National Academies Press: OpenBook

Effects of Past Global Change on Life (1995)

Chapter: CAUSE OF OCEANOGRAPHIC AND CLIMATE CHANGE

« Previous: CAUSE OF MASS EXTINCTION IN DEEP SEA
Suggested Citation:"CAUSE OF OCEANOGRAPHIC AND CLIMATE CHANGE." National Research Council. 1995. Effects of Past Global Change on Life. Washington, DC: The National Academies Press. doi: 10.17226/4762.
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TERMINAL PALEOCENE MASS EXTINCTION IN THE DEEP SEA: ASSOCIATION WITH GLOBAL WARMING 102 abundance of these taxa resulted from an increase rather than a decrease in organic productivity, or a decrease in the oxygen content of deep waters, resulting in decreased oxidation of organic matter. A general consensus exists that the mass extinction was caused by a rapid temperature increase of deep waters or the environmental effects associated with higher temperatures, including reduction in oxygen concentrations (Miller et al., 1987; Thomas, 1989, 1990, 1992; Kennett and Stott, 1990, 1991; Stott et al., 1990). Deep waters at the time of the excursion warmed to ~18°C. At such temperatures, deep waters would almost certainly have been depleted in oxygen even if oceanic primary production was lower and atmospheric oxygen levels were higher (Kennett and Stott, 1991). Widespread dysaerobism occurred in the deep ocean. However, there is no evidence that deep waters became completely anoxic, which would have caused an increase in accumulation of organic carbon during the excursion. Furthermore, infaunal benthic foraminifera remained abundant, which would not have been the case if there had been complete anoxia (Bernard, 1986). The dysaerobism was not as extreme as during the Cretaceous "oceanic anoxic events" (Schlanger and Jenkyns, 1976). CAUSE OF OCEANOGRAPHIC AND CLIMATE CHANGE The available data point to a mass extinction at the end of the Paleocene, resulting from physicochemical environmental changes in the deep-sea, especially rapid warming and a decrease in oxygen concentrations. What ocean process could have created such widespread and rapid change? Consensus has developed (Miller et al., 1987; Thomas, 1990, 1992; Kennett and Stott, 1990a, 1991; Katz and Miller, 1991) that the rapid deep-sea warming resulted from a rapid change to near dominance in the deep ocean, of warm saline deep waters (WSDW) produced in the middle-latitude regions (see also Mead et al., 1993). At the same time it is believed that there was a severe reduction in the production of deep waters produced at high latitudes. Such ocean circulation, drastically different from that of the modern ocean, was termed Proteus by Kennett and Stott (1990b; Figure 5.4). In the present ocean, most bottom waters are formed at high latitudes where cold temperatures, in combination with moderately high salinities, cause waters to become dense and sink (for summary see Broecker and Peng, 1982). These waters are relatively oxygen rich. At the same time, warm saline dense waters are formed in the modern Mediterranean and Red Seas as a result of high net evaporation. Because of low buoyancy fluxes, these waters sink only to thermocline depths. These waters do not represent large volumes in the modern deep ocean, although warm saline waters produced in the Mediterranean eventually become an important component of North Atlantic deep water (NADW) (Reid, 1979). Brass et al. (1982) suggested that this warm deep water (>10°C) of the Cretaceous and Early Paleogene reflected production of warm saline deep waters in middle-latitude areas. FIGURE 5.4 General model for deep and intermediate water circulation proposed for the time of the terminal Paleocene isotopic excursion and mass extinction. This model has been termed Proteus by Kennett and Stott (1990b). This is compared with the general circulation of the modern ocean (Oceanus). NOTE: WSDW = warm saline deep water; AAIW = Antarctic intermediate water; AABW = Antarctic bottom water; NADW = North Atlantic deep water; Med = Mediterranean. (Figure modified from Kennett and Stott, 1990b.) The rapid ocean warming associated with the isotope excursion and mass extinction must have occurred from the deep ocean upward. We believe that this was caused by the elimination of deep water formation at high latitudes and the incursion of warm water from middle latitudes. Synchroneity (Figure 5.3) of the mass extinction, the negative δ18 O shift, and the negative δ C shift in the deep- 13 dwelling planktonic foraminifer Subbotina patagonica is apparent. Calculations of rates of sedimentation suggest that the mass extinction occurred in less than about 3000 yr, the oxygen isotopic shift in less than 4000 yr, and the carbon isotopic shift in less than 6000 yr. The mass extinction occurred at the beginning of the oceanographic and climatic changes that mark the excursion. Most of the

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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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