CARBON ISOTOPE MEASUREMENTS OF FE(III) CARBONATE IN GOETHITE FROM A LATE PALEOCENE LATERITE AT 55°N

Chemical cleaning, selective dissolution and incremental vacuum dehydration experiments were used to deconvolve carbon isotope information from a Late Paleocene laterite in Northern Ireland. Delta C-13 values of organic matter removed from the samples range from -25‰ to -29‰. Such negative C-13 values are indicative of a humid, tropical climate atypical of modern Ireland.

The extent of Al substitution for Fe(III) in goethite indicates at least two different populations of goethite: low Al (~0 mol %) and high Al ( 12-24 mol %) goethites. However, the C-13 and mol fraction (Xm) values of Fe(III) carbonate in goethite indicate there are two different populations among the low Al goethites: one population with C-13 and Xm values of ~-5‰ and 0.0071, respectively, and a second population with values of ~-17‰ and 0.0056, respectively. The high Al goethite Fe(III) carbonate component has C-13 and Xm values ranging from about -19‰ to -21.5‰ and 0.0077 and 0.0059, respectively. Thus, there appears to be three populations of Fe(III) carbonate in solid solution in goethite in this paleosol profile. Low C-13 Fe(III) carbonate in the high Al goethites is of particular interest, because it appears to represent 2-component mixing of two isotopically distinct carbon dioxide components in the Late Paleocene soil: (1) carbon dioxide from oxidation of biological carbon and (2) atmospheric carbon dioxide.

Combination of Xm values for the high Al goethites with the Henry’s Law equation for pedogenic goethites indicates that Late Paleocene soil carbon dioxide concentrations were 29,000-37,000 ppmV, which is well within the expected range for lateritic profiles. Fe(III) carbonate in the high Al-goethites implies coeval oxidizing soil organic matter with C-13 values no more positive than -27‰. This calculated value is in reasonable accord with carbon isotope ratios of peroxide-oxidizable organic matter removed from these samples, suggesting the possible preservation of some Late Paleocene organic matter in this system. These data suggest that the high Al-goethites retain carbon isotope ratios relevant to the determination of ancient atmospheric carbon dioxide pressures.