FORMATION OF HISTORICAL PEDOGENIC SIDERITE IN PAH-CONTAMINATED ALLUVIAL CLAY SOILS, TENNESSEE, USA: PART II: STABLE ISOTOPIC DATA ON MODERN ANALOGUE FOR A DEEP-TIME PALEOCLIMATE PROXY

The sphaerosiderite paleoclimate proxy has been used to quantify isotope mass balances of Mesozoic hydrologic cycles. In the original formulation (Ludvigson et al., 1998), siderite-water ¹⁸O fractionations were based on published lab experiments at temperatures ranging from 33-197 ºC (Carothers et al., 1988), posing lingering questions about applications to lower sedimentary temperatures. The historic pedogenic siderite at Chattanooga Coke Plant Site (CCPS) provides an opportunity to empirically test several different published laboratory experiments on siderite-water ¹⁸O fractionations. Micron-scale siderite crystals were separated from soil matrix by centrifugation in water-soluble heavy liquid, and FeCO₃ was confirmed by XRD. Siderites from five depth intervals ranging from 92-200 cm in CCPS soil cores were analyzed for carbon & oxygen isotopes. Collectively, the data conform to a meteoric sphaerosiderite line trend (sensu Ludvigson et al., 1998) with average δ¹⁸O value of -4.8±0.6‰ VPDB. Individual horizons have average δ¹⁸O values ranging between -5.4±0.1‰ to -4.1±0.2‰ VPDB. The δ¹³C values of these siderites range from less than -17‰ to greater than +5‰ VPDB. The positive δ¹³C values are strongly suggestive of siderite formation involving microbial methanogenesis, an idea that we will test in upcoming field work. On-site field measurements of groundwater δ¹⁸O values and soil temperatures are needed to determine the most appropriate siderite-water ¹⁸O fractionation equation. Preliminary estimates can be made using the mean annual temperature of Chattanooga (15.7 ºC), and estimated groundwater δ¹⁸O values using Bowen’s (2009) OIPC 2.2 website. Given local relief in the watershed, and location near the edge of the Cumberland Plateau, the estimated δ¹⁸O of infiltrated groundwater from mean annual rainfall ranges between -7.2 to -5.1‰ VSMOW. There are still uncertainties about warming of local soil temperatures by exothermic microbial activity, but preliminary analysis suggests that the siderite-water ¹⁸O fractionation equation of Zhang et al. (2001) is more appropriate than that of Carothers et al. (1988) for equilibrium siderite precipitation at the CCPS site.