Mixing of CO_2 in surficial environments as recorded by the concentration and δ~(13)C values of the Fe(CO_3)OH component in goethite

摘要

Measured values of mol fraction (X) and δ~(13)C in the Fe(CO_3)OH component of 49 samples of goethite (α-FeOOH) from a wide variety of environments range from 0.0015 to 0.0182 and from -21.7 to +3.3%, respectively. The distribution of δ~(13)C values appears to be bimodal. δ~(13)C values are > -12‰ for marine hydrothermal goethites and continental goethites crystallized as secondary minerals in host rocks. With the exception of a young soil (DRR), δ~(13)C is < -14‰ for pedogenic/oolitic and bog ore goethites. Values of X and temperature determined for pedogenic goethite can be combined with the modern, global relation between soil respiration (Q) and average annual temperature to estimate diffusion coefficients (D_S) for steady-state Fickian diffusion of CO_2 in soils. Although there is considerable scatter in the relationship between Q and T, an illustrative calculation for a characteristic depth of 0.20 m, yields calculated values of D_S that range from about 0.002 to 0.011 cm~2/s among seven goethite-bearing soils. Steady-state diffusive transport of CO_2 in soils with one in situ source of CO_2 (oxidation of organic matter) yields a soil CO_2 mixing equation for two isotopically distinct CO_2 components (oxidized organic matter and atmospheric CO_2). With pedogenic goethite as a proxy for the soil CO_2, the positive slope of this twocomponent mixing equation can be used for determinations of CO_2 pressures in the Earth's atmosphere. If, however, dissolution of carbonates is concurrent with goethite crystallization, the mixing equation (at Z z~*) becomes three-component with a negative slope that precludes estimates of atmospheric Pco_2. An example is the DRR soil in which in situ, low-pH dissolution of relict, marine invertebrate fossils produced a negative mixing slope and goethite δ~(13)C values that range from -14.2 to -4.1‰. These results indicate that only goethites which crystallized in carbonate-free soils (highly leached or formed on initially carbonate-poor rocks) should be used to deduce ancient atmospheric CO_2 pressures. Addition of CO_2 from dissolution of carbonates also appears to explain the range of δ~(13)C values of most of the secondary continental goethites presumably formed in saturated groundwater systems. However, some of the variation of X and δ~(13)C of the Fe(CO_3)OH component in these goethites may be a consequence of variations in ambient pH. This possible dependence on pH seems to be manifested in a suite of goethites pseudomorphed after pyrite in a marine limestone.