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CRETACEOUS-TERTIARY (K/T) MASS EXTINCTION: EFFECT OF GLOBAL CHANGE ON CALCAREOUS 86 MICROPLANKTON the K/T boundary; instead, there is a gradual elimination of more specialized and generally rare taxa below the K/T boundary. Moreover, all dominant species survive well into the Tertiary and die out as the diversity of new Tertiary species increases (Zones P1b and P1c) The major faunal turnover, however, begins 5 cm below the K/T boundary with a sharp decline in abundances of H. globulosa from 40 to 4% by K/T boundary time, the decline and disappearance of H. complanata, and gradual declines in G. aspera and G. subcarinatus and hedbergellids. The most dramatic change is seen in Chiloguembelina waiparaensis, which increased from 15 to 90% in the 4 cm below the K/T boundary as triserial taxa (guembelitrids) increased in Zone P0 and new Tertiary species evolved. All Cretaceous taxa are dwarfed beginning 4 cm below the K/T boundary and through Zone P0, and reach maturity at less than half their usual size. Indian Antarctic Ocean Site 738C thus illustrates a record of long-term environmental deterioration beginning well before and continuing well after the K/T boundary. It also shows that the demise of the cosmopolitan latest Maastrichtian assemblage cannot have been caused by a K/T boundary bolide impact, but was related to the environmental changes that resulted in a change of sediment deposition from calcareous ooze to laminated clay. Clearly, there was no sudden and catastrophic mass extinction at Site 738C (or in the northern-high latitude Nye Klov section; Keller et al., 1993). In fact, it seems impossible to differentiate the effects of the proposed bolide impact from the continued environmental changes in progress. At most, the bolide impact may have hastened the demise of an already declining latest Cretaceous cosmopolitan fauna. Stable isotope measurements of planktic and benthic foraminifera indicate that there is no negative Î´13C shift across the K/T boundary. In fact, Î´13C values remain stable or increase slightly (0.2%o, Barrera and Keller, 1994). This contrasts with the 3.0%o negative shift observed in low latitudes and supports the interpretation that the effects of the K/T boundary event may have been limited to low latitudes. MAGNITUDE OF K/T DISTURBANCE Planktic Foraminifera The magnitude of the K/T disturbance can be estimated from faunal and floral abundance changes. In Figure 4.14, planktic foraminiferal abundance data are summarized for all four sections. Species and their relative numerical abundances are grouped into (a) species extinct at or before the K/T boundary, (b) survivor species except Guembelitria, (c) Guembelitria, and (d) evolving Tertiary species. The most dramatic change among the five sections is seen in the relative abundances of all species extinct at or before the K/T boundary. For instance, in the Brazos Core (deposited in a middle to inner neritic environment), species extinctions account for less than 5% of the individuals in the total fauna, although one- third of the species disappear. At El Kef, Caravaca, and Site 528, which were deposited in outer neritic to upper bathyal and bathyal depths, respectively, only between 10 and 30% of the individuals are affected, whereas two-thirds of the species disappear. Moreover, Figure 4.14 shows that at El Kef and Site 528 there is a gradual decline in numerical abundance of the disappearing fauna beginning below the K/T boundary. In contrast to these low- and middle-latitude sites, almost all species survive the K/T boundary in the Indian Antarctic Ocean Site 738C (Keller, 1993). These data illustrate that a taxonomic census alone significantly overestimates the environmental effects of the K/T boundary crisis. However, these data also imply that the effect on faunal populations was variable between shallow nearshore and open marine environments, and between low or middle and high latitudes. The numerical abundance of Cretaceous survivors (excluding G. cretacea) shows a similar pattern between El Kef and Caravaca, with a relatively rapid decline above the K/T boundary over about 10,000 yr and a more gradual decline over about 30,000 to 40,000 yr at Brazos. No data are available for Site 528 because of a hiatus. At Site 738C the decline of Cretaceous survivors is masked by the concurrent increase in C. waiparaensis. The Cretaceous survivor Guembelitria generally thrived after the K/T boundary as illustrated at El Kef, but is not present in great abundance at Caravaca in Zones P0 and basal P1a. This absence may be due, however, to a short hiatus at this interval at Caravaca (Figure 4.3). A peak in Guembelitria abundance within Zone P1a is present, however, in both Caravaca and Site 528 (Figure 4.13), as well as El Kef (Figure 4.9), but not at high latitude Site 738C. At Brazos, Guembelitria is common throughout the Maastrichtian and increases in abundance in the early Tertiary. Thus, Figure 4.14 illustrates unequivocally that only a small group of the planktic foraminiferal population became extinct at the K/T boundary, that the dominant group gradually declined and became extinct well after the boundary, and that the adverse effects of the K/T boundary event diminished toward high latitudes. The magnitude of the K/T boundary disturbance is thus significantly overestimated if based on taxic diversity alone or if low- to high latitude differences are disregarded. Calcareous Nannoplankton Quantitative data to estimate the magnitude of the effect of the K/T boundary disturbances are available for El