Does G. sacculifer record surface water conditions in the south Atlantic subtropical gyre?

The oxygen isotopic composition (δ¹⁸O) of planktonic foraminifera is frequently used in paleoceanographic studies to reconstruct sea surface temperature (SST) and the isotopic composition of sea-water. In tropical and subtropical locations, Globigerinoides ruber is most commonly used, as it is known to live in the surface mixed layer. However, for Pliocene-Pleistocene studies, Globigerinoides sacculifer is often used instead, as it is more common and is present throughout the past 5 million years. For example, at ODP site 1264 in south Atlantic tropical gyre (28.5°S; 2.8°E, 2505 m water depth), a previous study (Wojcieszek et al., in prep) used Mg/Ca and δ¹⁸O of G. sacculifer to reconstruct SST and sea surface salinity through the Pliocene-Pleistocene climate transition because G. sacculifer is present throughout the past 4 Ma while G. ruber is scarce past 1 Ma. The challenge with using G. sacculifer is that sediment trap and down-core work in the eastern equatorial Pacific indicates that different size fractions of this species may represent different water depths (Spero et al., 2003). The δ¹⁸O composition of smaller (250-350 μm) G. sacculifer was heavier (higher δ¹⁸O), indicating colder temperatures, compared to G. ruber. This implies that G. sacculifer samples in the 250-350 μm size fraction calcified their shells in cooler subsurface waters. This is in contrast to larger G. sacculifer (>650 μm), which were isotopically lighter (lower δ¹⁸O) than the smaller size fraction and very similar to δ¹⁸O of G. ruber. However, the eastern equatorial Pacific has a shallow thermocline (~50m), while the subtropical south Atlantic has a deeper thermocline (~ 100m) in the modern ocean. We will present δ¹⁸O data from both small and large G. sacculifersize fractions as well as G. ruber for the last 1 million years at ODP site 1264 to test if the 250-350 m G. sacculifer δ¹⁸O and Mg/Ca records represent the surface mixed layer.