SEASONAL VARIABILITY IN THE delta O-18 AND delta C-13 OF PLANKTONIC FORAMINIFERA FROM AN UPWELLING ENVIRONMENT: SEDIMENT TRAP RESULTS FROM THE SAN PEDRO BASIN, SOUTHERN CALIFORNIA BIGHT

Stable isotopic analyses (delta O-18 and delta C-13) on four species of planktonic foraminifera collected from high-resolution time series sediment trap sampling indicate that rapid and significant seasonal variability occurred in response to rapid changes in both the hydrographic environment and the isotopic composition of ambient seawater. Specimens were collected at a depth of 500 m for 28 consecutive 1-week periods in the San Pedro Basin, within the Southern California Bight, a region seasonally influenced by upwelling. Hydrographic monitoring of the study area revealed a 6-week period (late April to mid-June) during which pulses of upwelling occurred, causing significant increases in the oxygen isotopic composition of surrounding waters, while decreasing the carbon isotopic composition. Analyses of Globigerina bulloides, Orbulina universa, Neogloboquadrina dutertrei, and Neogloboquadrina pachyderma (right-coiling) indicate that during the upwelling period these species live, respectively, at the surface, in the surface mixed layer, within the thermocline, and below the thermocline. Globigerina bulloides ascends from well below the thermocline to the surface at the onset of upwelling and increases production to its maximum simultaneously as its morphology changes to less-calcified, open forms. Both N. dutertrei and N. pachyderma follow isothermal ranges, migrating to shallower waters during upwelling and subsequently descending after upwelling ceases. Neogloboquadrina dutertrei appears to inhabit the region associated with both the thermocline base and chlorophyll maximum when it is most abundant, following upwelling. Orbulina universa increases its depth habitat as solar irradiance increases, but this species decreases in abundance during upwelling. The delta O-18 signal of O. universaa ppearsto be a useful indicator of seasonal temperature variations above the thermocline, whereas the delta O-18 of G. bulloides documents near-surface temperatures during upwelling. The delta C-13 signal of both N. dugergrei and N. pachyderma appears to record the delta C-13 of equilibriumc alcite for the regions of the thermocline and just below the thermocline, respectively. Due to the relative preservation potentials of the four species analyzed, only N. dutertrei and N. pachyderma are likely to be preserved in the sedimentary record in regions of oxygenated bottom waters.