The spectral analysis suggests that eccentricity controls sedimentation. To elucidate that relationship, a Gaussian filter was applied to the insolation records at eccentricity and obliquity periodicities and to the TCMS age model (Fig. F7). The filtered MS signal is dominant at times of reduced filtered obliquity (4000-3000 ka) (Fig. F7A) and at times of increased dominance of the filtered eccentricity signal (4000-3000 ka) (Fig. F7B). The onset of Northern Hemisphere glaciation (e.g., 3.1 to 2.5 Ma; Haug and Tiedemann, 1998) is clearly associated with obliquity. The obliquity signal is strong between 3000 and 2500 ka; whereas, the eccentricity signal is reduced. Yet the 41-k.y. periodicity is not a dominant signal in the MS record in the Cape Basin despite the fact that Site 1085 is influenced by both Northern and Southern Hemisphere high-latitude climate changes (e.g., North Atlantic Deep Water impacts Antarctic Intermediate Water, but Antarctic glaciation impacts the position of the polar front, where Antarctic Intermediate Water is formed). One major problem may be the dual climate influences at this site. For example, glacial-interglacial periods are linked to changes in high-latitude Northern Hemisphere changes in obliquity. However, the Southern Hemisphere does not always experience synchronous changes in mean insolation and is sometimes out of phase with the Northern Hemisphere. Thus, different phases of the obliquity cycle forcing an oceanographic response in the Southern Hemisphere might thereby obscure the Northern Hemisphere connection.
Another problem may be that forcing at eccentricity periodicities may play a more important role in the onset of Northern Hemisphere glaciation than previously thought. The record of eccentricity changes from the more broad-shouldered events before and after (>5500 to 4300 ka and 3000 to 2000 ka), to events with higher frequency and greater amplitude (4300 to 3000 ka) (Fig. F6A, F6B, F6C). There is also a correlation between the change in character of the eccentricity curve and increased sedimentation rates (Fig. F4) that suggests a depositional response to orbital variation at the eccentricity periodicity. Close examination of the filtered record of insolation at eccentricity periodicities reveals a higher frequency oscillation at greater magnitude between ~4000 and 2800 ka (Fig. F7B), coincident with a decreased dominance of the obliquity signal (Fig. F7A) and an increase in the dominance of the TCMS record (Fig. F7A, F7B). This time period is associated with fluctuations in Northern Hemisphere ice-sheet growth. Haug and Tiedemann (1998) have suggested that the onset of Northern Hemisphere glaciation began as early as 4.1 Ma but proceeded as a series of false starts, with ice-sheet growth waning in response to reduced-amplitude obliquity fluctuations. They find potential growth events occurring between 4.1 and 3.9 Ma and 3.5 and 3.3 Ma, periods of high dominance of the insolation at the eccentricity periodicity (Fig. F7B) and of increased dominance in the MS record (Fig. F7A, F7B).