Seasonal Variability in the δ¹⁸O and δ¹³C of Planktonic Foraminifera from an Upwelling Environment: Sediment Trap Results from the San Pedro Basin, Southern California Bight

Abstract

Stable isotopic analyses (δ18O and δ13Crpar; 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 δ18O signal of O. universa appears to be a useful indicator of seasonal temperature variations above the thermocline, whereas the δ18O of G. bulloides documents near‐surface temperatures during upwelling. The δ13C signal of both N. dutertrei and N. pachyderma appears to record the δ13C of equilibrium calcite 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. Thus an actual record of hydrographic conditions during upwelling, as would be found in tests of G. bulloides, is not preserved; instead, conditions before (N. pachyderma) and just after (N. dutertrei) are more likely recorded.

Planktonic foraminifera in the sea of Okhotsk : population and stable isotopic analysis from a sediment trap

“…The lack of coherent trends in the G. bulloides size/mass-isotope analysis runs counter to the findings of Matthews (1981), Sautter andThunell (1991), Kroon and Darling (1995) and Spero and Lea (in press), all of whom find increasing 6180 and 6 13 C values with size. The lack of correlation in the 8 18 0-size and 6 18 0-mass relationships suggests no size-or mass-related preferred depth habitat, as is suggested by the N. pachyderma (1) data.…”

“…Coretop work has also shown N. pachyderma to be either light in 813C (Charles & Fairbanks, 1990;Labeyrie & Duplessy, 1985) or completely out of equilibrium (Keigwin & Boyle, 1989). Recent sediment trap work of Sautter and Thunell (1991) suggests that N. pachyderma does calcify in equilibrium for both isotopes. (Kahn & Williams, 1981 and N. pachyderma (1) 81 3 C records, however, suggest that G. bulloides is 1%o low in carbon isotopes.…”

“…Previous studies have shown increasing 6180 and 81 3 C with increasing size in N. pachyderma (1) (Donner & Wefer, 1994;Keigwin & Boyle, 1989;Sautter & Thunell, 1991). Keigwin and Boyle (1989) shallower than the smaller uncrusted individuals (Carstens & Wefer, 1992).…”

Planktonic foraminiferal ecozones: response of the pelagic environment to palaeoclimatic changes in the eastern Mediterranean Sea

“…Phytoplankton in the tropical oceanic regions is highly abundant in Deep Chlorophyll Maximum (DCM) layers and highest abundances of tropical neogloboquadrinids, therefore, occur in the DCM layer (FAIRBANKS et al, 1982;RAVELO et al, 1990). Also cooler-water morphotypes seem to exploit food sources, which are associated with the (seasonal) DCM layer (REYNOLDS & THUNELL, 1986;SAUTTER & THUNELL, 1991). Elevated percentages of sinistrally coiled morphotypes correlate with glacial isotope stages (RUDDIMAN et al, 1989), whereas dextrally coiled morphotypes show no such a relationship.…”

“…Globigerina bulloides is an eurythermic species which flourishes wherever surface waters are eutrophicated, such as in coastal upwelling areas (e.g. THIEDE, 1983;ZHANG, 1985;SAUTTER & THUNELL, 1991), regions off major river mouths ( VAN LEEUWEN, 1989) and nutrient-rich marginal basins (BARMAWIDJAJA et al, 1989).…”