National Academies Press: OpenBook

Effects of Past Global Change on Life (1995)


Suggested Citation:"HOW COMPLETE ARE K/T BOUNDARY SECTIONS?." National Research Council. 1995. Effects of Past Global Change on Life. Washington, DC: The National Academies Press. doi: 10.17226/4762.
Page 74
Suggested Citation:"HOW COMPLETE ARE K/T BOUNDARY SECTIONS?." National Research Council. 1995. Effects of Past Global Change on Life. Washington, DC: The National Academies Press. doi: 10.17226/4762.
Page 75

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CRETACEOUS-TERTIARY (K/T) MASS EXTINCTION: EFFECT OF GLOBAL CHANGE ON CALCAREOUS 74 MICROPLANKTON Figure 4.1 Locations of K/T boundary sections examined for this study plotted on a paleogeographic reconstruction of continental positions at the time of the K/T boundary (66.4 Ma). White = ocean basins; light stipple = continental platforms; black = inferred extent of terrestrial exposure. HOW COMPLETE ARE K/T BOUNDARY SECTIONS? To test competing causal hypotheses or to evaluate the effect of global change on marine microplankton, accurate estimates of the stratigraphic and temporal completeness of individual K/T boundary sections are needed. In current stratigraphic analysis the blueprint for judging complete and continuously deposited K/T boundary sequences seems to be the presence and relative thickness of a boundary clay, the presence of nannofossil zones Micula prinsii and Neobiscutum romeinii and planktic foraminiferal Zone Guembelitria cretacea (P0) followed by the Parvularugoglobigerina eugubina Zone (Pla) (Perch-Nielsen et al., 1982; Smit, 1982; Smit and Romein, 1985). By this definition, many K/T boundary sections have been judged as "relatively complete" even though the earliest Tertiary Zone P0 or the N. romeinii Zone are missing. Moreover, this method of judging chronostratigraphic completeness does not allow the recognition of the existence of intrazonal hiatuses, and may thus lead to erroneous interpretations of the nature and rate of environmental change. Foraminiferal workers have generally examined deep-sea sections, which were believed to be more complete than shallower continental shelf sections, and found that virtually all planktic foraminiferal species disappeared simultaneously at the K/T boundary. This pattern of species extinctions is illustrated in Figure 4.2 for DSDP Site 528, which contains a relatively continuous sedimentary record with a thin laminated boundary interval (in the core catcher) (D'Hondt and Keller, 1991). Yet, as in nearly all deep-sea sections, the basal Tertiary Zone P0 and probably part of Zone Pla are missing. By judging DSDP Site 528, as well as all other low- to mid-latitude deep-sea sites that show the same sudden mass extinction and absence of the basal Tertiary Zone P0 as representing a temporally complete record, the obvious interpretation was a geologically instantaneous catastrophic event such as a bolide impact. This interpretation has even been applied to the stratigraphically more complete K/T boundary sequences at El Kef and Caravaca by Smit (1982, 1990), based on the assumption that all Cretaceous species present above the K/T boundary (except G. cretacea) are reworked (see Canudo et al., 1991, for a discussion of this problem). Among nannofossil workers, Worsley (1974) speculated that major hiatuses characterized all deep-sea sections and shorter hiatuses were present in marine-shelf sections. This observation tended to be supported by the later discovery of the new uppermost Maastrichtian nannofossil Zone Micula prinsii and an unnamed basal Tertiary interval below the Neobiscutum romeinii subzone at El Kef by Perch-Nielsen (1979a). Despite this early recognition of an incomplete K/T boundary record in the deep-sea by both foraminiferal and nannofossil workers, few attempts at systematic chronostratigraphic analysis have been made and only for a single or a few sections (Thierstein, 1982; Herbert and D'Hondt, 1990), largely because high-resolution stratigraphic records have not been available. Thus, what has been missing from the K/T controversy is a comprehensive chronostratigraphic and biostratigraphic data synthesis for all K/T transitions in marine depositional settings that would allow determination of the temporal completeness of each section and thereby clarify the nature of the extinction record. Such an analysis has recently been completed by MacLeod and Keller (1991a,b), who used the graphic correlation method of Shaw (1964) to compare the distribution of biostratigraphic and lithostratigraphic datums in 15 relatively complete boundary sections. These comparisons resulted in a composite estimate of 76 latest Maastrichtian to Early Paleocene (Zones P0-P1c) biostratigraphic datums for planktic foraminiferal and calcareous nannofossil species that corrects for intersequence diachroneity and allows correlations for individual sections to be made within a common chronostratigraphic model. Based on this method, analysis of 29 K/T boundary sections suggests that short, global, intrazonal hiatuses of varying duration are present in virtually

CRETACEOUS-TERTIARY (K/T) MASS EXTINCTION: EFFECT OF GLOBAL CHANGE ON CALCAREOUS 75 MICROPLANKTON all sections previously considered relatively complete. Moreover, dramatically different patterns of sediment accumulation appear to characterize neritic and deep-sea depositional environments following the K/T boundary. Figure 4.2 Stratigraphic ranges of planktic foraminifera at DSDP Site 528 (data from D'Hondt and Keller, 1991). Note that the abrupt termination of all Cretaceous species and sudden appearance of Tertiary species are the result of a hiatus that encompasses Zone P0. Figure 4.3 illustrates this pattern for the sections discussed in this chapter along with the sea-level curve of Brinkhuis and Zachariasse (1988) based on dinoflagellate data from El Kef. Minimum estimates for hiatuses are given in black; maximum estimates are shown in stippled pattern. At DSDP Sites 528 and 577 as well as 10 other deep-sea sections examined (MacLeod and Keller, 1991a,b) an interval of nondeposition or short hiatus, which includes Zone P0 and the lower part of Zone P1a, characterizes the earliest Tertiary. This hiatus is coincident with a rapid transgression following the pre-K/T boundary maximum regression. Sediment accumulation occurs at this time, mainly in mid to outer shelf and upper bathyal environments as indicated by the presence of Zone P0, although a condensed interval or very short hiatus may be present in these sections also (Brazos, Caravaca, Agost). This sediment pattern suggests that the rapid sea-level transgression in Zone P0 trapped terrigenous sediment and organic carbon on continental shelves and temporarily deprived deep ocean basins of an inorganic sediment source and enhanced carbonate dissolution (Berger, 1970; Berger and Winterer, 1974; Loutit and Kennett, 1981; Haq et al., 1987; Donovan et al., 1988). Increased carbonate dissolution in the basal Tertiary (Zone P0) is widely recognized in

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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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