Research Training Program

Smithsonian Institution
National Museum of Natural History

PROJECT SUMMARY
1996

Ryan M. Houston
University of California - Berkeley
Berkeley, California
Brian T. Huber, Ph.D.
Supervising Scientist
Department of Paleobiology
"As I continue in my academic career, my experience here at the National Museum of Natural History will always remain as one of the most inspiring and memorable periods of my undergraduate education."

Stable Isotope Analysis of Late Cretaceous Planktic Foraminifera and the
Evolution of Algal Endosymbiosis

ABSTRACT
In paleoceanography, oxygen (18O) and carbon (13C) isotopes from the calcite of marine microfossils are some of the most important tools for estimating paleoceanographic conditions such as salinity, temperature, primary productivity, and carbon exchange. In ancient seas, stable isotope ratios are often recorded in the shells of planktic foraminifera. Using fossil foraminifera from deep sea cores, we can measure stable isotope ratios to obtain information about paleoceanographic conditions. However, the incorporation of stable isotopes into the planktic foraminifera shell can be changed by a variety of "vital effects" including algal endosymbionts, metabolic rates, and depth habitat. In order to accurately interpret the stable isotope ratios present in fossil planktic foraminifera, the magnitude of these vital effects must be understood and their impact upon the stable isotope ratios in the shells of planktic foraminifera must be determined. In this study, stable isotope analysis was performed on four species of planktic foraminifera in an effort to understand the role of algal endosymbiosis in altering the 18O and 13C ratios in Late Cretaceous planktic foraminifera. Using a new method of shell dissection, stable isotope analyses were performed on four taxa of low latitude Late Cretaceous planktic foraminifera including Planoglobulina acervulinoides, Planoglobulina multicamerata, Pseudoguembelina palpebra, and Racemiguembelina fructicosa. 18O and 13C analyses indicate distinct depth paleohabitat differences between P. multicamerata and the other three taxa. Ontogenetic increases in 18O signals suggest vertical migrations from shallow surface waters to deep surface waters in all taxa studied. Ontogenetic 13C changes include sharp juvenile increases in 13C signals, a decrease in the rate of 13C change through intermediate size intervals, and negative 13C excursions in terminal size intervals. The 13C increases observed in early growth stages suggest that photosymbionts may have been present in the Late Cretaceous. If so, these results document the earliest occurrence of photosymbiosis in planktic foraminifera.

This research was supported by a grant from the National Science Foundation Research Experiences for Undergraduates program, Award Number DBI-9531331.

Letter of Gratitude