Other
Scientific paper
Nov 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001gecoa..65.4115y&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 65, Issue 22, pp.4115-4130
Other
6
Scientific paper
Measured values of mol fraction (X) and 13 C in the Fe(CO 3 )OH component of 49 samples of goethite ( -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 13 C values appears to be bimodal. 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), 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 two-component 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 P 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 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 13 C values of most of the secondary continental goethites presumably formed in saturated groundwater systems. However, some of the variation of X and 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.
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