively short succession (ca 1.5 m) documents a change in energy of the transport media by the sharp erosive base, coarse-grained lithology, the incorporation of plant fragments and a fauna of mixed terrestrial and marine origin. This anomalous succession is herein interpreted to represent a tsunami deposit, possibly generated by tectonic activity or by an asteroid impact, the Mjølnir impact in the Barents Sea being a possible candidate. The Cretaceous-Paleogene boundary event relates to the Chicxulub impact in Mexico. The Upper Cretaceous Barton Creek Dolomite and the overlying two-layered ejecta deposit, collectively spanning the K-T boundary, were sampled at several sites along the Mexico-Belize border. The employed sedimentological and geochemical investigations document the chaotic circumstances following the impact. The geochemical analyses provided a genetic link between the proximal ejecta, the target rocks and K-T boundary deposits at other sites. There has been considerable debate as to whether this particular impact in fact took place at the K-T boundary. The results of this study envisage that the Chicxulub impact produced the global K-T boundary layer.

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