National Academies Press: OpenBook

Effects of Past Global Change on Life (1995)

Chapter: Marine Biotas

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Suggested Citation:"Marine Biotas." National Research Council. 1995. Effects of Past Global Change on Life. Washington, DC: The National Academies Press. doi: 10.17226/4762.
Page 125
Suggested Citation:"Marine Biotas." National Research Council. 1995. Effects of Past Global Change on Life. Washington, DC: The National Academies Press. doi: 10.17226/4762.
Page 126

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NEOGENE ICE AGE IN THE NORTH ATLANTIC REGION: CLIMATIC CHANGES, BIOTIC EFFECTS, AND FORCING 125 FACTORS distribution because of temperature changes but nonetheless persisted over broad areas because aridification was less pervasive. Evidently, many species of European mammals were insensitive to changes in the composition of forests. Perhaps exemplifying the greater specificity of extinction in Europe was the disappearance of Paraliurus anglicus, a close relative of the extant lesser panda, which feeds exclusively on bamboo (Kurtén, 1968). Eastern North America Unfortunately, Pliocene terrestrial floras of eastern North America have not been well dated or extensively studied. Floras of the Great Plains, however, reveal that climates were becoming drier during the Miocene Epoch, long before the global climatic changes of mid-Pliocene time. By the Late Miocene (10 to 5 Ma), grasslands were widespread in central North America, while forests were greatly restricted (Axelrod, 1985; Leopold and Denton, 1987). As discussed later, the early spread of grasslands can be related to tectonic events in the American West. Global climatic changes of the Pliocene presumably compounded these effects, but had a less severe impact on floras in central North America than in Europe or, especially, Africa because aridification had already progressed quite far during the Miocene. Similarly, late Cenozoic events of mammalian extinction in North America spanned a considerable interval of time: there were six such events during the past 10 m.y. (Webb, 1984). One of these was a minor episode that occurred early in Pliocene time. A more severe event took place near the end of the Pliocene, eliminating about 55 genera. This one was approximately coincident with the heaviest pulse of Pliocene extinction in Europe, but it is not clear that they shared the same cause. A striking pattern of the North American event was that many species of large mammals, among them peccaries and capybaras, survived by retreating to low latitudes. For some species the geographic shift occurred in stages. Extinction and emigration resulted in a severe impoverishment of modern mammalian faunas in the temperate zone of North America. The northward decline in diversity is especially pronounced today at latitudes above 38° (Webb, 1984). Marine Biotas The onset of the ice age had profound consequences for shallow water marine life of the North Atlantic, especially adjacent to North American and in the Caribbean Sea. Losses were relatively minor for planktonic species, most of which retained access to suitable biogeographic provinces, but heavy extinction left an impoverished bottom-dwelling fauna that has never recovered its diversity. Changes in sea-surface temperatures were the primary agent of extinction. We have noted that relatively warm marine climates with low seasonality characterized mid-Pliocene waters adjacent to the eastern United States, with temperatures reaching the subtropical range as far north as Virginia near the end of Yorktown deposition, when the Gulf Stream was strengthened by closure of the Isthmus of Panama about 3.1 Ma. Thus, benthic faunas of the Western Atlantic were warm adapted and stenothermal, which means that they were highly vulnerable to climatic deterioration. The Pliocene fate of the bivalve mollusks, which has been studied in some detail (Stanley, 1986), presumably typifies the history of the benthic fauna in general. Whereas in California and Japan, mid-Pliocene bivalve faunas contain about 70% extant species, in the Atlantic Coastal Plain only about 20% of mid-Pliocene species survive to the present. Although it might otherwise be tempting to attribute the heavy Western Atlantic extinction to the lowering of sea-level and reduction of shallow seafloor that accompanied glacial expansion during Late Pliocene time, the fact that Pacific faunas experienced no major pulse of extinction rules out eustatic change as a primary agent of extinction. In fact, a broad depositional ramp borders the west coast of Florida, whereas only narrow shelves fringe the Pacific coast of North America. Even during glacial maxima, a large area of shallow seafloor was available for colonization west of Florida; yet extinction here was as heavy as along the Atlantic coast and much heavier than in California. If we take the rate of extinction shared by California and Japan to represent a normal or "background" rate, then the Western Atlantic crisis removed about 65% of all mid-Pliocene species, whereas a much smaller fraction died out through normal attrition (Stanley, 1986). There is strong evidence that the cooling and accompanying increase in seasonality of shallow waters in the Western Atlantic described earlier constituted the dominant cause of heavy extinction. The mid-Pliocene bivalve fauna of west-central Florida, like any marginally tropical biota, included some species restricted to very warm climates and others that ranged into the temperate zone. As it turns out, however, every one of the nearly 60 species surviving from this fauna today range into the temperate zone: around the Gulf coast to Texas or northward along the Atlantic coast at least to the Carolinas. Thus, a thermal filter removed all stenothermal species, leaving a modern fauna dominated by forms with broad thermal tolerances (see Figure 7.4). The Plio-Pleistocene strata exposed along the Atlantic Coastal Plain represent only high stands of sea-level. Although discontinuous, the record here is consistent with the hypothesis that steps of extinction occurred during

NEOGENE ICE AGE IN THE NORTH ATLANTIC REGION: CLIMATIC CHANGES, BIOTIC EFFECTS, AND FORCING 126 FACTORS lowstands representing glacial maxima. Extinction had ended by late Pleistocene time, however. By this time, forms unable to tolerate conditions during the pronounced glacial maxima that began about 0.9 Ma had died out (Stanley, 1986). This pattern of change paralleled that for plants in Europe. What remains is an impoverished bivalve fauna of largely eurythermal species. Many range from the temperate zone to the tropics. In addition, many bivalve species that occupied lagoonal settings before the onset of the ice age are restricted to offshore shelf areas today. Apparently these species cannot tolerate the increased seasonality (in particular, the colder winter temperatures) that now characterize nearshore waters. Only a modest number of new bivalve species evolved during Late Pliocene and Pleistocene time. The extent to which climatic changes may have initiated some speciation in the Bivalvia remains to be investigated, but the onset of the ice age has been credited with triggering the origin of several new species of Western Atlantic ostracods (Cronin, 1988). Figure 7.4 Elongate species (scale bars are 1 cm) of the family Anadaridae that lived in shallow tropical seas of southern Florida at about 3 Ma. Only two of these species ( E and K) survive today, and they range into temperate waters. A: Arca williamsi; B: Arca wagneriana; C: Barbatia floridana; D: Barbatia irregularis; E: Barbatia dominigensis; F: Barbatia leonensis; G: Barbatia taeniata; H: Anadara notoflorida; I:Anadara campsa; J: Anadara improcera; K: Anadara lienosa; L: Anadara propatula. The molluscan fauna of the Caribbean, which was largely distinct from that of eastern North America, experienced a decline that was more or less as severe, although the details remain to be brought to light. In contrast, the molluscan fauna on the Pacific side of the Isthmus of Panama has maintained its very high diversity to the present day. It may be that some of the Caribbean extinctions resulted from reduced upwelling and productivity following the uplift of the isthmus (Vermeij and Petuch, 1986), but the Caribbean fauna suffered losses as far north as the Bahamas at 2.65 Ma (McNeill et al., 1988). This occurrence and the extinction during early Pleistocene time of three species of planktonic foraminifera that were narrowly adapted to the tropical Caribbean (Stanley et al., 1988) suggest that cooling was a major cause of extinction here, as it was in the Western Atlantic. While we do not know the temporal pattern of cooling in the Caribbean, the CLIMAP study showed that during the most recent glacial maximum the sea-surface temperature in the central Caribbean dropped to a level about 4° C below that of today (Prell and Hays, 1976).

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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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