Ecological and environmental consequences of Oceanic Anoxic Events and the Cretaceous-Paleogene mass extinction event: a molecular-isotopic approach

Abstract

The main aim of this thesis was to study the biotic and abiotic consequences of extreme environmental conditions during Cretaceous Oceanic Anoxic Events (OAE), and the recovery of primary production at the Cretaceous-Paleogene (K-Pg) mass extinction event through the use of lipid biomarkers, compound-specific stable isotopes, and bulk geochemistry. This multiproxy approach contributes new evidence for understanding the intricate environmental and biological interactions occurring during these important events in Earth's history, which were responsible for rapid biological turnover that greatly impacted biogeochemical cycles.Two OAEs covering the Late Cretaceous were studied - the Coniacian-Turonian Boundary OAE2 (~ 93.5 million years ago, Ma) in an intra-shelf basin of the Levant Platform in Central Jordan, and the Coniacian OAE3 (~ 88-89 Ma) at the western equatorial Atlantic off the coast of Surinam (ODP Site 1259, Demerara Rise). OAE2 at Jordan was characterized by sea-level changes that resulted in water column stratification with a fluctuating chemocline, hypersalinity, and oxygen depletion, whereas evidence of photic zone euxinia (PZE) was only found after the termination of this event. Abundant steranes and hopanoids, including 2-methyl hopanes (2-MeH), and 13C enriched aryl isoprenoids suggest that the observed environmental changes were accompanied by ecological successions of planktonic assemblages dominated by algae, including dinoflagellates, cyanobacteria and green-sulfur bacteria. The synchronous occurrence of 2-MeH and d15N values around 0' provides evidence for the importance of cyanobacteria and N2-fixation fueling primary production at this stratified/anoxic continental platform. OAE3 at Demerara Rise was characterized by apparently cyclic, concomitant variations of stable isotopic composition of carbon and hydrogen (d13C and dD, respectively) of marine- and terrestrial-derived n-alkanes. This pattern suggested a tight coupling between marine and terrestrial systems. Intervals of enhanced marine productivity were evidenced by positive d13C excursions of the algal marker n-C17, likely related to increased growth rates and primary productivity. Parallel 13C enrichments in C29 and C31 n-alkanes of higher land plant waxes suggest simultaneous changes of terrestrial ecosystems. Lowered concentrations of atmospheric CO2 resulting in increased importance of C4 plants was one possible scenario explaining the observed molecular-isotopic patterns. These intervals were also accompanied by dD enrichments in n-C17 suggestive of changes in dDwater, likely due to variations in the evaporation/precipitation balance and continental runoff. Overall, these results revealed a complex interplay of the climatic and oceanographic regime and a potential coupling of marine and terrestrial environmental changes.In addition, the mass extinction event at the K-Pg (~ 65.5 Ma) was studied in an exceptionally expanded section of the renowned "Fish Clay" layer at Stevns Klint, Denmark. At this location, decreased photosynthesis resulting from the low solar transmission after the meteorite impact may have lasted less than 50 years. A highly diminished contribution of algal biomarkers and increased heterotrophic bacterial activity was characteristic of the 2-mm-thick organic-rich layer deposited immediately after the boundary. This period preceded the onset of recovery in algal production. This result strongly supported models suggesting a rapid resurgence of carbon fixation and ecological reorganization after this major impact event, and provided a more comprehensive view of the biotic recovery compared to more traditional microfossil studies.