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

Abstract

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 Earth9s 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. Visually well-preserved ammonite, bivalve, and gastropod shells were collected from one locality in the GCP and examined using SEM. To evaluate microstructural preservation, we used a previously published scale based on WIS specimens for nacreous shell material and a complementary scale we developed for crossed lamellar microstructure. The stable carbon and oxygen isotope compositions of specimens across the preservational scale from both regions were analyzed. Isotopic composition does not change systematically with degrading preservation in the GCP specimens, a result that contrasts strongly with patterns in WIS specimens where isotopic composition systematically decreases with decreasing microstructural preservation. Dissimilar tectonic regimes that led to unique post-depositional conditions for each region are invoked to explain this difference. The WIS was a foreland basin subjected to significant meteoritic groundwater flow driven by active uplift, while the GCP was a passive margin that has remained close to sea level, with low hydraulic potential, since the time of deposition. Thus, during diagenesis the WIS shells were exposed to fluids that differed greatly from the seawater in which the organisms grew, whereas diagenetic fluids in GCP settings likely were similar to marine waters. As expected, shells with the best-preserved microstructure provide the most consistently reliable isotopic data for paleoenvironmental reconstructions, but our comparisons show that similar degrees of microstructural alteration can express very different diagenetic shifts in isotopic values.