DELTA18O VALUES OF EOCENE BIVALVES REVEAL HIGH-LATITUDE SHELF COOLING, SEYMOUR ISLAND, ANTARCTIC PENINSULA
We present a record of δ18O values and inferred paleotemperatures derived from bivalve carbonate collected throughout the formation at high stratigraphic resolution. A sampling design in which multiple samples are taken from multiple individuals of two different genera from multiple localities along strike and multiple horizons arrayed in time, allows us to statistically assess the significance of differences we see among samples at each of these hierarchical levels so as to be assured of the validity of trends we see through time.
Significant compositional differences between genera are found to result from differences in season of growth, and emphasize the importance of understanding the biology of organisms from which geochemical data are extracted. Significant variation exists within individual shells, reflecting temperature variation experienced during the lifespans of individuals. Comparisons of shells within collecting localities occasionally revealed variation, indicating a degree of time-averaging within shell beds that subsumes longer-term inter-annual variation. Samples from different collecting localities within the same horizon, however, are not significantly different, ensuring that variation within localities is consistent from place to place and will not obscure temporal patterns.
The long-term trend in δ18O and temperature shows overall cooling from about 19ºC near the base of the section to ~11ºC in the uppermost shell bed, consistent with published proxy data but with a greater level of complexity through time than has been suggested by previous work done at lower resolution. We believe the major aspects of this curve will be shown to reflect the global signal, with early Eocene warmth, middle Eocene relative stability, and a shift toward cooler conditions perhaps superimposed on limited ice growth in the upper Eocene.