Stable carbon isotope signatures preserved in authigenic gibbsite from a forested granitic–regolith: Panola Mt., Georgia, USA

Abstract

Six samples, selected from a weathering profile developed on a granite in the Panola Mountain Research Watershed in the Georgia Piedmont were studied for their stable carbon isotopic properties. The purpose was to understand the relationship between the stable carbon isotopic composition of the organic matter pool and the preservation of carbon in the authigenic soil minerals. The method for yielding carbon from authigenic phases is based on that of Yapp and Poths [ 13C/ 12C ratios of the FE(III) carbonate component in natural goethites. In: Taylor, H.P., O'Neil, J.R., Kaplan, I.R. (Eds.), Stable Isotope Geochemistry: A Tribute to Samual Epstein, The Geochemical Society. Special Publication No. 3, 257–270.] and uses the following sample treatments: (1) Separation of the <2 μm fraction; (2) Four 10-day oxidation cycles with 30% H 2O 2; (3) 12-h open-system vacuum dehydration at 110°C; (4) 1-h close-system oxidation with 0.3 bar oxygen at 200°C; (5) Timed step-wise open-system extractions at 230° to 240°C; and (6) A final 15-min closed-system combustion at 850°C with 0.2 bar oxygen. The evolved carbon dioxide, cryogenically collected during step 5, is operationally defined as hydroxide-bound carbon. X-ray diffractometry (XRD) and differential-scanning-calorimetry (DSC) indicate that the predominant phase transition at 230°C is the dehydroxylation of gibbsite. Gibbsite content of the bulk soil/saprolite ranges from 3 to 12% by weight. Acid Fe-extraction analysis as well as XRD, DSC and color show that goethite occurs at levels of <1% by weight. C/H ratios measured during incremental extraction did not produce time-dependent trends. This ‘non-stoichiometric’ water is attributed to the dehydration and dehydroxylation of coexisting halloysite. The stable carbon isotopic release curves for samples at depth (>28 cm) initially yield values from −15 to −17‰ (PDB) and then with each time step lighten to values from −18 to −22‰. Samples near the surface (<16 cm) yield initial values from −14 to −16‰. With successive time steps they initially become heavy (−13 to −11‰) and then become lighter with values from −16 to −17‰. The average stable carbon isotopic values for the six samples show a gradient trend from −21‰ at depth (236 cm) to −14‰ near the surface (6 cm). These results indicate that gibbsite offers a similar potential as the pedogenic minerals calcite and goethite to be a recorder of soil carbon systematics.