SUMMARY AND CONCLUSIONS

A sequence within the early to middle Quaternary productivity record off southwest Africa (Core 175-1085A-7H) is strongly influenced by climate cycles at periods of 41 k.y. (obliquity driven) and also 100, 70, 23, and 19 k.y. Earlier sequences (Cores 175-1085A-9H and 10H) show less distinct periodicity. Modular analysis of various productivity-related proxies at the obliquity period within Core 175-1085A-7H shows that the proxies BF/g and EDA are not in phase. Organic matter production, as recorded in BF/g, closely follows the climate state, with maxima during glacial times. But diatom production, as seen in EDA, does not follow this pattern. Instead, it shows a low level of response to climate fluctuations, with equal influence from the climate state (as seen in ¹⁸O) and its derivative (change in ¹⁸O). The reason is, presumably, the depletion of thermocline waters with silicate, as also seen elsewhere in the glacial ocean, notably the North Pacific (Berger and Lange, 1998).

In the present ocean, the Si/P ratio of subsurface waters may be taken as a measure of thermocline fertility, that is, nutrient concentration in subsurface waters (Berger and Lange, 1998). From this relationship we can derive an estimate for (relative) nutrient abundance, setting Si/P = EDA/(BF/g). Once variation in nutrient abundance is captured, we can derive an estimate of relative strength of upwelling (wind) as the ratio between productivity and nutrient content of upwelled waters (Berger et al., 1994; Herguera and Berger, 1994). In the present case, the "wind" is proxied by (BF/g)²/EDA. From this formulation, "wind" maxima emerge during maximum glaciation, and vice versa. This is not surprising, since the intensity of winds owes much to planetary temperature gradients, which may be taken as especially strong during glacials (Arrhenius, 1952; Flohn, 1985; Leinen and Sarnthein, 1989; Berger and Herguera, 1992).