RECONSTRUCTING SEASONALITY DURING THE LAST INTERGLACIAL USING THE BIVALVE MOLLUSK EPILUCINA CALIFORNICA, CHANNEL ISLANDS, CALIFORNIA

Understanding patterns of seasonality during the last interglacial (80-130 ka), the last time in Earth history that temperatures were at least as warm as today, may help us predict ecosystem responses in an anthropogenically warmed world. Marine terraces from San Nicholas Island, located in the Channel Islands of the Southern California Bight, contain well-preserved marine fossil assemblages from the last interglacial. Previous work has shown that these assemblages contain so-called non-analog communities (i.e., communities with species compositions unlike modern counterparts). Because these fossil assemblages contain a mix of both warm and cold water species, changes in seasonality may be responsible for the non-analog assemblages. To test the hypothesis that seasonality was different during the last interglacial, we analyzed the stable oxygen isotope (d¹⁸O) variation from modern and Pleistocene bivalve mollusk shells.

Epilucina californica is a common bivalve mollusk found on San Nicholas Island today. Fossil specimens are also found in marine terrace assemblages which date to Marine Isotope Stage (MIS) 5a, dated to approximately 80 ka, and MIS 5e dated ~120 ka. Modern and fossil specimens were sectioned along the axis of maximum growth and mounted on microscope slides. Point samples were drilled from the prismatic layer of the shell and analyzed on a Delta Advantage mass spectrometer equipped with a Gas Bench II (Union College). Oxygen isotope profiles from all three samples (modern, 5a, and 5e) all show sinusoidal variation suggesting we captured maximum and minimum d¹⁸O values during the year.

Stable oxygen isotope values from modern specimens (n=8) have a larger range (1.75 ‰) than their Pleistocene counterparts. The d¹⁸O range from 5e (n=4) and 5a (n=3) terraces is 1.09 and 1.16 ‰, respectively. If the range of d¹⁸O variability is solely a function of temperature, then seasonality was reduced by approximately 3 °C (modern: ~8.3 °C; 5e: ~5.3 °C; 5a: ~5.6 °C). Of course, shell d¹⁸O is also a function of the oxygen isotope composition of the water, which may have changed as a function of alterations in local ocean current circulation and/or upwelling. Ongoing analysis of modern and fossil shells will likely shed additional light on the causes isotopic variation.