MICROSTRUCTURAL PRESERVATION AND THE EFFECTS OF DIAGENESIS ON THE CARBON AND OXYGEN ISOTOPE COMPOSITION OF LATE CRETACEOUS ARAGONITIC MOLLUSKS FROM THE GULF COASTAL PLAIN AND THE WESTERN INTERIOR SEAWAY

摘要

The carbon and oxygen isotope compositions of calcium carbonate shells are widely accepted and applied proxies for tracking changes in paleoenvironmental conditions such as temperature, salinity and productivity. In order to accurately interpret isotopic measurements, diagenetic alteration must first be assessed. The occurrence of aragonitic shells is often taken as a first order sign of unaltered material because aragonite at the Earth's surface is metastable and converts to calcite. However, even specimens that retain an aragonitic composition and are macroscopically well preserved often exhibit subtle to considerable signs of alteration when shell microstructure is examined using scanning electron microscopy (SEM). To determine the textural and isotopic effects of progressive alteration on differing types of shell microstructure, we undertook a comparative study of aragonitic shells from two regions frequently used in paleoenvironmental reconstructions and diagenetic studies - the Upper Cretaceous of the Gulf Coastal Plain (GCP) and the Western Interior Seaway (WIS). Because the GCP is within a passive margin setting and the WIS has experienced tectonic processes, we also evaluate how diagenetic history affects isotopic composition.