A total of 463 sediment samples were taken from three cores from Site 1143 (9°21.72´N, 113°17.11´E; water depth = 2722 m; penetration = 516 meters composite depth [mcd]) (Wang, Prell, Blum, et al., 2000) for opal analysis. Samples were taken at intervals from 0.6 to 1.5 m with an average time resolution of ~20 k.y. (Table T1).
The opal analysis was performed in the Laboratory of Marine Geology of the Ministry of Education, Tongji University, People's Republic of China, following the methods proposed by Mortlock and Froelich (1989). Opal concentration values in the 463 samples range from 0.33 to 23.35 wt%, with an average value of 2.16 wt%. Precision and reproducibility of this method were estimated from the duplicate analyses of 47 random samples. The discrepancy between the two values of the same samples ranges from -1.68% to 14.76%, with an average of ±4.75%. The replicated measurements of 47 samples indicate that the average discrepancy of duplicate analyses is relatively low and document high accuracy and reproducibility. Therefore, we believe that this method may be used for values <5%, although the low opal values (<5%) should be regarded with suspicion (Mortlock and Froelich, 1989).
Mass accumulation rates (MARs) of opal and carbonate are calculated according to the following equation:
where
The latter is calculated by interposition of the DBD data at this site (Wang, Prell, Blum, et al., 2000). The data for carbonate and total mass accumulation rates used in the present study are from Wang, Prell, Blum, et al. (2000).