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

Six samples, selected from a weathering profile developed on a granite in t he 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 orga nic matter pool and the preservation of carbon in the authigenic soil miner als. The method for yielding carbon from authigenic phases is based on that of Yapp and Poths [C-13/C-12 ratios of the FE(III) carbonate component in natural goethites. In: Taylor, H.P., O'Neil, J.R., Kaplan, I.R. (Eds.), Sta ble Isotope Geochemistry: A Tribute to Samual Epstein, The Geochemical Soci ety. Special Publication No. 3, 257-270.] and uses the following sample tre atments: (1) Separation of the < 2 mu m fraction; (2) Four 10-day oxidation cycles with 30% H2O2; (3) 12-h open-system vacuum dehydration at 110 degre es C; (4) l-h close-system oxidation with 0.3 bar oxygen at 200 degrees C; (5) Timed step-wise open-system extractions at 230 degrees to 240 degrees C ; and (6) A final 15-min closed-system combustion at 850 degrees C with 0.2 bar oxygen. The evolved carbon dioxide, cryogenically collected during ste p 5, is operationally defined as hydroxide-bound carbon. X-ray diffractomet ry (XRD) and differential-scanning-calorimetry (DSC) indicate that the pred ominant phase transition at 230 degrees C is the dehydroxylation of gibbsit e. Gibbsite content of the bulk soil/saprolite ranges from 3 to 12% by weig ht. Acid Fe-extraction analysis as well as XRD, DSC and color show that goe thite occurs at levels of < 1% by weight. C/H ratios measured during increm ental extraction did not produce time-dependent trends. This 'non-stoichiom etric' water is attributed to the dehydration and dehydroxylation of coexis ting halloysite. The stable carbon isotopic release curves for samples at d epth (> 28 cm) initially yield values from - 15 to - 17 parts per thousand (PDB) and then with each time step lighten to values from - 18 to - 22 part s per thousand. Samples near the surface(< 16 cm) yield initial values from - 14 to - 16 parts per thousand. With successive time steps they initially become heavy (-13 to -11 parts per thousand) and then become lighter with values from -16 to - 17 parts per thousand. The average stable carbon isoto pic values for the six samples show a gradient trend from -21 parts per tho usand at depth (236 cm) to - 14 parts per thousand near the surface (6 cm). These results indicate that gibbsite offers a similar potential as the ped ogenic minerals calcite and goethite to be a recorder of soil carbon system atics. (C) 1999 Elsevier Science B.V. All rights reserved.