ISOTOPE PALEOECOLOGY OF GLOBOQUADRINA DEHISCENS ACROSS THE MIOCENE CLIMATE OPTIMUM IN THE SOUTHWEST PACIFIC, IODP SITE U1510

Globoquadrina dehiscens is a species of planktic foraminifera which lived during the Miocene Epoch. While this species has an easily recognizable test, the paleoecology of G. dehiscens from prior studies indicate it has a variable position in the water column. Little work has been done to determine if and how G. dehiscences changed location in the water column throughout its life history. We inferred the dwelling depth of Globoquadrina dehiscens using stable isotopic analyses (δ¹³C, δ¹⁸O) of fossil specimens in six different size fractions from samples that span the Miocene Climatic Optimum at IODP Site U1510. From smaller to larger size fractions, the δ¹⁸O and δ¹³C values increase with increasing size fractions. We also compared the stable isotopic values of G. dehiscens to those of mixed-layer and thermocline-dwelling species from the same samples. The G. dehiscens stable isotopic data fell between and outside of the thermocline and surface dwelling data. Taken together, these results indicate that G. dehiscens descends to deeper, more nutrient-rich depths during its life cycle, but its paleoecological signal may be obscured by high seasonality in the mid-latitudes. Overall, the data indicate that G. dehiscens may not be a reliable species to use in paleoceanographic geochemical reconstructions.