IMPLICATIONS FOR LATE PALEOCENE ATMOSPHERIC CO2 FROM STABLE CARBON ISOTOPE MEASUREMENTS OF FE(CO3)OH IN SOLID SOLUTION IN GOETHITE FROM A LATERITE IN NORTHERN IRELAND

A Late Paleocene (58.3-61.0Ma) laterite from Northern Ireland is primarily composed of authigenic goethite, gibbsite, smectite and ferrihydrite-like material. Chemical cleaning, selective dissolution and incremental vacuum dehydration experiments were used to deconvolve isotope information contained within the different mineral systems of this profile. The extent of Al³⁺ substitution for Fe³⁺ in the goethite indicates at least two different populations of goethite: those with low Al³⁺ (~0-3 mol %) and those with high Al³⁺ ( 12-24 mol %). These two goethite populations are refelected in a dichotomy of delta ¹³C values of Fe(CO₃)OH in solid solution in the goethites. The delta ¹³C values were determined from incremental vacuum dehydration-decarbonation spectra of goethites "concentrated" by selective dissolution. The low-Al³⁺ goethites (and the Fe(CO₃)OH component therein) break down in vacuum at temperatures of ~155°C and evolve CO₂ with delta ¹³C values of about —5‰. In contrast, the high-Al³⁺ goethites break down in vacuum at temperatures from 190° to 270°C, and the delta ¹³C values of the Fe(CO₃)OH range from —18.9 to —21.0‰. The low ¹³C Fe(CO₃)OH in the high Al³⁺ goethites appears to represent 2-component mixing of two isotopically distinct CO₂ components in the Late Paleocene soil: (1) CO₂ from oxidation of biological carbon and (2) atmospheric CO₂. In contrast, the higher ¹³C content of the Fe(CO₃)OH in the low Al³⁺ goethites suggests that, subsequent to Late Paleocene pedogenesis, crystallization of this goethite occurred in a phreatic environment characterized by mixing of 3 isotopically distinct CO₂ components: (1) oxidized biological carbon (2) atmospheric CO₂ and (3) CO₂ from dissolution of carbonate.

Two-component soil CO₂ mixing arrays can be used to deduce the isotopic composition of oxidized biological carbon and concentrations of atmospheric CO₂ during Fe(CO₃)OH crystallization. The delta ¹³C value of biological carbon contributing CO₂ to the Late Paleocene 2-component CO₂ mixing system was ~—28‰. Such negative carbon isotopic values of biological carbon are typical of low-latitude tropical ecosystems. Data from the high-Al³⁺ lateritic goethites will be discussed as a source of proxy information for Late Paleocene atmospheric CO₂.