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Trona, rather than , is the major carbonate in the coeval Green River Formation, Green River Basin, Wyoming. The Elevated Eocene Atmospheric CO2 and reaction þ 2NaHCO3 ðnahcoliteÞþ2H2O þ Na $ Its Subsequent Decline þ NaHCO3&Na2CO3&2H2O ðtronaÞþH Tim K. Lowenstein* and Robert V. Demicco indicates that forms instead of nahcolite þ þ uantification of the atmospheric con- 1A). The preponderance of trona in modern under conditions of high activity of Na (a Na ) E ^ centration of carbon dioxide ( CO2 atm) systems indicates that sodium carbonate depo- and high pH, suggesting different brine chem- Qduring warm periods of Earth_shistory sition follows thermodynamic predictions. istries in the Green River and Piceance Creek is important for predicting global warm- During the EECO, long-lived lakes in the basin lakes. ing, because over the next 100 years burning of western United States deposited oil shale and The only other economic accumulation of E ^ fossil fuels may produce CO2 atm approaching sodium carbonate of the Wilkins nahcolite is the Anpeng deposit, Henan Province, 1000 parts per million by volume (ppm) (1). Peak member of the Green River Formation China, also Eocene in age (SOM text). Trona is One such warm period, the early Eocene EÈ56 and equivalents. The dominant sodium carbon- the principal sodium carbonate in younger depos- ^ E ^ to 49 million years ago (Ma) , had the highest ate of the Piceance Creek Basin, Colo- its (Fig. 1B), indicating atmospheric CO2 atm prolonged global temperatures of the Cenozoic, rado, is nahcolite up to È300 m thick, which in dropped below 1125 ppm after the Eocene. È E ^ peaking 52 to 50 Ma during the early Eocene places occurs as microcrystalline chemical mud Atmospheric CO2 closer to modern values by climatic optimum (EECO) (2). However, the re- finely interlayered with (NaCl). This nah- the Miocene is suggested by the trona in the

lation between atmospheric CO2 concentrations colite, confirmed by x-ray diffraction analysis, Beypazari deposit, Turkey (21.5 Ma). Trona is and greenhouse climates of the early Eocene is contains textures diagnostic of precipitation in also the dominant sodium carbonate in the

uncertain because proxy measurements from contact with atmospheric CO2 at the air-water Pleistocene deposits of Searles Lake, California. paleosols (3, 4), marine boron isotopes (5), and interface of a perennial lake (fig. S1). The min- Primary nahcolite in the Green River evap- E ^ E ^ leaf stomatal indices (4) give estimated CO2 atm imum CO2 at which pure nahcolite precip- orites gives firm geochemical evidence for ele- È E ^ 9 concentrations between 100 and 3500 ppm. itates, determined experimentally, is 1330 ppm; vated CO2 atm ( 1125 ppm) in the early Eocene. E ^ Estimates of ancient CO2 atm can be deter- however, coprecipitation with halite (Fig. 1A) These data support a causal connection between E ^ mined from the equilibrium assemblage of requires a lower minimum CO2 , anchoring CO2 andglobalwarmthintheEECOandclarify E ^ 9 sodium carbonate precipitated from minimum early Eocene CO2 atm at 1125 ppm the history of atmospheric CO2 over the past 60 waters in contact with the atmosphere (6). At (Fig. 1B) (6). Estimates of paleotemperatures million years (My) (Fig. 1B). Estimates of early E ^ È & present CO2 atm of 380 ppm, trona (NaHCO3 from the Green River basin before and after EoceneatmosphericCO2 from Green River sodi- & Na2CO3 2H2O) crystallizes at temperatures deposition suggest surface water tem- um carbonates are in the same range as those pre- above È25-C in at least a dozen modern alkaline peratures varied seasonally from È20- to 35-C dicted by geochemical models (7). By È20 Ma, & E ^ E ^ saline lakes worldwide. Natron (Na2CO3 10H2O) Supporting Online Material (SOM) text .Pre- all available data (8) suggest CO2 atm was at or forms at lower temperatures, but nahcolite cipitation of nahcolite plus halite at those tem- near modern concentrations. E ^ (NaHCO3) is rare because it is predicted to peratures fixes minimum early Eocene CO2 atm E ^ References and Notes precipitate only under elevated CO2 (Fig. between 1125 and 2985 ppm. 1. Intergovernmental Panel on Climate Change, Climate Change 2001: The Science of Climate Change (Cambridge Univ. Press, Cambridge, 2001). AB 2. J. Zachos, M. Pagani, L. Sloan, E. Thomas, K. Billups, 3500 Science 292, 686 (2001). 3500 Alkenones 3. C. J. Yapp, Geochim. Cosmochim. Acta 68, 935 (2004). Nahcolite 3000 Boron isotopes Paleosols 4. D. L. Royer, R. A. Berner, I. P. Montan˜ez, N. J. Tabor,

+Halite (ppm) 2500 Leaf stomata NAHCOLITE D. J. Beerling, GSA Today 14, 4 (2004). 2 GEOCARB lll 5. P. N. Pearson, M. R. Palmer, Nature 406, 695 (2000). 2000 6. H. P. Eugster, J. Geophys. Res. 71, 3369 (1966). Eocene 7. R. A. Berner, Z. Kothavala, Am. J. Sci. 301, 182 (2001). 1500 0 49 56 8. M. Pagani, J. C. Zachos, K. H. Freeman, B. Tipple, Trona 1000 S. Bohaty, Science 309, 600 (2005); published online TRONA 16 June 2005 (10.1126/science.1110063). 500 Modern Atmospheric CO 9. We thank J. Dyni, U.S. Geological Survey, for providing content of gas (ppm) 2 0 samples and information, and D. Royer, Wesleyan 0102030405060University, for compiled data on ancient [CO ] . This

CO Natron 2 atm Age (Ma) work was partially supported by NSF Earth Sciences grants 0085371, 0433802, and 0447167. Temperature (°C) Supporting Online Material www.sciencemag.org/cgi/content/full/313/5795/1928/DC1 Fig. 1. (A) Stability fields of sodium carbonates as a function of [CO2] and temperature. Minerals are in SOM Text equilibrium with solution and gas at 1 atm total pressure (6). Dashed line shows that addition of halite Fig. S1 - lowers [CO2] of the gas in equilibrium with nahcolite plus trona to 1125 ppm at 20 C(6). Shaded areas 4 May 2006; accepted 14 July 2006 mark environmental boundaries (È20- to 35-C) for precipitation of sodium carbonate minerals today 10.1126/science.1129555 È ([CO2]atm 380 ppm) and during the early Eocene. (B)Atmospheric[CO2]overthepast60My,estimated 11 13 from d Bofforaminifera(5), alkenone d C(8), stomatal densities [compilation of (4)], paleosol Department of Geological Sciences and Environmental Studies, carbonates [(3), compilation of (4)], and predicted GEOCARB III values (7). (Inset) Details for 56 to 49 Ma. State University of New York at Binghamton, Binghamton, NY Thehorizontallineat1125ppmistheminimum[CO2]atm necessary to precipitate nahcolite. Probable 13902, USA. T [CO2]atm values during deposition of Green River nahcolites (51.3 to 49.6 Ma), Beypazari trona (21.5 Ma *To whom correspondence should be addressed. E-mail: 0.9 Ma), and Searles Lake trona (G1 Ma) are shaded red. Errors shown are discussed in (3–5)and(8). [email protected]

1928 29 SEPTEMBER 2006 VOL 313 SCIENCE www.sciencemag.org