F Stable isotope study of fluid inclusions in fluorite from Idaho: Implications for continental climates during the Eocene: Comment and Reply COMMENT Scott L. Wing Department of Paleobiology, National Museum of Natural History, NHB 121, Smithsonian Institution, Washington, D.C. 20560 Jack A. Wolfe Department of Geosciences, University of Arizona, Tucson, Arizona 85721 There at least four reasons to doubt the conclusion of Seal and Rye (1993) that their study". . . provides compelling support for the climate modeling study of Sloan and Barron (1990)," which argued for cool and seasonal climates in the interior of North America dur- ing the Eocene. First, there is a large and internally consistent body of paleontological data that demonstrate warm (mean annual tem- perature >15 ?C) and equable (mean annual range of temperature <15 ?C) climates in the continental interior during the Eocene (e.g., Archibald, 1991; Hickey, 1977; Wing, 1991; Wing et al., 1991). Sec- ond, Seal and Rye did not give any Eocene temperature estimates, they only implied that the Eocene might have been similar to the modern day! If the isotopic data are so compelling, why not give estimates? Third, much of the debate about the Sloan and Barron model concerns seasonal!ty, not mean annual temperature (MAT). MATs generated by the Eocene simulations are much warmer than modern MATs and are close to those inferred from paleobotanical data (Sloan and Barron, 1992; Wing and Greenwood, 1993). The controversial aspect of the Sloan and Barron (1990) simulations was the low cold-month temperatures, a climate parameter difficult to infer from isotopic data. Finally, paleobotanical and geological ev- idence indicates a substantial Eocene altitude for the sites studied by Seal and Rye (Fields et al., 1985; Axelrod, 1968). Axelrod (1990) estimated a paleoaltitude of 1555 m for the nearby Eocene Thunder Mountain flora, and much of the region from northeastern Wash- ington State to northern Nevada has been considered an Eocene volcanic highland (Axelrod, 1966; Wolfe and Wehr, 1987, 1992). Thus, even if the isotopic data have been interpreted correctly, they may reflect the paleoaltitude of the site rather than a cool continental interior "decoupled" from the warm oceans. Seal and Rye (1993) also stated that with regard to Eocene con- tinental climate "... the paleobotanical data are largely restricted to continental margins" (p. 219). To the contrary, data for the con- tinental interior were summarized in Geology (Archibald, 1991; Wing, 1991), and since 1990 there have been two additional articles in Geology that discuss the paleoclimatic implications of interior North American Eocene floras (Gregory and Chase, 1992; Wing et al., 1991). Paleoclimate inferences based on general climate model simulations or isotopic data must recognize and respond to the pa- leontological conclusions on the subject, or proper scientific debate will be stymied. REFERENCES CITED Archibald, J.D., 1991, Comment on " 'Equable' climates during Earth his- tory?": Geology, v. 19, p. 539. Axelrod, D.I., 1966, The Eocene Copper Basin flora of northeastern Ne- vada: University of California Publications in Geological Sciences, v. 59, p. 1-125. Axelrod, D.I., 1968, Tertiary floras and topographic history of the Snake River Basin, Idaho: Geological Society of America Bulletin, v. 79, p. 713-734. Axelrod, D.I., 1990, Environment of the Middle Eocene (45 Ma) Thunder Mountain flora, central Idaho: National Geographic Research, v. 6, p. 355-361. Fields, R.W., Tabrum, A.R., Rasmussen, D.L., and Nichols, R., 1985, Ce- nozoic rocks of the intermontane basins of western Montana and eastern Idaho: A summary, in, Flores, R.M., and Kaplan, S.S., eds., Cenozoic paleogeography of west-central United States: Rocky Mountain Sec- tion, Society of Economic Paleontologists and Mineralogists, p. 9-36. Gregory, K.M., and Chase, C.G., 1992, Tectonic significance of paleobo- tanically estimated climate and altitude of the Late Eocene erosion sur- face, Colorado: Geology, v. 20, p. 581-585. Hickey, L.J., 1977, Stratigraphy and paleobotany of the Golden Valley For- mation (early Tertiary) of western North Dakota: Geological Society of America Memoir 150, 181 p. Seal, R.R., and Rye, R.O., 1993, Stable isotope study of fluid inclusions in fluorite from Idaho: Implications for continental climates during the Eocene: Geology, v. 21, p. 219-222. Sloan, L.C., and Barron, E.J., 1990, "Equable" climates during Earth his- tory?: Geology, v. 18, p. 489-492. Sloan, L.C., and Barron, E.J., 1992, A comparison of Eocene climate model results to quantified paleoclimatic interpretations: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 93, p. 183-202. Wing, S.L., 1991, Comment on " 'Equable' climates during Earth history?": Geology, v. 19, p. 539-540. Wing, S.L., and Greenwood, D.R., 1993, Fossils and fossil climate: The case for equable continental interiors in the Eocene, in Allen, J.R.L., et al., eds., Palaeoclimates and their modelling with special reference to the Mesozoic Era: Royal Society of London Philosophical Transactions, ser. B (in press). Wing, S.L., Sown, T.M., and Obradovich, J.D., 1991, Early Eocene biotic and climatic change in interior western North America: Geology, v. 19, p. 1189-1192. Wolfe, J.A., and Wehr, W., 1987, Middle Eocene dicotyledonous plants from Republic, northeastern Washington: U.S. Geological Survey Bul- letin 1597, p. 1-25. Wolfe, J.A., and Wehr, W., 1992, Significance of Eocene fossil plants at Republic, Washington: Washington Geology, v. 19, p. 18-24. REPLY Robert R. Seal II U.S. Geological Survey, 954 National Center, Reston, Virginia 22092 Robert O. Rye U.S. Geological Survey, MS 963, Federal Center, Denver, Colorado 80225 We agree with Wing and Wolfe that "compelling" may be too strong a word to describe the support of our study for the modeling study of Sloan and Barron (1990) and that "tentative" might be more accurate. Our intention was to offer data as a paleoclimate reference point, and the data can be interpreted to support the conclusions of Sloan and Barron (1990). Regardless of the ultimate relation of our study to the climate modeling study, the comments of Wing and Wolfe do not challenge the fundamental conclusions of our work nor do they undermine the usefulness of fluid inclusions as paleoclimate indicators. On the basis of climate modeling studies, Sloan and Barron (1990) concluded that the continental interior climates during the GEOLOGY, November 1993 1051 Eocene were probably more similar to those today than to the warm marine climates recorded by the oxygen isotope signatures of deep- sea forams. They also made a plea for proxy data from continental interiors to aid in the calibration of their model calculations. Wing (1991) and Archibald (1991), among others, have answered this re- quest with paleontological data that questioned some of the results of Sloan and Barron (1990). The significance of our work centers on the fact that the fluid inclusions that we studied formed in a continental interior setting during the critical interval of Earth's history between 50 and 55 Ma when Cenozoic ocean temperatures are known to have been their highest. The fluid-inclusion record preserved by the Bayhorse flu- orite deposits is of exceptional quality. With these observations in mind, our study resulted in three main conclusions: (1) The Eocene fluid inclusions are samples of ancient heated meteoric (ground) wa- ters that did not undergo significant water-rock interactions to alter their isotopic compositions; (2) the oxygen and hydrogen isotope compositions of these ancient meteoric waters are identical to present-day meteoric waters; and (3) the equivalence of the Eocene and modern meteoric water stable isotope values implies that the general climatic conditions during the Eocene were similar to those today. The isotope data for the Bayhorse district most directly re- flect the mean annual temperature of precipitation, which is one of the most important factors in determining meteoric water composi- tions. By analogy with the present, our data indicate Eocene mean annual temperatures at Bayhorse of approximately 10 ?C. In our paper, we did not elaborate upon the two main factors that control the present mean annual temperature of precipitation: latitude and elevation. The magnitude of these two factors in the Bayhorse district bears heavily on the significance of our fluorite inclusion fluid data. As pointed out by Wing and Wolfe, elevation was probably an important contributing factor in determining the temperature of precipitation at our locality, and thus indicates at least a local control on temperature. This observation is consistent with the approximately 670 m of subaerial volcanic cover which we reported from the literature (Ross, 1937) in our paper. Regarding the other points of Wing and Wolfe, we did not in- tend to downplay the significance of the paleontological data avail- able from the Cenozoic of western North America. The paleobo- tanical data that we chose to reference as most applicable were those from Ross (1937) from the Bayhorse area. Like Axelrod (1990), who reported data for the Thunder Mountain caldera, 90 km northwest of the Bayhorse district, Ross (1937) reported Pinus, Sequoia, Alnus, Juglans, Quercus, and Salix. Axelrod (1990) concluded that the mean annual temperature was 8.5 ?C, favorably compared to our mean annual temperature of precipitation of 10 ?C. In summary, fluid inclusions hosted by minerals, such as fluorite, quartz, and adularia, among others, from epithermal mineral deposits provide important paleoclimate reference points in continental set- tings that are readily obtainable. However, these data and all other paleoclimate indicators must be evaluated in the proper context with respect to their temporal and spatial setting. Thus, Cenozoic epither- mal mineral deposits in western North America hold great potential for providing a detailed picture of Cenozoic meteoric water compo- sitions and climates, as first suggested by O'Neil and Silberman (1974). REFERENCES CITED Archibald, J.D., 1991, Comment on " 'Equable' climates during Earth his- tory?": Geology, v. 19, p. 539. Axelrod, D.I., 1990, Environment of the Middle Eocene (45 Ma) Thunder Mountain flora, central Idaho: National Geographic Research, v. 6, p. 355-361. O'Neil, J.R., and Silberman, M.L., 1974, Stable isotope relations in epither- mal Au-Ag deposits: Economic Geology, v. 69, p. 902-909. Ross, C.P., 1937, Geology and ore deposits of the Bayhorse region, Custer County, Idaho: U.S. Geological Survey Bulletin 877, 161 p. Sloan, L.C., and Barron, E.J., 1990, 'Equable' climates during Earth histo- ry?: Geology, v. 18, p. 489-492. Wing, S.L., 1991, Comment on " 'Equable' climates during Earth history?": Geology, v. 19, p. 539-540. 1052 GEOLOGY, November 1993