Oceanic regions such as those along the western edges of continents are characterized by upwelling of nutrient-enriched intermediate waters (500-1500 m). The high surface productivity in the oceans is dependent on the rate of supply of nutrients to the surface water where they can be utilized by phytoplankton. Upwelling of nutrient-enriched intermediate waters represents one of the two major sources of nutrients for phytoplankton. The second source of nutrients for the Iberia Abyssal Plain is provided by the nutrient store in the sediment deposited on the Galicia and Lusitanian margins (shelves) during sea-level highstand. As the sea-level falls during glacial intervals, these nutrients return to the sea. With increased supplies of nutrients and increased mixing rates (upwelling), increased productivity in calcareous nannoplankton may be expected. Quantitative studies on calcareous nannofossil abundance correlated to the sea-level curve of Haq et al. (1987) indicate that nannofossil assemblage variations could be mainly related to the sea-level variations (Gaboardi et al., 1994).
On the basis of a detailed quantitative analysis we draw abundance curves of nannofossil species and observe several trends in the nannofossil association that have been related to the sea-level variation.
Changes in species diversity are correlated to the sequence chronostratigraphy in Figure 18. We plotted the total abundance and the nannofossil species diversity to the sea-level curve of Haq et al. (1987, 1988) using both direct and indirect correlations with the transgressive-regressive cycles of the third-order sequence. The first kind of correlation was made possible by drawing bioevents previously correlated to the magnetostratigraphy (e.g., Backman 1987, Rio et al., 1990, and Gartner 1992). The second kind of correlation, indirect, was obtained by positioning other bioevents (Fig. 19) proportionally between two directly correlated bioevents. A general trend (Fig. 18) shows that important decreases in species diversity (e.g., in NP25, NN2, NN4, NN5) occur during short periods of time and in the regressive phase of highstand system tract.
In Figure 19, three classes of environmentally controlled forms were distinguished. Most of the helicolith events occur during a regressive period, most of the sphenolith events are recorded during a transgressive period, and some other species events coincide with the maximum flooding surface (T/R boundary in Fig. 10, Fig. 11, Fig. 12, Fig. 13, Fig. 14, Fig. 15, Fig. 16, Fig. 17) or with the lowstand system tract.
Several trends are observed in the calcareous nannofossil distribution patterns (Fig. 10, Fig. 11, Fig. 12, Fig. 13, Fig. 14, Fig. 15, Fig. 16, Fig. 17). A general signal, corresponding to the maximum flooding surface of the third-order cycle, is determined by a maximum increase in the number of calcareous nannofossil families and genera (e.g., Calcidiscus, Discoaster, Helicosphaera, and Sphenolithus). The correlations with the sea-level curve indicate that Discoaster species with thin rays and small central area have their FO in the lowstand phase of the third-order cycle. Conversely, Discoaster taxa with thick rays and a large central area have their FO in the highstand phase of the third-order cycle (Fig. 15). In the middle Miocene, the first marked increase in Discoaster with a central knob occurs during the regressive phase. The late Miocene hiatus at Holes 897C and 900A does not allow us to conclude the presence or absence of a central knob with sea level variation.
The presence of common helicoliths in the association was often referred to shallow-water, near-shore conditions (Bukry et al., 1971). In the Iberian Abyssal Plain the sea-floor depth of the four studied Sites is nearly 5000 m. Although the water depth at the time of the sedimentation was probably shallower than at present day, occurring above the CCD, as proved by the nannofossil preservation and abundance, it is presumable that the basin conditions were not shallow water. On the correlation of peaks of abundance of helicoliths (Fig. 11) with regressive phases of the third-order cycle, we interpret the common abundance of helicoliths related to nutrient supply from sediments from the margins during the sea level falls.
Another group of calcareous nannofossils often related to shallow-water conditions is represented by holococcoliths (Kleijne, 1991). The correlation with the sea-level curve (Fig. 10) shows that peaks in abundance of holococcoliths coincide with regressive sequence, and the same explanation as the one above suggested for helicoliths could be true also for holococcoliths.
Abundance patterns of selected sphenoliths correlated with the sea level curve (Fig. 13) evidence peaks in abundance in coincidence of the maximum flooding surface or within the transgressive system tract of the third-order cycle, suggesting a preference for more open ocean conditions for this group.
Moreover, reworked Cretaceous specimens (Fig. 12) are plotted against the sea level curve and their abundance increases in the regressive phase of a third-order cycle. Gaboardi et al. (1994) in the south central Pyrenees show that reworked Cretaceous taxa increase and reach a maximum during the highstand system tract of fourth-order units, influenced by the prograding sediments that induce a relative sea-level fall.
At the scale of third-order sequences, several preferences in the nannofossil association were recognized in relation to different physical phases. A statistical approach would be necessary to recognize more univocal significance of calcareous nannofossil abundance patterns. Such an approach could be the subject of further work.