WHAT'S THE LOW DOWN ON SOIL-FORMED SIDERITE?: MODERN PROCESS STUDIES AND VALIDATION OF A DEEP TIME TERRESTRIAL PALEOCLIMATE PROXY

Pedogenic Siderite (PS) is common in hydromorphic mudstone paleosols from zonal belts with strong precipitation-evaporation balances. Siderite-bearing paleosols are closely associated with histosols in terrestrial deposits spanning the Late Paleozoic - Cenozoic. Common morphologies of PS include mm-scale spherulites, concretions of microcrystalline siderite, and micron-scale crystals dispersed in soil matrix. PS are used as proxies for δ¹⁸O of ancient soil groundwaters and thus paleoprecipitation from hosting paleosols, an application pioneered by Ludvigson et al. (1998, Geology 26:1039-1042). Groundwater δ¹⁸O values calculated from PS traditionally used the ¹⁸O fractionation equation from abiotic laboratory synthesis experiments of Carothers et al. (1988, GCA 52:2445-2450), but modern process studies of PS in surface soils show that the equation from microbial laboratory synthesis experiments of Zhang et al. (2001, GCA 65:2257-2271) more accurately predict δ¹⁸O values of groundwaters at earth surface temperatures. Driese et al. (2010; JSR 80:943-954) reported a meteoric sphaerosiderite line (MSL) value with δ¹⁸O value of -4.8±0.66‰ VPDB from historic PS in a surface soil in Chattanooga, TN. Field measurements of soil temperatures and groundwater stable isotopes at this site in March & August 2010 clearly show that the Zhang et al (2001) siderite-water fractionation equation produces the closest fit to observed conditions. An analysis of mean monthly records of soil temperatures, precipitation rates, and mean monthly δ¹⁸O values of precipitation (Bowen, 2011, OIPC 2.2) carried out to calculate theoretical mean monthly PS δ¹⁸O values, when compared with the MSL δ¹⁸O value at the site, indicates a strong seasonal bias toward peak precipitation rate, and PS formation coinciding with the seasonal high water table. An upcoming shift from the Carothers et al. (1988) to the Zhang et al. (2001) siderite-water fractionation equation for future paleoclimatic applications of PS will result in about a 2 per mil shift toward higher calculated water δ¹⁸O values in the Cretaceous meridional transect of the Americas shown by Suarez et al. (2011; Palaeo-3 307:301-312), and thus will force an upcoming reevaluation of the stable isotope mass balance of the Early Cretaceous hydrologic cycle.