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Effects of Past Global Change on Life
The Cretaceous-Tertiary boundary transition is marked by one of the most dramatic environmental changes in the Earth's history, with both cause and effect still vigorously disputed. Presently the most popular theory of the cause of this global change is a large extraterrestrial bolide impact. Supporters of this theory cite anomalously high concentrations of noble elements and shocked mineral grains in a thin boundary clay layer in the marine and terrestrial realm as sufficient evidence of a bolide impact. In addition, Alvarez et al. (1980) viewed the reported sudden extinction of all but one Cretaceous planktic foraminiferal species as evidence of the catastrophic effect of this impact on Earth's biota. Alternative theories invoke long-term Earth-derived causes such as large-scale volcanism (McLean, 1985; Officer and Drake, 1985), mantle plume activity (Loper and McCartney 1986, 1988), sea-level fluctuations, and climate changes (Hallam, 1989) as seen in the geological record spanning the K/T boundary transition, as well as the decline and extinction of many fossil groups during the late Maastrichtian (Kauffman, 1984; Sloan et al., 1986; Keller, 1888, 1989b; Keller and Barrera, 1990; Canudo et al., 1991). Although no evidence presented to date conclusively supports either extraterrestrial or Earth-derived causes, the magnitude of this global change remains undisputed. What is unclear, however, is the nature, tempo, and geographic extent of this mass extinction.
Regardless of the ultimate cause of this global change, the effect on life and especially on calcareous marine microplankton can be evaluated from the geological record. Shells of calcareous nannofossils and planktic foraminifera contribute the bulk of marine sediments and provide a record not only of species extinction and evolution, but also of population dynamics in response to environmental perturbations. A record of climate and productivity changes is retained in the carbon and oxygen isotope ratios of their shells. No other fossil group provides as much information on the changing environment across the K/T boundary transition.
Despite the rich information contained in the calcareous marine microplankton, specialists still differ considerably in their interpretation of the extinction record, with one group arguing for catastrophic extinctions of nearly the entire fauna and flora at the K/T boundary and the other group arguing for an extended extinction period. Among nannofossil experts, the difference in opinion centers on whether Cretaceous species in Tertiary sediments are reworked or survivors (Perch-Nielsen, 1979a,b; Perch-Nielsen et al., 1982; Thierstein, 1982; Jiang and Gartner, 1986). Unfortunately, the very small size of nannofossils, and their easy transportation and redeposition, make determination of in situ assemblages a nontrivial problem, and neither view can be proven beyond reasonable doubt.
Among foraminiferal experts, differences rest largely on two apparently contradictory extinction records, the deep-sea versus continental shelf or neritic environments. Deep-sea records show all but one Cretaceous species extinct at the K/T boundary, apparently supporting a catastrophic impact (e.g., Smit and Romein, 1985; D'Hondt and Keller, 1991). In contrast, neritic sections show an extended period of extinctions beginning before the K/T boundary and continuing well into the Tertiary; thereby providing only weak support for a geologically instantaneous catastrophic event (Keller, 1988, 1989a,b; Canudo et al., 1991; Huber, 1991). Impact supporters reconcile these different records by interpreting Cretaceous species present above the K/T boundary as reworked (e.g., Smit, 1982, 1990; Smit and Romein, 1985).
Unlike the unresolved problem of reworked nannofossils, reworked Cretaceous foraminiferal species in Tertiary deposits can be positively identified by their physical and preservational characteristics, as well as by their d13C isotope signal (Barrera and Keller, 1990; Keller et al., 1993). Based on these criteria, up to one-third of the Cretaceous species may have survived the K/T boundary event. In contrast, the near total extinction of Cretaceous species in the deep-sea appears to be an artifact of a temporally incomplete stratigraphic record as discussed below (MacLeod and Keller, 1991a,b). Recently, the study of marine K/T sections in northern and southern high latitudes has revealed no significant species extinctions and the survival of nearly all species well into the Tertiary, which indicates that the effects of the K/T mass extinction may have been limited to lower latitudes (Keller, 1993; Keller et al., 1993).
In this chapter we examine the effects of global environmental changes on calcareous marine microplankton across the K/T boundary in deep-sea and continental shelf sequences. We have chosen the temporally most complete sections known to date: El Kef in Tunisia, Caravaca in Spain, Brazos River in Texas, Deep-Sea Drilling Program (DSDP) Site 528 in the South Atlantic, and Ocean Drilling Program (ODP) Site 738C in the Indian Antarctic Ocean (Figure 4.1). These four sections span bathyal to neritic environments from high to low latitudes and thus provide a cross section of marine ecosystems. We address four critical and interrelated topics:
How complete are K/T boundary sequences?
What is the nature of species extinctions?
What is the effect on dominant species populations?