In Figure F2 we present the opal values (in weight percent) from 137.52 to 266.13 mcd at Site 1082, the interval analyzed by the two laboratories. The two records show the same general pattern but show offsets in absolute numbers. OLin values tend to be higher, averaging 23.6% (N = 50), whereas OPer values have an average of 18.3% (N = 50), comparing the closest neighbors (<4 cm offset) in the overlapping portion of the record.
Reproducibility in the two procedures was estimated from both duplicate and replicate determinations. The pooled precision was ±1.37% opal for duplicate determinations of 23 OLin samples and ±0.6% opal for duplicate determinations of 42 OPer samples. Replicate determinations of nine OLin samples with an average content of ~15%–20% opal yielded a standard deviation of ±1.42% opal; replicate measurements of three OPer samples with ~8%–15% opal gave a standard deviation of ±0.55% opal. Table T1 and Figure F3 show the differences between the two techniques for selected OLin and OPer samples (opal-poor and opal-rich samples). The error is larger for samples with the lower opal content.
We tested whether a regression of OPer data points vs. OLin data points was sufficiently precise to transform OPer to OLin values and vice versa. Because OLin and OPer opal measurements were not done on aliquots from the same samples, we followed two approaches: the closest neighbor method and the linear interpolation method. For the closest neighbor method, only those samples taken at nearby depth intervals (<4 cm offset) were considered in the correlation (N = 50). Alternatively, estimates of the OPer opal content for depth intervals corresponding to OLin samples were obtained by linear interpolation from the OPer values (N = 70) (Fig. F4A, F4B). The data points outside the 95% confidence interval were removed for the calculation of the regression. We found that a linear regression had the highest correlation coefficient of several possible transforms in both approaches (r2 = 0.84 and r2 = 0.86, respectively).
By using the linear regression equation given by the linear interpolation method (Fig. F4B), OPer data were converted to OLin data. Subsequently, the estimated OPer data set and the measured OLin data set were merged to obtain a more detailed and extended opal record of Site 1082. The merged data set could then be compared to the opal record of Site 1084, which was analyzed with only the OLin method. Figure F5A shows a similar pattern at both sites with a distinct opal maximum during the late Pliocene and early Quaternary. Because of the location of Site 1084, close to the Lüderitz upwelling cell, opal values were higher at this site than at Site 1082.
DAI, as estimated in Lange et al. (1999), yielded the same patterns at both sites (Fig. F5B, F5C). The diatom maximum is characterized by high abundances of Thalassiothrix and other pelagic species in addition to the Chaetoceros spores indicating coastal upwelling (Lange et al., 1999).