GRA bulk and log density data obtained at Sites 1081, 1082, and 1084 allowed the calculation of high-resolution SR in the depth and time domain. Our profiles generally agreed with those from biostratigraphic analyses. We believe that the upwelling history of the BCUS can be explained in part by orbital forcing by influencing indirectly the complex ocean and atmospheric circulation patterns along the southwest African coast over time (Diester-Haass et al., 1992). Accordingly, upwelling vigor varied spatially and temporally, causing a heterogeneous distribution of bioproductivity. This controlled the variability of the SR history along the BCUS, which was further modified by coastal runoff during glacial and interglacial periods (Meyers, 1992).
Results from modeling cyclic variations in simplified synthetic density compaction curves reminded us that caution is required when interpreting periodicity in real geologic records. Our analyses of wet bulk density from log and GRA bulk density records obtained at Sites 1081, 1082, and 1084 (Figs. F7, F8) certainly exhibited some of the phenomena demonstrated with our synthetic density models (Figs. F9, F10, F11, F12). Our theoretical results corresponded with similar studies (Herbert, 1994), suggesting that rapid fluctuations in accumulation rates along the southwest African coast may have caused irregularities in evolutionary power spectra. These, in return, must have affected the accuracy of our method to deduce a high-resolution SR history from spectral analyses. Synthetic accumulation rate profiles, perturbed by a known cyclicity, revealed irregular deviations in power spectra as the result of aliasing (not enough cycles were resolved at a given sampling rate), effects of certain autocorrelation and FFT window lengths, and the choice of smooth factors. The presence of very strong density gradients in our geologic records (delta spikes) introduced broadband noise into the spectral profiles and thus complicated a clear separation of the individual cycles.
In summary, we feel comfortable about our interpretation of wet bulk density data obtained at Sites 1081, 1082, and 1084 because of a reasonable agreement with biostratigraphic analyses (Figs. F5, F6). Conclusions about the upwelling history of the BCUS drawn from previous studies (e.g., Diester-Haass et al., 1999) and about cyclicity from this and other locations (e.g., Meyers, 1992; Clark et al., 1999) confirm the reliability of our results.