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CRETACEOUS-TERTIARY (K/T) MASS EXTINCTION: EFFECT OF GLOBAL CHANGE ON CALCAREOUS 84 MICROPLANKTON low surface productivity prevails during most of Zone P1a, and increasing but fluctuating values mark the top of Zone P1a (Figure 4.11; Keller, 1989a; Barrera and Keller, 1990). Calcareous nannofossil abundance changes in the nearby Brazos-1 section show a similar pattern of rapid decline beginning near the K/T boundary and ending about 1 m above (Jiang and Gartner, 1986; Figure 4.5). The decline of the Cretaceous species is accompanied by increased abundance of Thoracosphaera spp. followed by Braarudosphaera and N. romeinii. As at El Kef and Caravaca there are some differences in the succession of N. parvulum and F. petalosa abundance distributions. At Brazos, abundance peaks of these taxa overlap with the maximum abundance of C. primus, whereas at Caravaca and El Kef the C. primus and F. petalosa peaks follow after the abundance peak of N. parvulum. It is possible that these differences are due primarily to differing taxonomic concepts among nannofossil workers especially for N. parvulum, a very small species that is difficult to identify with an optical microscope. DSDP Site 528, South Atlantic Relative abundances of planktic foraminiferal species are illustrated in Figure 4.12 for Site 528. Although the K/T boundary is located between two cores, a laminated boundary transition was recovered in the core catcher (D'Hondt and Keller, 1991). As noted earlier, Site 528 has a hiatus or interval of nondeposition at the K/T boundary and Zone P0, and the lower part of Zone P1a is missing (Figure 4.3). Moreover, nannofossil assemblages lack the characteristic basal Tertiary taxa N. romeinii, N. parvulum, and F. petalosa (Manivit, 1984; Manivit and Feinberg, 1984). Instead, a typical high latitude Zone NP1 assemblage with common Thoracosphaera and Braarudosphaera is present immediately above the K/T boundary. Thus, it is because of a hiatus, rather than catastrophic extinctions, that all planktic foraminiferal species truncate at the K/T boundary and Tertiary species suddenly appear. Because of this hiatus, which is present in virtually all deep-sea sections (Worsley, 1974; MacLeod and Keller, 1991a,b), the ecological response to the K/T boundary disturbance cannot be evaluated from such deep-sea sections. However, it is interesting to note that as in continental shelf sections, a similar species assemblage consisting of small cosmopolitan heterohelicids, globigerinellids, and hedbergellids dominates the late Cretaceous at Site 528, and there seems to be a trend toward decreased abundance in the heterohelicids, globotruncanids, and globotruncanellids in the terminal Cretaceous, indicating a changing environment. ODP Site 738C, Indian Antarctic Ocean Relative abundances of planktic foraminifera for Site 738C are illustrated in Figure 4.13. In this southern high latitude section the K/T transition is preserved within a 15-cm-thick laminated layer (which has been analyzed at 1-cm intervals), with the K/T boundary and Ir anomaly 2 cm above the base of the laminated layer (Schmitz et al., 1991; Thierstein et al., 1991; Keller, 1993). Planktic foraminiferal changes are dramatic across the K/T transition, but do not coincide with the Ir anomaly and K/T boundary. No significant species extinctions coincide directly with Figure 4.12 Abundance of dominant planktic foraminiferal species (percent) across the K/T boundary in DSDP Site 528 (data from D'Hondt and Keller, 1991). Note the sudden faunal change at the K/T boundary is due to a hiatus that encompasses Zone P0.
MICROPLANKTON Figure 4.13 Planktic foraminiferal turnover at the centimeter-scale in the laminated interval that spans the K/T boundary in the Antarctic Indian Ocean ODP Site 738C. Note the CRETACEOUS-TERTIARY (K/T) MASS EXTINCTION: EFFECT OF GLOBAL CHANGE ON CALCAREOUS absence of significant species extinctions at the K/T boundary, and the survivorship of all dominant Cretaceous species well into the Tertiary, but an onset of decline in their relative abundances beginning below the K/T boundary. Data from Keller (1993). 85