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CRETACEOUS-TERTIARY (K/T) MASS EXTINCTION: EFFECT OF GLOBAL CHANGE ON CALCAREOUS 78 MICROPLANKTON hence concentrates these extinctions near the boundary.) Fourteen species or 33% disappear at 25 cm (6 species) and 7 cm (8 species) below the K/T boundary. Twelve species extinctions (28%) coincide with the K/T boundary, and 6 species (14%) disappear 15 cm above the boundary. Of the remaining 16 Cretaceous species (38%), 8 species disappear in Zones P0 or basal Zone P1a; the remaining 8 species die out gradually in Zones P1a or basal P1b. This extended species extinction pattern is not unique to El Kef, but has also been observed at other low latitude sections including Brazos River (Keller, 1989a,b), Agost, and Caravaca (Canudo et al., 1991). Such similar extinction patterns in regions as widely separated as Tunisia, Spain, and Texas argue against this being an artifact of sample spacing or reworking. Moreover, species extinctions appear to be systematic, affecting tropical complex, large, and highly ornamented morphologies first (globotruncanids, racemiguembelinids, planoglobulinids) followed by subtropical and somewhat smaller morphologies (pseudotextularids, rugoglobigerinids; Figure 4.4). Still, smaller, less ornamented, robust cosmopolitan species with triserial, biserial, and rotalid morphologies (guembelitrids, heterohelicids, globigerinellids) survive the K/T boundary event and disappear gradually during the first 200,000 yr of the Tertiary. Within Tertiary sediments, these survivor species are generally dwarfed, with specimen size averaging about half the size of their Cretaceous ancestors. Their preservation is indistinguishable from the Tertiary fauna in the same samples, indicating that their origin is not likely due to redeposited Cretaceous sediments (Keller, 1988, 1989a,b). Moreover, stable isotopic signatures of the Cretaceous species Guembelitria cretacea and Heterohelix globulosa, unambiguously measure Tertiary values (Barrera and Keller, 1990; Keller et al., 1993). However, despite their ubiquitous presence in lower Danian sediments of continental shelf sections, these Cretaceous foraminifers have been routinely interpreted as reworked and consequently ignored or eliminated from faunal distribution lists (Hofker, 1960; Olsson, 1960; Berggren, 1962; Smit, 1982, 1990; Olsson and Liu, 1993). This interpretation has led to erroneously depicting a sudden extinction of all but one (G. cretacea) Cretaceous species exactly at the K/T boundary, as most recently illustrated by Smit (1990) and challenged by Canudo et al. (1991). Moreover, recent studies of high latitude sequences in Denmark (Nye Klov) and the Indian Antarctic Ocean (ODP Site 738C) have shown that virtually all species survived the K/T boundary and thrived 200,000 to 300,000 yr into the Tertiary (Keller, 1993; Keller et al., 1993). This Cretaceous survivor fauna includes the same cosmopolitan taxa that survived the K/T boundary in low latitudes. In contrast to the more specialized tropical and subtropical taxa, these cosmopolitan species were probably tolerant of wide-ranging temperature, oxygen, salinity, and nutrient conditions. Evolution of Tertiary species begins immediately after the K/T boundary with the appearance of very small, unornamented, and "primitive" morphologies (Figure 4.4). In these aspects they are similar to the surviving Cretaceous fauna. Cretaceous survivors gradually disappeared as Tertiary species diversified and somewhat larger morphotypes appeared. Species diversity, which declined near the K/T boundary from an average of 45 species to about 10 species in the earliest Tertiary Zone P0 and increased to about 15 species in Zone P1a (Figure 4.4) failed to recover until Zone P1c or about 300,000 yr after the K/T boundary. In high latitudes, this recovery coincides with the disappearance of the Cretaceous survivor taxa (Keller, 1993; Keller et al., 1993). Calcareous Nannoplankton Detection of the calcareous nannoplankton species extinction pattern across the K/T boundary transition is seriously hampered by the ease with which these specimens are reworked and the fact that there is usually no way to tell a reworked from an in situ nannofossil based on the state of preservation. For instance, Jiang and Gartner (1986) noted that of nearly 70 species considered to be "vanishing Cretaceous species," only 2 were not found in at least one of up to 60 samples from the 6 m of sediments representing Tertiary Zones NP1 and NP2. The relative abundance of common Cretaceous species, however, decreases gradually up-section as illustrated in Figure 4.5 (Jiang and Gartner, 1986). Recently, a quantitative study of Antarctic Ocean ODP Site 690 by Pospichal and Wise (1990) has revealed a very similar gradual decrease of Cretaceous species in the early Tertiary (Zone CPla; Figure 4.6). A similar pattern is also observed at El Kef where a rapid decrease of Cretaceous species also occurs in nannofossil Zone NP1, but few are still present in Zones NP2 to NP3 (Figure 4.7; Perch- Nielsen et al., 1982). No quantitative data are available from DSDP Sites 577 and 528. However, at the South Atlantic Site 524, Percival (1984) also noted a gradual decline and subsequent disappearance of Cretaceous species in Zone NP1. These patterns of gradual decline of Cretaceous taxa in Tertiary sediments in sections spanning from low to high latitudes are very similar to those observed in planktic foraminifera and strongly suggest survivorship of at least some Cretaceous taxa. As among planktic foraminifera, the vanishing Cretaceous species in lower Tertiary sediments are essentially the same group of species that are common in terminal Cretaceous sediments: Arkhangelskiella cymbiformis, Cretarhabdus crenulatus, Eiffellithus turriseiffelii, Micula
CRETACEOUS-TERTIARY (K/T) MASS EXTINCTION: EFFECT OF GLOBAL CHANGE ON CALCAREOUS 79 MICROPLANKTON decussata, Prediscosphaera cretacea, Nephaolithus frequens, Kamptnerious magnificus, and Watznaueria barnesae. An exception is Prediscosphaera quadripunctata (P. stoveri of other authors), which becomes rare above the boundary. Species of Braarudosphaera and the calcareous dinoflagellate genus Thoracosphaera are the only persistent species that become considerably more common above the boundary in the T. imperforata subzone. Figure 4.5 Abundance of dominant calcareous nannofossils (percent) across the K/T boundary in a Brazos River section (modified from Jiang and Gartner, 1986). Measurements of the size distribution of several Maastrichtian species (Arkhangelskiella cymbiformis, Eiffellithus turriseiffelii, Micula decussata, Prediscosphaera cretacea s. ampl.) from samples just below and just above the boundary in El Kef indicate no significant differences in the size of the specimens, but stable isotope values of Î´13C and Î´18O from fine fraction carbonate show dramatic changes across the boundary (Perch-Nielsen et al., 1982). Since the fine fraction carbonate consists primarily of Maastrichtian calcareous nannofossils (100% below the boundary and >90% above the boundary), it is difficult to explain how reworked Maastrichtian nannofossils could carry a Tertiary isotope signature. This originally led to the suggestion that most Maastrichtian nannoplankton species survived the K/T boundary crisis (Perch-Nielsen, 1981; Perch- Nielsen et al., 1982). More studies of this nature will be necessary to differentiate Cretaceous survivors from reworked populations in Tertiary sediments. Figure 4.6 Abundance of dominant calcareous nannoplankton (percent) across the K/T boundary at ODP Site 690, Weddel Sea, Antarctic Ocean (data from Pospichal and Wise, 1990).