Atmospheric carbon dioxide levels for the last 500 million years

Abstract

The last 500 million years of the strontium-isotope record are shown to correlate significantly with the concurrent record of isotopic fractionation between inorganic and organic carbon after the effects of recycled sediment are removed from the strontium signal. The correlation is shown to result from the common dependence of both signals on weathering and magmatic processes. Because the long-term evolution of carbon dioxide levels depends similarly on weathering and magmatism, the relative fluctuations of CO2 levels are inferred from the shared fluctuations of the isotopic records. The resulting CO2 signal exhibits no systematic correspondence with the geologic record of climatic variations at tectonic time scales.

Figure 1

Data for ɛ_toc (red filled circles) (3) and ⁸⁷Sr/⁸⁶Sr (blue open squares) (4). The time scale for ɛ_toc has been revised from the original to match the scheme (32) used for the strontium data. The capital letters correspond to the following geologic periods: Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, and Tertiary.

Figure 2

Contours of R_λμ as a function of μ, the fraction of sedimentary Sr deriving from the memory flux, and t_1/2 = −(ln 2)/λ, the half-life of sedimentary Sr. R was calculated for μ = 0, 0.01, … , 0.99 and t_1/2 = 1, 2, … , 99 My. For large half-life, the contours decrease from left to right as follow: −0.60, −0.65, −0.70, −0.75, −0.79. The symbol × marks the minimum, R_λ*μ* = −0.80, obtained under the constraint that g(t) > 0.7035. The area shaded dark gray on the right does not satisfy the constraint. The rectangular area labeled “Brass” corresponds to previous estimates obtained by geochemical arguments (17); its horizontal extent has not been explicitly computed.

Figure 3

The function g (blue squares) obtained by removing the memory flux from the ⁸⁷Sr/⁸⁶Sr data of Fig. 1, along with ɛ_toc (red circles). Note that the range of the ⁸⁷Sr/⁸⁶Sr curve is approximately five times greater than in Fig. 1. The data are plotted such that the mean of both time series lies on the same horizontal line and their rms fluctuations have the same vertical extent.

Figure 4

Fluctuations of pCO₂ for the last 500 My, normalized by the estimate of pCO₂ obtained from the most recent value of ζ. The solid line is obtained from Eq. 12 by using ɛ₀ = 36‰. The lower and upper limits of the gray area surrounding the pCO₂ curve result from ɛ₀ = 38 and 35‰, respectively. The gray bars at the top correspond to periods when Earth's climate was relatively cool; the white spaces between them correspond to warm modes (18).