Coexisting goethite and gibbsite from a high-paleolatitude (55°N) Late Paleocene laterite:: Concentration and 13C/12C ratios of occluded CO2 and associated organic matter

A Late Paleocene (similar to 60 Ma BP) lateritic soil from Northern Ireland (the Antrim paleosol, herein referred to as Nire) contains coexisting goethite, gibbsite, phyllosilicate, and hematite. The Fe(III) oxides exhibit pisolitic and Liesegang-type morphologies that are mutually exclusive in hand specimens. X-ray diffraction (XRD) measurements of Al substituted for Fe in goethite indicate two populations: (1) low-Al, Liesegang-type goethites (similar to 0 mol% Al) and (2) high-Al, pisolitic goethites (similar to 9 to similar to 24 mol% Al). Selective dissolution and incremental vacuum dehydration-decarbonation were used to determine the concentration and delta C-13 values of CO2 occluded in the respective structures of the goethites and gibbsites in this complex mixture of Nire lateritic minerals. The Fe(CO3)OH component in the high-Al goethites appears to retain a proxy carbon isotopic record of vadose zone CO2 in the ancient soil. The delta C-13 values of CO2 occluded in coexisting goethites and gibbsites indicate that these minerals did not form in equilibrium with the same environmental CO2. The measured mole fractions (X) of Fe(CO3)OH in the high-Al goethites range from 0.0059 (+/- 0.0005) to 0.0077 (+/- 0.0006) and correspond to soil CO2 concentrations of similar to 28,000 to similar to 37,000 ppmV. The average values of X and delta C-13 for the four high-Al goethites are 0.0067 +/- 0.0007 and -20.1 +/- 0.5 parts per thousand, respectively. The delta C-13 value of the organic matter undergoing oxidation in this midlatitude (similar to 55 degrees N) Late Paleocene soil appears to have been similar to -28.2 parts per thousand. Taken together, these data indicate an atmospheric CO2 concentration of similar to 2400 ppmV (+/- similar to 1200 ppmV) at similar to 60 Ma BP. The inferred high concentration of atmospheric CO2 would have been coincident with the warm global climate of the Late Paleocene and is consistent with the idea that CO2 plays an important role in climate variation. Copyright (c) 2005 Elsevier Ltd.