National Academies Press: OpenBook

Effects of Past Global Change on Life (1995)

Chapter: Planktic Foraminifera

Suggested Citation:"Planktic Foraminifera." National Research Council. 1995. Effects of Past Global Change on Life. Washington, DC: The National Academies Press. doi: 10.17226/4762.
Page 76
Suggested Citation:"Planktic Foraminifera." National Research Council. 1995. Effects of Past Global Change on Life. Washington, DC: The National Academies Press. doi: 10.17226/4762.
Page 77

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CRETACEOUS-TERTIARY (K/T) MASS EXTINCTION: EFFECT OF GLOBAL CHANGE ON CALCAREOUS 76 MICROPLANKTON virtually all marine K/T boundary sequences (Keller, 1988, 1993; Canudo et al., 1991; D'Hondt and Keller, 1991; Schmitz et al., 1992). A second short hiatus is frequently present near the P1a/P1b Zone boundary (Figure 4.3), and like the P0/P1a, a hiatus may mark a sea-level lowstand. With the possible exception of El Kef, these two hiatuses appear to be present in the Spanish, Texan, and Israeli sections (Keller et al., 1990; MacLeod and Keller, 1991a,b). At El Kef an interval of reduced sedimentation (stippled pattern) may be present. A similar hiatus and dissolution pattern was observed at ODP Site 738 in the Indian Antarctic Ocean, where deposition occurred at about 1000-m depth (Keller, 1993). Figure 4.3 Extent and temporal distribution of intrazonal hiatuses plotted against the eustatic sea-level curve of Brinkhuis and Zachariasse (1988). Biostratigraphic zonation from Keller (1988) as modified by Keller and Benjamini (1991). It is evident from these analyses that neritic sequences such as those at El Kef and Brazos River, upper bathyal to outer neritic sequences such as those at Agost and Caravaca, and the upper slope Antarctic Ocean Site 738 contain the most detailed records of biological and environmental changes for the K/T transitions. Therefore, it is imperative that the effect of global change on marine microplankton be examined in these temporally most complete K/T boundary transitions. SPECIES RESPONSE TO K/T DISTURBANCE Planktic Foraminifera Are species extinctions across the K/T boundary geologically instantaneous or successive and taxonomically selective? If species extinctions appear geologically instantaneous as illustrated for planktic foraminifera in deep-sea Site 528 (Figure 4.2), then the environmental change must have been caused by a major catastrophe, provided there is no hiatus in the sediment record. However, as discussed in the previous section, a geologically instantaneous mass extinction at the K/T boundary must now be ruled out because this pattern is a result of truncation by a hiatus (Figures 4.2 and 4.3). If species extinctions appear systematic, sequentially eliminating certain morphologies, geographically limited taxa, or habitats over an extended time interval, then long-term environmental changes such as changes in climate, sea-level, rates of seafloor spreading, volcanism, and atmospheric pCO2 are likely causes. However, an extended species extinction pattern does not necessarily rule out the possibility that an extraterrestrial impact (albeit of lesser magnitude than that proposed by the impact theory) occurred some time during this interval. However, recognizing such an impact, and separating its effects on marine microplankton from already ongoing environmental changes, are difficult, if not impossible. The pattern of planktic foraminiferal species extinctions in low latitudes across the K/T boundary transition is best exemplified by the Tunisian El Kef section, which contains the most complete boundary sequence known to date. Figure 4.4 illustrates the sequence of species extinctions at El Kef along with illustrations of species and their sizes relative to each other. By comparing the apparent species extinction pattern in deep-sea Site 528 (Figure 4.2) with that of the mid to outer neritic El Kef section, it is immediately obvious that the patterns of extinction and evolution are dramatically different. Whereas species first and last appearances cluster at the K/T boundary at Site 528, an extended pattern beginning at least 25 cm below the boundary and ending about 4 m above it (Zone P1a) is observed at El Kef. (Note that the scale is reduced for the late Maastrichtian interval, relative to the Paleocene and

MICROPLANKTON CRETACEOUS-TERTIARY (K/T) MASS EXTINCTION: EFFECT OF GLOBAL CHANGE ON CALCAREOUS Figure 4.4 Stratigraphic ranges of planktic foraminifera across the K/T boundary at El Kef. Note the early disappearance of tropical, large, complex, deep, and intermediate water dwellers at or before the K/T boundary and the survival of smaller cosmopolitan surface water dwellers. Evolving Tertiary taxa are small, simple, and unornamented. 77

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What can we expect as global change progresses? Will there be thresholds that trigger sudden shifts in environmental conditions—or that cause catastrophic destruction of life?

Effects of Past Global Change on Life explores what earth scientists are learning about the impact of large-scale environmental changes on ancient life—and how these findings may help us resolve today's environmental controversies.

Leading authorities discuss historical climate trends and what can be learned from the mass extinctions and other critical periods about the rise and fall of plant and animal species in response to global change. The volume develops a picture of how environmental change has closed some evolutionary doors while opening others—including profound effects on the early members of the human family.

An expert panel offers specific recommendations on expanding research and improving investigative tools—and targets historical periods and geological and biological patterns with the most promise of shedding light on future developments.

This readable and informative book will be of special interest to professionals in the earth sciences and the environmental community as well as concerned policymakers.

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