Elevated Eocene Atmospheric CO2 and Its Subsequent Decline

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Science  29 Sep 2006:
Vol. 313, Issue 5795, pp. 1928
DOI: 10.1126/science.1129555


Quantification of the atmospheric concentration of CO2 ([CO2]atm) during warm periods of Earth's history is important because burning of fossil fuels may produce future [CO2]atm approaching 1000 parts per million by volume (ppm). The early Eocene (~56 to 49 million years ago) had the highest prolonged global temperatures of the past 65 million years. High Eocene [CO2]atm is established from sodium carbonate minerals formed in saline lakes and preserved in the Green River Formation, western United States. Coprecipitation of nahcolite (NaHCO3) and halite (NaCl) from surface waters in contact with the atmosphere indicates [CO2]atm > 1125 ppm (four times preindustrial concentrations), which confirms that high [CO2]atm coincided with Eocene warmth.

Quantification of the atmospheric concentration of carbon dioxide ([CO2]atm) during warm periods of Earth's history is important for predicting global warming, because over the next 100 years burning of fossil fuels may produce [CO2]atm approaching 1000 parts per million by volume (ppm) (1). One such warm period, the early Eocene [∼56 to 49 million years ago (Ma)], had the highest prolonged global temperatures of the Cenozoic, peaking ∼52 to 50 Ma during the early Eocene climatic optimum (EECO) (2). However, the relation between atmospheric CO2 concentrations and greenhouse climates of the early Eocene is uncertain because proxy measurements from paleosols (3, 4), marine boron isotopes (5), and leaf stomatal indices (4) give estimated [CO2]atm concentrations between 100 and 3500 ppm.

Estimates of ancient [CO2]atm can be determined from the equilibrium assemblage of sodium carbonate minerals precipitated from waters in contact with the atmosphere (6). At present [CO2]atm of ∼380 ppm, trona (NaHCO3·Na2CO3·2H2O) crystallizes at temperatures above ∼25°C in at least a dozen modern alkaline saline lakes worldwide. Natron (Na2CO3·10H2O) forms at lower temperatures, but nahcolite (NaHCO3) is rare because it is predicted to precipitate only under elevated [CO2] (Fig. 1A). The preponderance of trona in modern systems indicates that sodium carbonate deposition follows thermodynamic predictions.

Fig. 1.

(A) Stability fields of sodium carbonates as a function of [CO2] and temperature. Minerals are in equilibrium with solution and gas at 1 atm total pressure (6). Dashed line shows that addition of halite lowers [CO2] of the gas in equilibrium with nahcolite plus trona to 1125 ppm at 20°C (6). Shaded areas mark environmental boundaries (∼20° to 35°C) for precipitation of sodium carbonate minerals today ([CO2]atm ∼ 380 ppm) and during the early Eocene. (B) Atmospheric [CO2] over the past 60 My, estimated from δ11B of foraminifera (5), alkenone δ13C (8), stomatal densities [compilation of (4)], paleosol carbonates [(3), compilation of (4)], and predicted GEOCARB III values (7). (Inset) Details for 56 to 49 Ma. The horizontal line at 1125 ppm is the minimum [CO2]atm necessary to precipitate nahcolite. Probable [CO2]atm values during deposition of Green River nahcolites (51.3 to 49.6 Ma), Beypazari trona (21.5 Ma ± 0.9 Ma), and Searles Lake trona (<1 Ma) are shaded red. Errors shown are discussed in (35) and (8).

During the EECO, long-lived lakes in the western United States deposited oil shale and sodium carbonate evaporites of the Wilkins Peak member of the Green River Formation and equivalents. The dominant sodium carbonate mineral of the Piceance Creek Basin, Colorado, is nahcolite up to ∼300 m thick, which in places occurs as microcrystalline chemical mud finely interlayered with halite (NaCl). This nahcolite, confirmed by x-ray diffraction analysis, contains textures diagnostic of precipitation in contact with atmospheric CO2 at the air-water interface of a perennial lake (fig. S1). The minimum [CO2] at which pure nahcolite precipitates, determined experimentally, is ∼1330 ppm; however, coprecipitation with halite (Fig. 1A) requires a lower minimum [CO2], anchoring minimum early Eocene [CO2]atm at >1125 ppm (Fig. 1B) (6). Estimates of paleotemperatures from the Green River basin before and after evaporite deposition suggest surface water temperatures varied seasonally from ∼20° to 35°C [Supporting Online Material (SOM) text]. Precipitation of nahcolite plus halite at those temperatures fixes minimum early Eocene [CO2]atm between 1125 and 2985 ppm.

Trona, rather than nahcolite, is the major sodium carbonate in the coeval Green River Formation, Green River Basin, Wyoming. The reaction Embedded Image Embedded Image indicates that trona forms instead of nahcolite under conditions of high activity of Na+ (aNa+) and high pH, suggesting different brine chemistries in the Green River and Piceance Creek basin lakes.

The only other economic accumulation of nahcolite is the Anpeng deposit, Henan Province, China, also Eocene in age (SOM text). Trona is the principal sodium carbonate in younger deposits (Fig. 1B), indicating atmospheric [CO2]atm dropped below 1125 ppm after the Eocene. Atmospheric [CO2] closer to modern values by the Miocene is suggested by the trona in the Beypazari deposit, Turkey (21.5 Ma). Trona is also the dominant sodium carbonate in the Pleistocene deposits of Searles Lake, California.

Primary nahcolite in the Green River evaporites gives firm geochemical evidence for elevated [CO2]atm (>1125 ppm) in the early Eocene. These data support a causal connection between CO2 and global warmth in the EECO and clarify the history of atmospheric CO2 over the past 60 million years (My) (Fig. 1B). Estimates of early Eocene atmospheric CO2 from Green River sodium carbonates are in the same range as those predicted by geochemical models (7). By ∼20 Ma, all available data (8) suggest [CO2]atm was at or near modern concentrations.

Supporting Online Material

SOM Text

Fig. S1


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