Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
TROPICAL CLIMATE STABILITY AND IMPLICATIONS FOR THE DISTRIBUTION OF LIFE 114 sity, Stehli and Wells (1971) describe the importance of tropical temperature by noting the decrease in diversity with temperature from 50 genera at a mean temperature of 27°C to half that number at only 24°C. Tropical warming, on the other hand, might expand the latitudinal range of some organisms or displace organisms outside the tropics if some upper limit in temperature or salinity were exceeded. Quantitative data on tolerances of organisms from the geologic record are obviously very limited. Experiments with modern organisms provide some guides. Vaughn and Wells (1943) performed a number of tank experiments on hermatypic corals for different temperatures and salinities. Tolerance limits were defined by evidence for notable damage or death due to exposure of only 12 to 24 hr. The optimum temperature range for corals was found to be 25 to 30°C, with minimum and maximum tolerances of 18 to 20°C and 35 to 37°C, respectively. The optimum salinity for growth was found to be 36%o, with minimum and maximum tolerances of 17 to 28%o and 40 to 48%o, respectively. Since, the minimum and maximum tolerances were defined by damage over a very short time (24 hr), logically longer exposures could well result in narrower temperature limits or competitive disadvantage. Knowledge of organism tolerances provides crucial data for examining the question of tropical temperature variation. A MID-CRETACEOUS CASE STUDY Four lines of evidenceâisotopic paleotemperatures, climate model results, the distribution of climate-sensitive organisms, and quantitative estimates of tropical tolerancesâprovide the basis for case studies of tropical climate sensitivity and the response of organisms during Earth history. From the viewpoint of both the availability of all lines of evidence and the interest in warm geologic climates for future global change considerations, the mid-Cretaceous is an interesting and valuable case study. Mid-Cretaceous atmospheric GCM experiments have been completed for Cretaceous geography with the present level of atmospheric CO2 and Cretaceous geography with four times the present-day levels of atmospheric CO2 (Barron and Washington, 1984). These atmospheric simulations were utilized to drive an ocean GCM (Barron and Peterson, 1990) that simulates surface ocean temperatures and salinities. Figure 6.4 illustrates the tropical areas that exceed the temperature optimum (30°C) and the salinity optimum (37%o) for each of the two simulations. A substantial portion of the Tethyan Ocean exceeds the optimum conditions for corals as described by Vaughn and Wells (1943). There are substantial differences between the two simulations. In the high CO2 simulation, the optimum is exceeded over much of the tropical latitudinal band. In the simulation with present-day CO2 levels, the optimum is exceeded largely within Tethys. Figure 6.4 Ocean GCM predictions above the optimum conditions for coral growth for temperature (â¥30°C) and salinity (â¥37%o): (a) mid-Cretaceous simulation; (b) mid-Cretaceous simulation with atmospheric CO2 concentrations at four times the present value. The region of warm temperatures and higher salinities corresponds closely to the Supertethys zone of Kauffman and Johnson (1988), which was dominated by rudistid bivalves. The history of reef-building rudists has been documented by numerous authors (Kauffman and Sohl, 1974; Scott, 1988; Kauffman and Johnson, 1988; Scott et al., 1990). During the Early Cretaceous, rudists became important in shallow water communities, tending to be more important in restricted environments such as lagoons and intrashelf basins. From the Hauterivian to the Albian, rudists became increasingly successful as reef dwellers and became the dominant tropical reef organism by the late Albian. Regionally, corals and rudists coexisted (e.g., Texas, Arizona margin of Tethys in the mid-Cretaceous), but rudists displaced corals in large measure. Corals decreased substantially within the tropics, and stromatoporoids almost disappeared. Many hypotheses have been offered, based on both competition and environment, to explain the displacement of corals by rudists. The model results presented here fit well with the speculation by Barron (1983)
