Environmental change across the Cretaceous-Paleogene boundary: An integrated, multi-proxy approach

The Cretaceous-Paleogene (K-Pg) boundary mass extinction event is inextricably linked to the direct consequences of the Chicxulub impact 66 million years ago. Yet, it has been argued that other environmental factors, such as climate change caused by pre-impact Deccan Traps mantle degassing, and sea-level change across the K-Pg boundary, could have contributed to this mass extinction event. This dissertation aims to clarify to what extent temperature shifts did occur during the late Maastrichtian, and if these temperature shifts are coeval and observed globally. Inorganic and organic geochemical proxies Mg/Ca, d18O and TEX86 are used to reconstruct past temperatures during this interval. Subsequently, ecological change during the late Maastrichtian interval is studied using benthic foraminiferal and dinoflagellate cyst (dinocyst) assemblage data, to demonstrate the possible link between late Maastrichtian climate change and ecology. Furthermore, sea-level changes before, during and after the K-Pg boundary are reconstructed using benthic foraminiferal, dinoflagellate cyst (dinocyst) assemblage and sedimentological data to substantiate if sea-level change potentially could have contributed to the mass  extinction event. Finally, to gain more understanding in the post-impact paleoecological and paleoceanographic consequences of the early Danian ‘Living Ocean’ condition,  benthic foraminiferal and dinoflagellate cyst (dinocyst) assemblage data from several locations are studied and compared.

The magnitude of the late Maastrichtian warming that coincided with the second phase of Deccan Traps outgassing, as well as the timing of the warming, are further constrained in this dissertation. Firstly, a robust method for integrating temperature records is proposed. Using this method, data from two proximal sites in New Jersey and Argentina suggests that the late Maastrichtian warming had a magnitude of 3.9±1.1 °C. Furthermore, this dissertation has substantiated that late Maastrichtian warming affected environment and ecology. Dinocyst assemblages of the New Jersey paleoshelf indicate severe algal blooms during the warming interval. These algal blooms have in turn led to a shift in benthic foraminiferal assemblages, indicating changes in food supply towards the seafloor and benthic foraminiferal ecosystem stress. In Argentina, benthic foraminiferal and dinocyst assemblages suggest an increased freshwater flux into a semi-restricted basin, possibly as a result of an expanded South American thermal low during the warming interval.  The link between climate change and the K-Pg boundary mass extinction event remains however unresolved. 

Benthic foraminiferal and dinocyst assemblages have proven to be less suited for reconstructing sea-level changes when the assemblages are severely disrupted, as was the case directly following the K-Pg boundary mass extinction. Yet, a maximum flooding  ~100 kyr before the K-Pg boundary, followed by a regression and a sea-level lowstand between the K-Pg boundary and Zone P1c is suggested by benthic foraminiferal, dinocyst and sedimentological data.

Directly following the K-Pg boundary, the benthic foraminiferal assemblages experience a ‘Disaster phase’. This phase is initially characterized by a bloom of endobenthic taxa as result of the post-impact flux of organic matter to the seafloor. After this short-lived initial phase, the second part of the ‘Disaster phase’ is characterized by reduced primary productivity as a result of the collapse of the biological pump. After the ‘Disaster phase’, increasing benthic foraminiferal diversity and species richness indicate a long-term recovery phase, although these do not recover to pre-impact values within the studied intervals (> 300 to 700 kyr) after the K-Pg boundary. This suggests a permanent shift in food supply towards the seafloor after the K-Pg boundary. Furthermore, the widespread occurrence of species such as Alabamina midwayensis and Anomalinoides acutus suggests that after the K-Pg boundary a more cosmopolitan benthic foraminiferal fauna arose from the surviving taxa of the more regionally disparate late Maastrichtian faunas.

Finally, compared to other marginal sites, the sites studied in this dissertation show similar benthic foraminiferal turnover rates across the K-Pg boundary. Yet, turnover rates between individual marginal records vary considerable. This difference is probably related to latitude, geographic setting and water depth, as sites with comparable settings show comparable turnover rates.