Paper No. 10
Presentation Time: 11:05 AM


LUDVIGSON, G.a.1, GONZALEZ, Luis A.2, FOWLE, D.A.3, ROBERTS, J.a.3, DRIESE, Steven G.4 and SUAREZ, Marina5, (1)Kansas Geological Survey, University of Kansas, Lawrence, KS 66047, (2)Department of Geology, University of Kansas, Lawrence, KS 66045-7613, (3)Geology, University of Kansas, Multidisciplinary Research Building, 2030 Becker Dr, Lawrence, KS 66047, (4)Terrestrial Paleoclimatology Research Group, Dept. of Geology, Baylor University, One Bear Place #97354, Waco, TX 76798-7354, (5)Dept. of Geological Sciences, U. of Texas, San Antonio, 1 UTSA Circle, San Antonio, TX 78249,

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.