Atmospheric carbon dioxide levels for the last 500 million years - PubMed (original) (raw)

Atmospheric carbon dioxide levels for the last 500 million years

Daniel H Rothman. Proc Natl Acad Sci U S A. 2002.

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.

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Figures

Figure 1

Figure 1

Data for ɛtoc (red filled circles) (3) and 87Sr/86Sr (blue open squares) (4). The time scale for ɛtoc 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

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

Figure 3

The function g (blue squares) obtained by removing the memory flux from the 87Sr/86Sr data of Fig. 1, along with ɛtoc (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

Figure 4

Fluctuations of _p_CO2 for the last 500 My, normalized by the estimate of _p_CO2 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 _p_CO2 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).

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