The calcification depth and Mg/Ca thermometry of Pulleniatina obliquiloculata in the tropical Indo-Pacific: A core-top study

A sub-surface dwelling planktonic foraminifer, Pulleniatina obliquiloculata, in particular its shell's geochemical properties such as stable oxygen isotopic composition and Mg/Ca ratios, are widely used to reconstruct past changes in the thermocline of tropical Indo-Pacific. However, the depth range that P. obliquiloculata population generally dwells or calcifies has been shown to significantly vary, depending on the regions and the seasons. In this study, δ18O and Mg/Ca of core-top P. obliquiloculata from the tropical Indo-Pacific are analyzed by means of comparison with modern hydrographic data. We apply a δ18O-based approach to decipher the varying apparent calcification depth (ACD) of this species for the tropical Indo-Pacific, and find that P. obliquiloculata's ACD varies between 50 and 150 m regionally, in connection with local oceanographic settings. At given ACD, exponential relationships between temperature and P. obliquiloculata Mg/Ca are determined as Mg/Ca = 0.245 ± 0.041 exp. (0.088 ± 0.009 T) for these core-top samples. The sensitivity of P. obliquiloculata Mg/Ca to temperature is in good accordance with most previous findings, particularly the recent work using core-top samples from the western tropical Pacific. Taken together, our results not only improve the understanding of the calcification conditions of P. obliquiloculata, but also make it possible to better estimate the sub-surface temperature of the Indo-Pacific Warm Pool (IPWP) using Late-Quaternary sedimentary materials.