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CRETACEOUS-TERTIARY (K/T) MASS EXTINCTION: EFFECT OF GLOBAL CHANGE ON CALCAREOUS 80 MICROPLANKTON Figure 4.7 Relative abundance of Cretaceous calcareous nannofossils in basal Tertiary sediments at El Kef. Data based on counts of Cretaceous specimens encountered during a 10-min examination per sample with an optical microscope at 1000x. POPULATION RESPONSE TO K/T DISTURBANCE Relative abundance changes in dominant species groups of both planktic foraminifera and nannofossils yield a more reliable measure of environmental disturbance than systematic diversity. For instance, at the K/T boundary, virtually all planktic foraminiferal species that became extinct were numerically rare (<1%) in the latest Cretaceous ocean, and even relatively minor disturbances of the ecosystem may have caused their demise. In contrast, survivor species dominated the faunal assemblages. In this group, disturbance of the ecosystem is reflected in relative abundance changes and particularly the terminal abundance decline. In each of the sections examined (except Site 528), there are strong similarities in the timing of faunal change among planktic foraminifera, although faunal compositions vary from one region to another. Among calcareous nannofossils the most commonly preserved taxa in the uppermost Maastrichtian are also still the most common in basal Tertiary deposits as also observed in planktic foraminifers (Figures 4.5 and 4.6). However, since these taxa did not give rise to the new Tertiary species and their presence in Tertiary deposits may be due to reworking, Jiang and Gartner (1986) labeled them ''Cretaceous species" (Figure 4.5). Taxa they considered "survivors" or persistent species are very rare in the Maastrichtian and become dominant only in early Tertiary sediments of middle to high latitudes in the Atlantic (Denmark, Biarritz, Zumaya, DSDP Site 524) and Antarctic Oceans (Figure 4.6). These taxa include relatively large extant and new species of the genera Biscutum, Cyclagelosphaera, Markalius, Neocrepidolithus, Zygodiscus, and Placozygus (Figures 4.5 and 4.6). In contrast, such survivors are rare in lower latitudes of the Tethyan Sea (Caravaca and El Kef), where very small new species appear and soon dominate the assemblages. In addition to these floral changes there are increases in the genera Thoracosphaera and Braarudosphaera in both realms, although blooms of the latter are not observed in all sections. The faunal and floral turnovers at El Kef, Caravaca, Brazos, DSDP Site 528, and ODP Site 738 are discussed below. El Kef, Tunisia Figure 4.8 shows the relative abundances of survivor species of planktic foraminifera at El Kef along with changes in δ13C values for planktic and benthic foraminifera (data from Keller, 1988; Keller and Lindinger, 1989). The K/T boundary, defined by the first appearance of Tertiary planktic foraminifera and nannofossils, coincides with an iridium anomaly and spinels (Kuslys and Krahenbuhl, 1983; Robin et al., 1991), a lithological shift from tan colored marls to black clay with a thin basal red layer, a shift of -3.0%o in δ13C values, and elimination of the surface-to deep δ13C gradient, which suggests a drop in surface productivity (Zachos and Arthur, 1986; Keller and Lindinger, 1989). At El Kef the terminal decline of survivor species begins about 10 cm (or about 8000 to 10,000 yr) after the K/T boundary and leads the to near disappearance (<1%) of this group within a few thousand years. The decline of the Cretaceous species and drop in δ13C values are accompanied by the rise and dominance of Guembelitria cretacea (a Cretaceous survivor) in Zone P0 and the evolution of Tertiary species. Figure 4.9 illustrates a sequence of rapidly evolving and changing dominant faunal components in the earliest Tertiary Zones P0 and P1a that seem to be related to low surface productivity as indicated by the very low δ13C values, also observed in numerous low latitude sections (Zachos and Arthur, 1986; Barrera and Keller, 1990). Increased surface productivity (higher δ13C values) at the top of Zone P1a (4 m above K/T) coincides with the disappearance of dominant earliest Tertiary species (G. conusa, P. eugubina), evolution of new species, and increased diversity (Keller, 1988, 1989b; Keller and Lindinger, 1989). This initial recovery of the ecosystem occurred about 250,000 to 350,000 yr after the K/T boundary (Keller, 1988, 1993). Among calcareous nannofossils, semiquantitative data from Perch-Nielsen (1981) indicate that the first group to bloom in the basal Tertiary are species of Thoracosphaera followed by Neobiscutum romeinii and N. parvulum. The interval with common N. parvulum also contains common Lanternithus minutus, Braarudosphaera bigelowii, Chiastozygus ultimus, Cruciplacolithus primus, and Placozygus sigmoides. This increased floral diversification corresponds to the initial increase in surface productivity and first major increase in foraminiferal diversity
CRETACEOUS-TERTIARY (K/T) MASS EXTINCTION: EFFECT OF GLOBAL CHANGE ON CALCAREOUS 81 MICROPLANKTON Figure 4.8 Abundance of Cretaceous planktic foraminiferal species (percent) across the K/T boundary at El Kef in relation to δ13C values of benthic foraminifera and sediment fine fraction. Note the terminal decline of Cretaceous taxa (except for G. cretacea) beginning 10 cm above the K/T boundary. Figure 4.9 Abundance of dominant planktic foraminiferal taxa (percent) in the early Danian at El Kef in relation to benthic (A. acuta) and planktic (<25 mm sediment fraction) δ13C values. Note the increase in species diversity coincident with increased surface productivity at the top of Zone P1a.
