Figure 4. Two spectral analyses using the Blackman-Tukey method. A. ¹⁸O spectrum of Site 607 shows that ¹⁸O variations indicative for glaciations occur at the primary Milankovitch frequencies. Obliquity, however, is the dominant frequency (Raymo et al., 1990). B. Gamma-ray spectrum of Miocene marl/limestone alternations at Site 1003 produces frequencies at 40, 23, 19, 16, and 11 k.y. The strongest peak occurs at 23 k.y., indicating the dominance of orbital precession on these sedimentary cycles (Bernet, 2000). For further discussion, see the text.