Calculated high-resolution SR values in the depth domain overall exhibited a strong correlation with those obtained by biostratigraphy (Fig. F5). Similar to biostratigraphic profiles, our SR values sometimes changed abruptly, but additionally showed much higher resolution excursions. The most pronounced drop in SR values occurred at Site 1084 (Fig. F5C) at ~200 mbsf, from ~315 to 180 m/m.y. Rates at Site 1082 (Fig. F5B) declined more gradually than at Site 1084, from 290 to 120 m/m.y. between 220 and 260 mbsf. In contrast, SR values at Site 1081 (Fig. F5A) on the Walvis Ridge were generally lower compared to Sites 1082 and 1084 in the Walvis Basin and varied only between 35 and 60 m/m.y. below 200 mbsf. However, a strong correlation in high SR peaks was found within the top 150 mbsf between all three sites. Below 200 mbsf, this correlation was not so strong, with the most pronounced SR variations found at Site 1084 between 300 and 600 mbsf (Fig. F5C).
Calculated SR values in the time domain generally followed the trend of the biostratigraphic profiles (Fig. F6). Age-depth relations (A_A to C_C, inset in Fig. F6), derived from our spectral analyses, yielded maximum ages of ~9.0 Ma at Site 1081 (Fig. F6A), 5.3 Ma at Site 1082 (Fig. F6B), and 4.9 Ma at Site 1084 (Fig. F6C). The largest discrepancy in the age-depth profiles was found at Site 1082 between 2.2 and 5.3 Ma (inset B_B in Fig. F6).
A strong correlation in SR values for GRA bulk density was observed at Sites 1081 and 1082 between 0.2 and 0.5 Ma (Fig. F6A, F6B). In contrast, computed data at Site 1084 showed a negative correlation for this time period with the other sites (Fig. F6C). Very pronounced pulses of high deposition rates (150 m/m.y. and greater) were found at all three sites between ~0.7 and 1.05 Ma. The highest variability in SR values (100 to 290 m/m.y.) and the broadest peak (~290 m/m.y.) was observed at Site 1084 (Fig. F6C). A rapid decline in deposition at all sites was found roughly between 1.1 and 1.7 Ma. A rapid increase in deposition around 1.7 Ma and pulses of high SR values until 2.6 Ma characterized deposition at all three sites, with the broadest peak identified at Site 1084 (Fig. F6C).
Before 2.7 Ma, a strong correlation between maximal and minimal SR values among the three sites could not be clearly defined. At Site 1082 (Fig. F6B), a pulse of high SR was found between 2.9 and 3.1 Ma, whereas at Site 1084 (Fig. F6C) an interval of high SR values existed between 3.1 and 3.8 Ma. Log density data obtained at Site 1081 did not correspond to this trend (Fig. F6A). In contrast, Site 1081 revealed relatively high SR values between 4.1 and 5.0 Ma, which was paralleled by a short burst of high deposition rate at Site 1082 only between 4.6 and 4.8 Ma (Fig. F6B). Data obtained at Site 1084 (Fig. F6C) suggested the opposite trend, with low SR values within this particular time period.
Evolutionary power spectra as a function of age were generated by applying our spectral analysis technique on residual GRA bulk and log density records in the time domain. Our results showed a strong presence of Milankovitch periodicity (Figs. F7, F8). During some intervals, all three Milankovitch cycles appeared contemporaneously in the spectral waveband, whereas other time periods were predominated only by one or two of the main cycles. Thus, the key observation in our spectral profiles was the waxing and waning of various cycles over time, shifting either abruptly or gradually from one frequency into another.
Strong eccentricity was recognized for the past 1.5 m.y., whereas frequencies in the obliquity and precession waveband appeared to be less pronounced (Fig. F7). Around 1.6 Ma, the dominance of eccentricity waned (Figs. F7A, F8C) and amplitudes of the obliquity cycles became more amplified. All three cycles coexisted in the waveband between 2.0 and 2.6 Ma, whereas before 2.6 Ma mainly eccentricity governed the spectral energy. Practically no obliquity and precession were observed between 3.5 and 4.0 Ma, with slight variations in their times of occurrence among the individual sites (Fig. F8). All three cycles showed strong spectral amplitudes at Site 1081 between 4.2 and 4.8 Ma (Fig. F8A). Before 4.8 Ma, spectral energy records at Site 1081 suggested a pronounced wax-and-wane pattern only between eccentricity and obliquity, but not a strong presence of precessional cycles.