Coexisting goethite and gibbsite from a high-paleolatitude (55°N) Late Paleocene laterite: Concentration and 13C/ 12C ratios of occluded CO 2 and associated organic matter

Abstract

A Late Paleocene (∼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 (∼0 mol% Al) and (2) high-Al, pisolitic goethites (∼9 to ∼24 mol% Al). Selective dissolution and incremental vacuum dehydration-decarbonation were used to determine the concentration and δ 13C values of CO 2 occluded in the respective structures of the goethites and gibbsites in this complex mixture of Nire lateritic minerals. The Fe(CO 3)OH component in the high-Al goethites appears to retain a proxy carbon isotopic record of vadose zone CO 2 in the ancient soil. The δ 13C values of CO 2 occluded in coexisting goethites and gibbsites indicate that these minerals did not form in equilibrium with the same environmental CO 2. The measured mole fractions (X) of Fe(CO 3)OH in the high-Al goethites range from 0.0059 (±0.0005) to 0.0077 (±0.0006) and correspond to soil CO 2 concentrations of ∼28,000 to ∼37,000 ppmV. The average values of X and δ 13C for the four high-Al goethites are 0.0067 ± 0.0007 and −20.1 ± 0.5‰, respectively. The δ 13C value of the organic matter undergoing oxidation in this midlatitude (∼55°N) Late Paleocene soil appears to have been ∼ −28.2‰. Taken together, these data indicate an atmospheric CO 2 concentration of ∼2400 ppmV (± ∼1200 ppmV) at ∼60 Ma BP. The inferred high concentration of atmospheric CO 2 would have been coincident with the warm global climate of the Late Paleocene and is consistent with the idea that CO 2 plays an important role in climate variation.