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Effects of Past Global Change on Life
and contains two short hiatuses (Figure 4.3), the ranges are truncated. Similar to El Kef, d13C values at Caravaca also show low surface productivity through Zone P0 and the lower part of Zone P1a, with initial recovery in the upper part of this zone (Figure 4.10).
Calcareous nannofossil distribution at Caravaca is also similar to El Kef, with a succession of dominant Thoracosphaera, B. bigelowii, and N. parvulum parallel with the major decrease of Cretaceous nannofossils in the basal Tertiary (Romein, 1979; Romein and Smit, 1981). Differences in floral successions include the possible absence of a bloom of N. romeinii, which has not yet been studied, and the occurrence of common Octolithus multiplus preceding an abundance peak of F.petalosa.
Brazos River sections do not have a well-defined boundary clay but contain a 2-mm thin red-brown layer that exhibits the iridium anomaly (Beeson et al., in preparation). The first appearance of Tertiary foraminifera and nannofossils places the K/T boundary at this thin red-brown layer and Ir anomaly (Jiang and Gartner, 1986; Keller, 1989a) and at the onset of the d13C shift in the Brazos Core section (Barrera and Keller, 1990) (Figure 4.11). The same group of Cretaceous survivors dominates foraminiferal assemblages, although because of the shallow neritic water depth, H. globulosa is most abundant. There is no major faunal change apparent below or at the K/T boundary. Above the boundary, Cretaceous species thrive well into Zone P1a (1 m above the K/T) similar to Nye Klov, Denmark (Keller et al., 1993), in contrast to Caravaca and El Kef where Cretaceous survivors become rare within or immediately above Zone P0. However at Brazos, H. globulosa populations also begin their terminal decline in Zone P0, parallel with a gradual decline in d13C values of H. globulosa and the benthic species Lenticulina (Figure 4.11; Barrera and Keller, 1990). This is the first time that the d13C shift has been observed as a gradual rather than a sudden change (Hsu et al., 1982; Zachos et al., 1986) and implies deteriorating environmental changes beginning within a few thousand years of the K/T boundary.
Above the K/T boundary the Cretaceous survivors G. cretacea and G. trifolia dominate (50 to 60%) through Zones P1a-P1b (Figure 4.11). Such prolonged dominance of this group has not been observed in other sections and seems to be the result of local ecological conditions. However, the evolutionary sequence of early Tertiary planktic foraminiferal species is similar to that of Nye Klov, ODP Site 738C, El Kef, and Caravaca, although P. eugubina is rare, presumably because of the shallow neritic environment. The d13C curve is similar to El Kef and Caravaca in that
FIGURE 4.11 Abundnace of planktic foraminifera (percent) and d13 C values of benthic (solid diamond = Lenticulina sp.; open diamond = analysis of single specimens) and planktic (triangles = H. globulosa) foraminifera across the K/T boundary in the Brazos Core section. Note the abundance of Cretaceous specimens in the basal Tertiary and their terminal decline parallel with the gradual decrease in d13C values of both planktic and benthic foraminifera. (Stable isotope data from Barrera and Keller, 1990.)