Abstract
The paired analyses of the Mg/Ca ratio and oxygen isotopic composition (δ 18Oc) of surface-dwelling planktonic foraminifera have become a widely used method for reconstructing the oxygen isotopic composition of ambient seawater (δ 18Osw) 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 δ 18Osw estimates for G. ruber using core-top sediments from the East China Sea (ECS). First, we determined a local δ 18Osw–salinity equation for the eastern part of the ECS in July [δ 18Osw = −7.74 + 0.23 × salinity]. Then, we calculated δ 18Osw from core-top δ 18Oc and Mg/Ca values in G. ruber using the δ 18Oc–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 δ 18Osw 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 δ 18Osw–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 δ 18Oc–temperature equation by Bemis et al. (Paleoceanography 13(2):150–160, 1998) are appropriate for calculating δ 18Osw in the ECS. Furthermore, we measured core-top Ba/Ca ratios of G. ruber (Ba/Ca G. ruber ), which ranged from 0.66 to 2.82 μmol mol−1. There was not a significant relationship between the salinity and Ba/Ca G. ruber ratios due to the highly variable Ba/Ca G. ruber data. Given the seawater Ba/Ca data and the published partition coefficient for Ba (D Ba = 0.15–0.22), pristine Ba/Ca G. ruber ratios at northern Okinawa Trough sites should be less than 0.84 μmol mol−1. Anomalously high Ba/Ca G. 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/Ca G. 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.
Highlights
An East Asian summer monsoon (EASM) precipitation zone is formed by the convergence of a westerly flow to the north of the Tibetan Plateau and a southerly monsoon flow over eastern China, which gradually moves northward from South China and reaches northern China in late July (Qian and Lee 2000)
The main findings of this study are as follows: (1)Shallow seawater samples (0–100 m) taken from the East China Sea (ECS) in early July showed a strong correlation between δ18Osw and salinity
Á 40 : This local δ18Osw–salinity equation might be representative of the northern Okinawa Trough during the EASM season
Summary
An East Asian summer monsoon (EASM) precipitation zone is formed by the convergence of a westerly flow to the north of the Tibetan Plateau and a southerly monsoon flow over eastern China, which gradually moves northward from South China and reaches northern China in late July (Qian and Lee 2000). Since the annually averaged precipitation and evaporation are near equilibrium in the ECS and Yellow Sea and the Changjiang River runoff accounts for 90 % of the total river discharge (with an annual mean of ~3.0 × 104 m3 s−1), the Changjiang River discharge is considered the dominant freshwater source in the ECS (Beardsley et al 1985; Chen et al 1994; Yanagi 1994) This Changjiang River water extends offshore, and the water mass characteristics of the offshore ECS can be roughly described by the binary mixing of the cooler, less saline CDW and the warmer, more saline Kuroshio water (Zhang et al 1990; Ichikawa and Beardsley 2002; Zhang et al 2007). In the central Okinawa Trough, the mean annual SST is 25.0 °C near site HR2, with a maximum in August (28.9 °C) and a minimum in February (21.5 °C) (Japan Oceanographic Data Center (JODC), available at http://www.jodc.go.jp/)
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