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CRETACEOUS-TERTIARY (K/T) MASS EXTINCTION: EFFECT OF GLOBAL CHANGE ON CALCAREOUS 90 MICROPLANKTON Moreover, recent studies of high latitude sections show that the effects of the K/T boundary event were negligible in high latitudes. DISCUSSION AND SUMMARY We have examined the effect of global change on calcareous marine microplankton across the K/T boundary with respect to four critical topics: (1) the temporal completeness of boundary sequences; (2) the nature of species extinctions; (3) the effect on dominant species populations; and (4) the specific habitats affected. Our investigation has not uncovered any simple answers. The negative effects of global environmental changes across the K/T transition appear complex, prolonged, and unlikely to result from a single cause. We summarize the results below. 1. MacLeod and Keller (1991a,b) have demonstrated that nearly all K/T boundary sequences are temporally incomplete, but that the hiatus patterns may vary between deep-sea and neritic environments. In deep-sea sections a short hiatus or interval of nondeposition that includes the basal Tertiary Zone P0 and part of Zone P1a is virtually always present. In continental shelf sequences, sediment deposition continued at this time, albeit at a reduced rate, and short hiatuses are usually present at the P0/P1a Zone boundaries. This hiatus pattern seems to be linked to the sea-level regression maximum just below the K/T boundary, followed by a rapid transgression across the K/T boundary and into Zone P0 (Schmitz et al., 1992; Keller et al., 1993), which trapped terrigenous sediment and organic carbon on continental shelves and temporarily deprived deep ocean basins of an inorganic sediment source and enhanced carbonate dissolution (Loutit and Kennett, 1981; Haq et al., 1987; Keller, 1988, 1989a, 1993; Keller et al., 1993). The two short hiatuses coincide with sea-level lowstands. In view of this information, the sudden, near-complete mass extinction observed in deep-sea sections reflects artificial truncation of species ranges due to a hiatus as illustrated here for DSDP Site 528. Moreover, the abrupt changes in geochemical tracers (iridium, shocked quartz, soot, Î´13C, CaCO3, total organic carbon) observed in many deep-sea sections also need to be reevaluated, and the possible effects of elemental concentrations during a time of no carbonate or terrigenous sediment deposition must be considered (see Donovan et al., 1988). 2. The record of species extinctions and evolution in the low latitude El Kef section, the temporally most complete K/T boundary sequence known to date, indicates that Cretaceous foraminiferal species extinctions occurred over an extended time period beginning well before and ending well after the K/T boundary. About one-third of the species disappeared before the boundary, nearly one-third disappeared at the boundary, and more than one-third survived and gradually disappeared during the early Tertiary. Moreover species extinctions appear systematic, affecting tropical complex, large, and highly ornamented morphologies first and favoring the survival of more cosmopolitan, smaller, and less ornamented morphologies. Furthermore, Cretaceous survivor species in Tertiary deposits are generally dwarfed. Evolution of Tertiary species begins immediately after the K/T boundary with the appearance of very small unornamented and primitive morphologies. Their increasing diversification and abundance seem to have contributed to the demise of the Cretaceous survivors. This species extinction pattern is representative of depositional sequences that are temporally complete across the K/T boundary in low latitudes (including Caravaca, Agost, Brazos) and illustrates the absence of a near-complete mass extinction as commonly reported from the deep-sea sections that contain a hiatus. Moreover, this gradual species extinction pattern cannot be reconciled with a single geologically instantaneous cause. The species extinction pattern in high latitudes (Site 738C, Nye Klov) is quite different from that in low latitudes. In contrast to low latitudes, no significant species extinctions occur at the K/T boundary and nearly all species survive well into the Tertiary. Yet, as in low latitudes, extinctions occur over an extended time, beginning well below and extending well above the K/T boundary. From low to high latitudes, Cretaceous survivor taxa are primarily small cosmopolitan heterohelicids, hedbergellids, guembelitrids, and globigerinellids. These taxa were able to tolerate changing environmental conditions, whereas the more specialized tropical and subtropical taxa went extinct. The emerging global pattern of species extinctions among planktic foraminifera suggests that the K/T boundary event may have been most severe in low latitudes but that its effect diminished into high latitudes. This interpretation is supported by the -3.0%o shift of Î´13C in low latitudes and absence in high latitudes (Keller et al., 1993; Barrera and Keller, 1994). 3. Changes in the numerical abundances of species are an important factor in evaluating the magnitude of global change. For instance, the numerical abundance of all planktic foraminiferal species extinct at or before the K/T boundary is relatively small, less than 5% in midneritic environments (Brazos) and between 10 and 30% in outer neritic and bathyal depths (El Kef, Caravaca, and Site 528), and this group declined gradually during the late Maastrichtian. Cretaceous survivors dominate during the late Maastrichtian and decline rapidly in the Tertiary over about 10,000 to 40,000 yr in low latitudes when maximum negative conditions are reached accompanied by low primary marine productivity. Thus, only a small group of the planktic foraminiferal population became extinct at or be