Sr/Ca-sea surface temperature calibration in the coral Porites lutea from subtropical northern South China Sea

Strontium to calcium (Sr/Ca) ratios in annual bands of Porites corals are commonly used as past sea surface temperature proxy in tropical oceans, whereas studies that refer to Sr/Ca-SST calibration in high-latitude corals are rather rare in general. Here we present a 50-year (1961-2010) near-fortnightly Sr/Ca record in two Porites lutea corals living near the northern limit of reef-building corals in the subtropical northern South China Sea, where winter temperatures are extreme for hermatypic corals. The Sr/Ca time series shows a clear annual cyclicity, with marked sharp/narrow winter peaks. Comparison with an instrumental SST record (since 1961) indicates that this Sr/Ca record is chiefly modulated by SST. Two different methods are discussed for calibrating Sr/Ca with SST. The correlation between interpolated Sr/Ca and SST is significant, but the coefficient is relatively low, mainly attributed to extreme cold winter SSTs (<similar to 16 degrees C) which cause a significant decrease or even cessation of coral skeletal extension. Another modified method using only annual low Sr/Ca values which could clearly be matched with the same features in the instrumental SST record is more reliable. The modified regression equation does not encompass SSTs below 24 degrees C. The fidelity of the coral-based Sr/Ca-SST calibration was assessed and the precision of the method is estimated to be 031 +/- 0.62 degrees C based on the calibration-verification exercise. The modified method is suggested in marginal coral areas with extreme intra-annual SST variation (e.g., from 13 to 30 degrees C). (C) 2013 Elsevier B.V. All rights reserved.