Stable carbon isotope signatures preserved in authigenic gibbsite from a forested granitic–regolith: Panola Mt., Georgia, USA
نویسندگان
چکیده
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 w 12 Ž . Poths Cr C ratios of the FE III carbonate component in natural goethites. In: Taylor, H.P., Ž . O’Neil, J.R., Kaplan, I.R. Eds. , Stable Isotope Geochemistry: A Tribute to Samual Epstein, The x Geochemical Society. Special Publication No. 3, 257–270. and uses the following sample Ž . Ž . treatments: 1 Separation of the -2 mm fraction; 2 Four 10-day oxidation cycles with 30% Ž . Ž . H O ; 3 12-h open-system vacuum dehydration at 1108C; 4 1-h close-system oxidation with 2 2 Ž . Ž . 0.3 bar oxygen at 2008C; 5 Timed step-wise open-system extractions at 2308 to 2408C; and 6 A final 15-min closed-system combustion at 8508C with 0.2 bar oxygen. The evolved carbon dioxide, cryogenically collected during step 5, is operationally defined as hydroxide-bound Ž . Ž . carbon. X-ray diffractometry XRD and differential-scanning-calorimetry DSC indicate that the predominant phase transition at 2308C is the dehydroxylation of gibbsite. Gibbsite content of the bulk soilrsaprolite 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. CrH 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 Ž . carbon isotopic release curves for samples at depth )28 cm initially yield values from y15 to Ž . y17‰ PDB and then with each time step lighten to values from y18 to y22‰. Samples near Ž . the surface -16 cm yield initial values from y14 to y16‰. With successive time steps they ) Corresponding author. Tel.: q1-706-542-2384; Fax: q1-706-542-2425; E-mail:
منابع مشابه
Apparent gibbsite growth ages for regolith in the Georgia Piedmont
–Carbon bound in gibbsite, collected from a residual weathering profile developed on a Paleozoic granite in the Georgia Piedmont, was examined for its C content and found to be geologically young. The study site, located at the Panola Mountain Research Watershed, has developed a granite–saprolite–soil regolith in which C-gibbsite model ages deep within the profile (C-horizon) average about 8000...
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