Goethite, calcite, and organic matter from Permian and Triassic soils: Carbon isotopes and CO2 concentrations

Pedogenic goethites in each of two Early Permian paleosols appear to record mixing of two isotopically distinct CO2 components—atmospheric CO2 and CO2 from in situ oxidation of organic matter. The C values measured for the Fe(CO3)OH component in solid solution in these Permian goethites are 13.5‰ for the Lower Leonardian ( 283 Ma BP) paleosol (MCGoeth) and 13.9‰ for the Upper Leonardian ( 270 Ma BP) paleosol (SAP). These goethites contain the most C-rich Fe(CO3)OH measured to date for pedogenic goethites crystallized in soils exhibiting mixing of the two aforementioned CO2 components. C measured for 43 organic matter samples in the Lower Leonardian (Waggoner Ranch Fm.) has an average value of 20.3 1.1‰ (1s). The average value yields a calculated Early Permian atmospheric PCO2 value of about 1 PAL, but the scatter in the measured C values of organic matter permits a calculated maximum PCO2 of 11 PAL (PAL present atmospheric level). Measured values of the mole fraction of Fe(CO3)OH in MCGoeth and SAP correspond to soil CO2 concentrations in the Early Permian paleosol profiles of 54,000 and 50,000 ppmV, respectively. Such high soil CO2 concentrations are similar to modern soils in warm, wet environments. The average C values of pedogenic calcite from 9 paleosol profiles stratigraphically associated with MCGoeth (Waggoner Ranch Fm.) range from 6.5‰ to 4.4‰, with a mean C value for all profiles of 5.4‰. Thus, the value of C between the pedogenic calcite data set and MCGoeth is 8.1 ( 0.9)‰, which is in reasonable accord with the value of 7.7‰ expected if atmospheric PCO2 and organic matter C values were the same for both paleosol types. Furthermore, the atmospheric PCO2 calculated for the Early Permian from the average measured carbon isotopic compositions of the paleosol calcite and organic matter is also analytically indistinguishable from 1 PAL, with a maximum calculated atmospheric PCO2 (permitted by one standard deviation of the organic matter C value) of 5 PAL. If, however, measured average C values of the plant organic matter are more positive than the original soil organic matter as a result of diagenetic loss of C-depleted, labile organic compounds, calculated Permian atmospheric PCO2 using these C-enriched organic values would underestimate the actual atmospheric PCO2 using either goethite or calcite. This is the first stratigraphically constrained, intrabasinal study to compare ancient atmospheric CO2 concentrations calculated from pedogenic goethite and calcite. These results demonstrate that the two different proxies record the same information about atmospheric CO2. The Fe(CO3)OH component in pedogenic goethite from a Triassic paleosol in Utah is significantly enriched in C relative to Fe(CO3)OH in goethites from soils in which there are mixtures of two isotopic CO2 components. Field-relationships and the C value ( 1.9‰) of the Triassic goethite indicate that this ancient paleosol profile experienced mixing of three isotopically distinct CO2 components at the time of goethite crystallization. The three components were probably atmospheric CO2, CO2 from in situ oxidation of organic matter and CO2 from in situ dissolution of preexisting calcite. Although mixing of three isotopically distinct CO2 components, as recorded by Fe(CO3)OH in goethite, has been described in modern soil, this is the first example from a documented paleosol. Its preservation affirms the need for careful, case-by-case assessment of ancient paleosols to establish that goethite in any particular soil is likely to be a valid proxy of atmospheric PCO2. Copyright © 2004 Elsevier Ltd