Stable isotope and sclerochronologic analysis of environmental and temporal resolution in modern and fossil bivalve mollusk shells

Abstract

Organisms that grow by skeletal accretion contain a geochemical record of environmental conditions--they are, in effect, biological chart recorders. Thus, shell-bearing organisms are an important source of data on modern and ancient environments. Geochemical analysis of shell material sampled along an ontogenetic profile can provide time-series of the environmental variation experienced when the organism was growing. However, variations in growth rates and complete cessations of growth can bias biogeochemical archives. Thus, careful calibration of environmental conditions with shell growth is critical if reliable records are desired. Here, I present several studies designed to understand the relationship between bivalve shell growth and environmental variation. This is accomplished through careful calibration of temperature, geochemical variability, and growth increment variation (sclerochronology). I then apply the findings of these calibration studies to address paleobiological and paleoclimatic questions. I conducted a cross-calibration study relating annual temperature variation with stable oxygen isotope (δ¹⁸O) variation. I used daily increments to assign dates to each δ¹⁸O sample. I then compared the geochemically based temperature estimates with the actual temperatures from the same dates. Results indicate that combined geochemical and sclerochronological analyses can provide reliable estimates of environmental variation, as well as shed light on aspects of the clam's biology, such as the rate and timing of shell growth. The results of this study were then incorporated into a more generalized investigation of the relationship between annual temperature variation and growth rate. This study indicates that the resolution and fidelity of geochemically based environmental reconstructions depends strongly on growth rate and duration. Together, the results of these studies were applied to address paleobiological and paleoclimatic issues. First, I used sclerochronologically calibrated annual isotope profiles to detect time-averaging and spatial mixing. This study indicates that short duration time-averaging (<50 years), previously undetectable using traditional dating techniques, can be identified using oxygen isotope variation. Results also suggest that within-habitat spatial mixing can be detected. Results of the calibration studies were also applied to paleoclimate questions. Seasonality is an important aspect of paleoclimate reconstruction and is often inferred from annual oxygen isotopic variation in fossil shells. However, because many organisms do not grow throughout the year, their shells do not record the full range of seasonal temperatures. This limitation can be overcome by using stable oxygen isotope variation from two species, each of which continues to grow while the other has shut down. The method was demonstrated using two common venerid bivalves from the eastern Pacific. The reconstructed estimate of seasonality matches published sea-surface temperature data from the same site. I also used differences in δ¹⁸O values and daily increment numbers from modern and Pleistocene (last interglacial, ∼125,000 ybp) bivalves to estimate temperature change in the shallow marine environment off the coast of southern California. Data indicate that temperatures were∼3°C cooler than present, however, more data are needed to confirm these initial results.