Oxygen isotopic composition of Globorotalia truncatulinoides as a proxy for intermediate depth density

Abstract

We investigate whether the oxygen isotope ratio in the test of Globorotalia truncatulinoides can serve as a proxy for intermediate depth (200-500 m) density. Since intermediate depth horizontal density gradients are associated with the vertical shear of upper ocean flows, this proxy could provide a tool for reconstructing past ocean circulation. The spatial pattern of core top Gr. truncatulinoides δ18O in the Atlantic Ocean mimics the upper ocean density gradients associated with the major ocean currents. To better constrain the controls on the calcification depth(s) of Gr. truncatulinoides, we attempt to simulate the surface sediment data set using water column temperature and salinity conditions above the core sites. We predicted foraminiferal δ18O for each core site assuming (1) the calcification occurs at a single depth and (2) the initial calcification is at the surface and the subsequent calcification is at 800 m water depth. The predicted δ18O best resembles measured δ18O of Gr. truncatulinoides when using (1) a single depth calcification at 350 m or (2) a two-depth approximation with 30% surface and 70% 800-m calcification. This result gives us confidence in the ability of δ18O in Gr. truncatulinoides to proxy lateral density gradients at the intermediate depths associated with upper ocean flow. Copyright 2004 by the American Geophysical Union.