Model Results

Results of rock abundance modeling (Berner and Canfield, 1989) and isotope mass balance modeling (Berner, 2001) for atmospheric oxygen over Phanerozoic time are compared in figure 6.5. The isotope modeling, designated here as RROD, is based on the carbon isotopic data of Veizer et al. (1999) (with a minor positive excursion during the Silurian ignored; see figure 3.3), the sulfur isotopic data of Strauss (1999), and rapid recycling (RR) and O2-dependent (OD) C and S isotope fractionation (Berner, 2001). There is excellent agreement between the results of the two approaches. This is partly due to adjusting the O2 dependence of carbon and sulfur isotopic fractionation to obtain a best fit to the rock abundance results. However, the resulting fractionations for carbon isotopes A13C are in rough agreement (except for the past 20 million years) with the data for the measured difference between 813C values for coexisting carbonates and organic matter in sediments (Hayes et al., 1999). This is shown in figure 6.6. Note that values of the adjustment parameters in the RROD model (J and n; see Berner, 2001, for details) are the same for both the O2 calculation (figure 6.5) and the A13C calculation (figure 6.6). Agreement of the plots in each figure gives some credence to the results of the RROD isotope mass balance method. Furthermore, measurements of carbon isotopic fractionations for plant fossils (discussed below) provide further agreement with the theoretically calculated Permo-Carboniiferous values of A13C centered around 300 Ma. (Much of global organic burial during the Permo-Carboniferous derived from land plants; Berner and Raiswell, 1983.)

Time my

Figure 6.5. Plots of % O2 versus time based on rock abundance modeling (Berner and Canfield, 1989) and RROD isotope mass balance modeling with rapid recycling and O2-dependent C and S isotope fractionation. The adjustable parameters J and n refer to the isotope dependence of fractionation for C and S isotopes, respectively (Berner, 2001).

Figure 6.6. Comparison of carbon isotope fractionation a13C over time for the RROD model with measurements of a13C by Hayes et al. (1999) on Phanerozoic carbonates and organic matter.

The most notable feature of figure 6.5 is the maximum in O2 concentration extending from 375Ma to 250Ma (Devonian-Permian). This is the result of an increased rate of production of O2 by an increased rate of burial of organic matter during this period. The increased burial rate is indicated by both the abundance of organic matter in sediments and by the carbon isotopic composition of the oceans. High values of oceanic 813C during this period (figure 3.3) indicate increased removal of light carbon from seawater via photosynthesis, resulting in increased burial of the produced organic matter in sediments. Increased burial of organic matter on land (e.g., in coal basin sediments) would also be recorded by heavy carbon isotopic values in seawater because of rapid isotopic exchange of the atmosphere with the oceans. The increased burial was brought about mostly by the rise of large woody land plants (i.e., trees). Woody plants contain lignin, which is relatively resistant to microbial decomposition, and deposition of lignin must have led to the addition of organic matter to terrestrial sediments and extra organic matter to marine sediments after transport there by rivers. (For further discussion of organic burial, see chapter 3.)

The evolution of atmospheric O2 during the past 150 Ma has been calculated by Hansen and Wallmann (2003), using an a priori method, and is compared in figure 6.7 to the RROD isotope model. There is amazingly good agreement considering the use by Hansen and Wallmann of f factors, as they affect weathering, which is not done in the RROD modeling. The disagreement between the two studies for the period 100-50 Ma

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Hansen and Wallmann o2 %

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Hansen and Wallmann o2 %

Time my

Figure 6.7. Comparison of results for % O2 for the past 150 million years between the model of Hanson and Wallmann (2003) and RROD isotope mass balance modeling (Berner, 2001).

can be explained by the lack of consideration of these factors by the simple isotope model.

The results for O2 level over Phanerozoic time, calculated via the a priori model of Bergman et al. (2003), are compared to the results of RROD modeling in figure 6.8. Both models show a large rise in percent O2 during the mid-Paleozoic, attaining a maximum near the Permian-Carboniferous boundary. However, the Bergman model shows another equally high maximum in the Cretaceous not obtained by RROD isotope modeling.

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