RECENT PROGRESS ON THE PEDOGENIC SIDERITE PALEOCLIMATE PROXY

Pedogenic Siderite (PS) is a mineral that forms in hydromorphic mudstone paleosols from zonal climatic belts with strong positive precipitation-evaporation balances. Siderite-bearing paleosols are closely associated with histosols in siliciclastic terrestrial deposits spanning from the Late Paleozoic through the Cenozoic. Petrographic studies show PS morphologies including mm-scale spherulites, macroscopic concretions of microcrystalline siderite, and micron-scale crystals dispersed in soil matrix. PS is used as a proxy for the δ¹⁸O values of ancient soil groundwaters (i.e. paleoprecipitation) from hosting paleosols, an application pioneered by Ludvigson et al. (1998, Geology 26:1039-1042). A new statement on the PS paleoclimate proxy is available in the 2013 issue of SEPM Special Publication 104. Groundwater δ¹⁸O values calculated from PS traditionally have used the ¹⁸O fractionation equation from abiotic laboratory 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 experiments of Zhang et al. (2001, GCA 65:2257-2271) better predict δ¹⁸O values of groundwaters over a range of earth surface temperatures. Field measurements of PS δ¹⁸O values, soil temperatures, and groundwater stable isotopes at a site in Tennessee (CCP) show that the Zhang et al (2001) equation produces a closer fit to observed conditions. Analyses of mean monthly soil temperatures, precipitation rates, and δ¹⁸O values of precipitation (Bowen, 2011, OIPC 2.2) have been used to calculate theoretical mean monthly PS δ¹⁸O values at CCP. Our analysis suggests seasonal bias in PS MSL values toward the rainy season, and PS formation coinciding with the seasonal high water table. A meridional transect of PS δ¹⁸O values in Cretaceous paleosols in North America shown by Suarez et al. (2011; Palaeo-3 307:301-312) tracks changes in the stable isotope mass balance of the hydrologic cycle during this greenhouse period. A shift from the Carothers et al. (1988) to the Zhang et al. (2001) siderite-water fractionation equation results in about a 2 per mil shift toward higher calculated water δ¹⁸O values in the Cretaceous meridional transect, and thus will force a reevaluation of the stable isotope mass balance of the mid-Cretaceous hydrologic cycle.