DEPOSITIONAL AND PALEOENVIRONMENTAL SETTINGS OF CRETACEOUS LIMESTONES IN THE GREATER ANTILLES

Abstract

The first part of this research examines uncertainties surrounding the middle Cretaceous Antillean rudist biostratigraphy from fossiliferous limestones in the Greater Antilles and adds chemostratigraphic controls to constrain their stratigraphic ranges. In particular, this study shows the practical uses of strontium isotope stratigraphy in tectonically complex areas that lack good biostratigraphic controls. Our results show similar strontium isotope (87Sr/86Sr) derived ages from the middle Cretaceous rudist specimens collected in Puerto Rico (112.43Ma), the Dominican Republic (112.62 Ma) and Jamaica (112.68 Ma). Moreover, these derived ages agree with the presumed depositional age of related rudist taxa from the Gulf of Mexico (114.03-111.55 Ma). Strontium isotope stratigraphy is also used to constrain the depositional age of the Late Cretaceous rudist assemblage from southwestern Puerto Rico. The 87Sr/86Sr derived numerical ages (84.45Ma) from these rudist specimens are consistent with the inferred stratigraphic range of related rudist taxa from Jamaica. In this study we present the first Aptian/Albian Antillean carbon isotope (d13C) chemostratigraphy from the Hatillo Limestone in the Dominican Republic with age control provided by 87Sr/86Sr derived ages, published ammonite, rudist, and foraminiferal biostratigraphy, and U/Pb zircon dates. The Antillean d13C profiles provide insights into middle Cretaceous ocean circulation within the Caribbean and Circum-Caribbean regions. The second part of this research examines the uncertainties surrounding the response of tropical oceans to Cretaceous greenhouse conditions and the effects of enhanced evaporation to calcite d18O signatures. For this study, the analyzed material consisted of Late Cretaceous rudist specimens collected in southwestern Puerto Rico. Fluid inclusions from marine cements precipitated within the inner cavities of rudists were used to derive paleosalinities (35.3 to 41.2 ppt). The effect of these salinities on the calcite d18O values was evaluated using temperature and salinity reconstructions based on the Railsback et al. (1989) model. The scenarios used to calculate sea surface temperatures resulted in paleotemperatures of up to 6°C higher than present day conditions. These results suggest that enhanced evaporative conditions dominated the paleotropics, and agree with global circulation and mass balance models, which predict warmer and saline conditions for tropical and sub-tropical regions during the Cretaceous. The final chapter of this dissertation explores the socioeconomic effect of present and future Caribbean reef degradation and utilizes the knowledge gained in this and other studies to suggest approaches for mitigation. During the Cretaceous, reef ecosystems experienced a major ecological shift that included domination by rudist bivalves and scleractinian corals during the Early Cretaceous and by rudist bivalves during the middle to Late Cretaceous. The Cretaceous ecological shift, however, took approximately 30 Ma to happen, while the current warming rates for tropical oceans suggest that the temperature tolerance of corals will be exceeded within a few decades. Because the timing for genetic adaptation is longer than the rate of climate change and global warming, the genetic mapping of corals and bivalve should be a priority in order to create heat resistant coral stocks.