The oxygen isotope composition of planktonic foraminifera from the Guaymas Basin, Gulf of California: Seasonal, annual, and interspecies variability

Abstract

Sediment trap samples collected over a seven-year period (February 1991–October 1997) from Guaymas Basin in the Gulf of California were used to study the oxygen isotope composition of five species of planktonic foraminifera, Globigerinoides ruber (white), Globigerina bulloides, Neogloboquadrina dutertrei, Pulleniatina obliquiloculata, and Globorotalia menardii. The δ 18O data were analyzed for temporal and interspecies variability and were compared to local hydrography to evaluate the use of each species in reconstructing past oceanographic applications. The two surface dwelling species, G. ruber and G. bulloides displayed the lowest δ 18O values (∼ 0.0 to − 5.0‰), while δ 18O values for the thermocline dwelling N. dutertrei, P. obliquiloculata, and G. menardii were higher (∼ 0.0 to − 2.0‰). The δ 18O of G. ruber most accurately records measured sea surface temperatures (SSTs) throughout the year. G. bulloides δ 18O tracks SSTs during the winter–spring upwelling period but for the remainder of the year records slightly colder, subsurface temperatures. The difference between the δ 18O of the surface dwelling species, G. ruber and G. bulloides, and that of the thermocline dwelling species, N. dutertrei, P. obliquiloculata, and G. menardii, was used to estimate the surface to thermocline temperature gradient. During the winter these δ 18O differences are small (∼ 0.50‰) reflecting a well-mixed water column. These interspecies δ 18O differences increase during the summer (∼ 1.90‰) in response to the strong thermal stratification that exists at this time of year.