Stable oxygen isotopes in planktonic foraminifera from surface sediments in the California Current system

Stable oxygen isotopes of Globigerinoides ruber s.l. and Globigerina bulloides have been widely used to infer the variability of the water column hydrographic structure over various time scales. However, the reliability of this geochemical tool requires a regional analysis since the inferred calcification depth and temperature of planktonic foraminifera depend on local environmental conditions. In the present study, δ¹⁸O values of G. ruber s.l and G. bulloides collected from 21 surface sediment samples were used to infer their calcification depth in the southwestern margin of the Baja California peninsula. The δ¹⁸O values of G. ruber s.l. represented oceanographic conditions from warm tropical water masses (summer-autumn), while G. bulloides represented upwelling conditions and cold-water masses (winter-spring). Estimated calcification depths ranged from 0 to 30 m for both species. The isotopic difference between them (1.2‰ in this study and 1.1‰ in the Guaymas basin sediment trap) suggests a sea temperature difference of ~5 °C, which is very similar to the difference inferred from the Mg/Ca ratio for both species 3 to 14 ka. This suggests that G. ruber s.l. and G. bulloides have calcified over the same depth interval but under contrasting seasonal oceanographic conditions i.e. upwelling (winter-spring) and stratification of the water column (summer-autumn).