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δ18Osw estimate for Globigerinoides ruber from core-top sediments in the East China Sea

Research paper by Keiji Horikawa, Tomohiro Kodaira, Jing Zhang, Masafumi Murayama

Indexed on: 07 Jul '15Published on: 07 Jul '15Published in: Progress in Earth and Planetary Science



Abstract

The paired analyses of the Mg/Ca ratio and oxygen isotopic composition (δ18Oc) of surface-dwelling planktonic foraminifera have become a widely used method for reconstructing the oxygen isotopic composition of ambient seawater (δ18Osw) as a robust proxy for surface salinity. Globigerinoides ruber (G. ruber) is a mixed-layer dweller, and its fossil shell is an ideal archive for recording past sea surface water conditions, such as those caused by variability in the East Asian summer monsoon (EASM). Here, we investigate the validity of shell-derived δ18Osw estimates for G. ruber using core-top sediments from the East China Sea (ECS). First, we determined a local δ18Osw–salinity equation for the eastern part of the ECS in July [δ18Osw = −7.74 + 0.23 × salinity]. Then, we calculated δ18Osw from core-top δ18Oc and Mg/Ca values in G. ruber using the δ18Oc–temperature equation of Bemis et al. (Paleoceanography 13(2):150–160, 1998) and the Mg/Ca–temperature equation of Hastings et al. (EOS 82:PP12B-10, 2001). The core-top δ18Osw and salinity were estimated to be in the ranges of −0.2 to +0.39 ‰ and 33.7 to 34.5, respectively, which fall close to the local δ18Osw–salinity regression line. The core-top data showed that the Mg/Ca–temperature calibration by Hastings et al. (EOS 82:PP12B-10, 2001) and the δ18Oc–temperature equation by Bemis et al. (Paleoceanography 13(2):150–160, 1998) are appropriate for calculating δ18Osw in the ECS. Furthermore, we measured core-top Ba/Ca ratios of G. ruber (Ba/CaG. ruber), which ranged from 0.66 to 2.82 μmol mol−1. There was not a significant relationship between the salinity and Ba/CaG. ruber ratios due to the highly variable Ba/CaG. ruber data. Given the seawater Ba/Ca data and the published partition coefficient for Ba (DBa = 0.15–0.22), pristine Ba/CaG. ruber ratios at northern Okinawa Trough sites should be less than 0.84 μmol mol−1. Anomalously high Ba/CaG. ruber ratios (>0.84 μmol mol−1) might be attributable to contamination by sedimentary barite adherent on fossil shells. Therefore, further evaluation of the Ba/CaG. ruber ratio as a paleo-salinity proxy requires diethylene triamine pentaacetic acid (DTPA)-cleaned Ba/Ca data that can minimize the influence of barite contamination.