Extracting paleoenvironmental records from molluscan carbonate.

Abstract

This dissertation explores the use of molluscan carbonate chemistry for paleobiological and paleoenvironmental reconstructions. This broad topic is addressed from several angles, including two chapters based upon molluscan fossil geochemistry to reconstruct paleobiology and paleoenvironmental conditions, one chapter that focuses upon modern environmental data and modeling to better understand assumptions inherent in reconstructions of this type, and one chapter that discusses the calibration of a new paleothermometer. The first chapter evaluates the paleodepth habitat of a dimotobelid belemnite collected from Late Cretaceous sediments outcropping on Seymour Island, Antarctica. delta<super> 18</super>O and delta<super>13</super>C of these belemnite specimens are statistically indistinguishable from those of benthic foraminifera in this middle to outer shelf environment and are significantly offset from values recorded by planktic foraminifera. This relation suggests that these belemnites were common inhabitants of benthic waters of the middle to outer shelf, and therefore likely underestimate sea-surface temperatures. The second chapter examines the biogeochemistry of several species of bivalves collected from this stratigraphic section to reconstruct climatic and environmental conditions across this critical boundary. Ontogentic trends in stable isotope compositions of these bivalves are used to determine patterns of seasonal growth for each bivalve genus. Three pulses of warmth are identified in the latest Maastrichtian, at &sim;600, &sim;400, and &sim;100 kyr prior to the K/T boundary. Decreased amplitude of seasonal variation in temperature is observed at the K/T boundary, while the delta<super>13</super>C data suggest that productivity was even stronger just prior to and during the mass extinction event. The third chapter compares precipitation and river water delta<super> 18</super>O to better understand the environmental signal that is recorded by freshwater bivalve delta<super>18</super>O in continental environments. This comparison reveals substantial differences in delta<super>18</super>O related to topography and seasonal precipitation amount over the western U.S. and the mid-continent, but little to no difference is observed across most of the eastern U.S. The final chapter describes a culturing study of the mussel <italic> Mytilus californianus</italic> that was conducted to calibrate the use of a Mg/Ca paleothermometer. The results suggest that the species <italic>M. californianus</italic> may preferentially exclude Mg in its calcitic shell to a greater degree than <italic>M. edulis</italic> or <italic>M. trossulus </italic>.