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CRETACEOUS-TERTIARY (K/T) MASS EXTINCTION: EFFECT OF GLOBAL CHANGE ON CALCAREOUS 87 MICROPLANKTON Kef, Brazos River (Perch-Nielsen et al., 1982; Jiang and Gartner, 1986), and ODP Sites 690, 738, and 761 (Pospichal and Wise, 1991; Wei and Pospichal, 1991; Pospichal and Bralower, 1992). However, as noted earlier, their interpretation is not unambiguous and depends on whether the so-called vanishing Cretaceous taxa are assumed to have survived at least partly into the basal Tertiary, or whether they are considered to be reworked above the K/T boundary. Figure 4.14 Faunal turnover across the K/T boundary in planktic foraminifera between midneritic and bathyal environments based on percentage of the population eliminated (number of individuals in all taxa that disappear at the K/T boundary) and percentage of the population surviving. Note the decreasing effect into shallower waters in percentage of fauna eliminated and the prolonged survivorship of the Cretaceous fauna. If we assume that the vanishing Cretaceous species survived into the early Tertiary, then their abundance rapidly decreases to less than 10% by the N. romeinii subzone in the Brazos section. This decline in Cretaceous nannofossils parallels that of Cretaceous planktic foraminifers in the basal 1 m of the Tertiary at Brazos. A similar rapid decline in vanishing Cretaceous species is observed at El Kef. In higher-latitude sections (including DSDP Site 524, ODP Sites 690C, 738, and 761, Biarritz (France) and the Danish sections), vanishing Cretaceous species seem to remain relatively common (10 to 30%) into the upper part of NP1 (Perch-Nielsen, 1979a,b; Pospichal and Wise, 1991; Wei and Pospichal, 1991; Pospichal and Bralower, 1992). Their decrease is accompanied by a rapid increase in Thoracosphaera and, in high latitudes, also by an increase in persistent species. If we assume that all vanishing Cretaceous species became extinct at the K/T boundary and their presence in Tertiary sediments is due to reworking, then the extinction rate exceeds 100 species per year, compared to an average of 1.5 species per million years (m.y.) and 5% per m.y. estimated by Perch-Nielsen (1986) and Roth (1987), respectively, for the Late Cretaceous. Given the uncertainties in this assumption, this estimate is not a satisfactory approximation of the magnitude of the K/T boundary event on the species level. Extinctions clearly occurred at a rate at least one order of magnitude higher than during the Late Cretaceous. On the genus level about 18 of some 50 to 60 Cretaceous genera survived into the Tertiary (Perch-Nielsen, 1985, 1986). Among these, 4 genera are still extant; the remainder disappeared during the Paleocene (7 genera) and Eocene (7 genera) at an average rate of about 0.5 genus/m.y. This rate of extinction is similar to that of Paleocene to Middle Eocene floras in general. Because of the present ambiguity in determining whether Cretaceous species in basal Tertiary deposits are reworked or in situ, an accurate estimate of the effect of the K/T crisis based on nannoplankton cannot be obtained. ARE SPECIFIC HABITATS SELECTIVELY DESTROYED? Most planktic foraminifera live in the upper 200 to 400 m of the water column and within this interval can be grouped into surface, intermediate, or deep dwellers based on their oxygen and carbon isotope ranking and the assumption that they grew their shells in isotopic equilibrium with the sea water in which they lived (see Table 4.1; Douglas and Savin, 1978; Boersma and Shackleton, 1981; Stott and Kennett, 1989; Lu and Keller, 1993). Species
CRETACEOUS-TERTIARY (K/T) MASS EXTINCTION: EFFECT OF GLOBAL CHANGE ON CALCAREOUS 88 MICROPLANKTON living in warm surface waters have the lightest δ18O and heaviest δ13C values, whereas species living in deeper, cooler waters have successively heavier δ18O and lighter δ13C values. Variations in the distribution of these groups may indicate environmental changes related to climate, ocean circulation, and water chemistry. Because stable isotope ranking of Cretaceous species is still preliminary and awaiting further analyses of individual species, Table 4.1 groups by genera. However, future isotope analyses may show that not all species within a genus inhabited the same depth environment, or that some species calcified their shells in disequilibrium with the seawater in which they grew. Heterohelix globulosa is one example of a species that calcified in isotopic disequilibrium or changed its habitat over time. In the Cretaceous, this species has been reported with relatively heavy δ18O values in the open ocean (although other heterohelicid species have light values), indicating that it lived at thermocline depths (Boersma and Shackleton, 1981). However, in the shallow water (<100 m) Brazos sections (where this species is dominant throughout the Late Cretaceous and into the early Tertiary (Keller, 1989a), it apparently adapted to living in a shallower environment. The same pattern has been observed in the shallow water Stevns Klint and Nye Klov sections (Schmitz et al., 1992; Keller et al., 1993). In this study H. globulosa has therefore been grouped with surface dwellers. In Figure 4.15, the Cretaceous fauna has been grouped into surface, intermediate, and deep dwellers based on two faunal parameters, the percentage of species in each group and the percentage of individuals in each group. The latter is a more sensitive parameter of environmental change because it weighs species according to their relative abundance in the total fauna, whereas the former gives each species the same significance regardless of whether it is represented by 1% or 80% in the faunal assemblage. These data are illustrated in Figure 4.15 along with the stable isotope record for each of the four sections, which are arranged from shallow (Brazos) to deep (Site 528). TABLE 4.1 Oxygen Isotope Ranking of Cretaceous Species Surface Intermediate Deep Pseudoguembelina Globotruncana Globotruncanella Rugoglobigerina Rugotruncana Racemiguembelina Heterohelix Hedbergella Contusotruncana Shakoina Pseudotextularia Gublerina Globigerinelloides Planoglobulina Both faunal parameters in the four sections show a strong trend toward decreasing deep and intermediate dwellers and increasing surface dwellers. About 15 to 20% of the species are deep dwellers at Site 528 (midbathyal) and Caravaca (upper bathyal), and a decrease to 8 and 12%, respectively, occurs in the latest Maastrichtian (Figure 4.15). At El Kef (outer neritic) an average of 10% of the species are deep dwellers, and at Brazos 0% (midneritic). The relative abundance of individuals in this group shows the same trend, but they comprise a significantly smaller part of the population except at Site 528. Abundance of deep dwelling individuals decreases from 15 to 20% at Site 528, to 5 to 10% at Caravaca and El Kef, and 0% at Brazos. All deep dwellers disappear at the K/T boundary. Intermediate dwellers show a similar trend. At Site 528, 5 to 10% of the species are in this group; at Caravaca 15 to 20%; at El Kef 10 to 15%; and at Brazos between 10 and 20% decreasing up-section and disappearing well below the K/T boundary (Figure 4.15). The relative abundance of individuals in this group is smaller. At Site 528, maximum abundance is 10%, decreasing to 2% well below the K/T boundary; at Caravaca maximum 5% decreasing up-section to <2%; at El Kef <2%; and at Brazos <1%. Like the deep water dwellers all intermediate dwellers disappear at the K/T boundary. Surface dwellers are the largest component of the late Maastrichtian fauna. Their abundance increases from 75 to 85% in the open marine Site 528, to 85 to 90% at Caravaca and El Kef, and 99 to 100% at Brazos. In all sections the dominant surface dwellers are cosmopolitan taxa and survive the K/T boundary event; no data are available for Site 528 because the earliest Tertiary is missing. The relative abundance of dominant survivor species rapidly declines in the boundary clay at Caravaca and El Kef, but at Brazos, Cretaceous survivors thrive well into Zone P1a. Rapid expansion of the evolving Tertiary fauna accompanies the decline of Cretaceous survivors. Figure 4.15 thus illustrates environmental conditions in four different geographic localities and at four different water depths. Some differences between these sections are due to changing water depths and some are due to other global environmental changes. For instance, the variations in the faunal assemblages among the four sites are due primarily to changing water depth and its effect on the thermocline and vertical stratification of species. Global sea-level changes and their effect on water mass stratification are indicated by the decrease and disappearance of deep and intermediate groups at each locality, especially in the middle to upper bathyal and outer neritic environments of Site 528, Caravaca, and El Kef. These gradual faunal changes indicate a changing environment beginning 100,000 to 300,000 yr before the end of the
CRETACEOUS-TERTIARY (K/T) MASS EXTINCTION: EFFECT OF GLOBAL CHANGE ON CALCAREOUS 89 MICROPLANKTON Maastrichtian (Keller and Barrera, 1990). The terminal disappearance of all deep and intermediate dwellers near the K/T boundary, however, indicates accelerated environmental changes that favored the survival of cosmopolitan surface dwellers. The effect of these changes may have included global warming, disruption of the vertical water mass stratification and thermocline, and rapid reduction of marine primary productivity. Stable isotope data indicate that both warming and a drop in marine productivity beginning at the K/T boundary were restricted to low latitudes. However, through the latest Maastrichtian, stable isotope data indicate high marine productivity. The gradual faunal changes prior to the K/T disturbance imply Earthderived causes and may be related to the latest Maastrichtian sea-level regression followed by a transgression across the K/T boundary clay (Schmitz et al., 1992; Keller et al., 1993), but large-scale volcanism and mantle plume activity may also have been contributing factors. However, the considerably smaller mass extinction in planktic foraminifera than generally assumed suggests that the effects of a bolide impact on marine plankton would have been significantly less catastrophic than proposed by current theory. Figure 4.15 Cretaceous planktic foraminiferal species grouped into surface, intermediate, and deep water dwellers based on stable isotope ranking. Data are presented in terms of percentage of species and fauna (individuals) in each category and are arranged from shallow to deep. Stable isotope data from Lindinger (1988), Keller and Lindinger (1989), and Barrera and Keller (1990). Note: Species data overemphasize the magnitude of the K/T boundary effect when compared to the fauna affected; there is a decreasing effect from deep to shallow water environments; there is a gradual decline in the Cretaceous deep and intermediate dwellers and all become extinct at the K/T boundary, only surface dwellers survive.
