Actualistic Calibration of the Pedogenic Siderite Paleoclimate Proxy

Pedogenic (soil formed) siderite (FeCO₃) is applied as a paleoclimate proxy for warm periods in Earth’s history, specifically to humid continental environments of the Cretaceous and Paleogene (145 to 65 Ma). Initial calibrations of this proxy are based on experimental laboratory studies. Actualistic studies of modern siderites are needed to substantiate laboratory findings.  Recently discovered modern (last 100 years) pedogenic siderite precipitation in Tennessee (Driese et al.,2010, JSR 80:943-954) provides such an opportunity.  Siderite is forming in a wetland site contaminated with coal tar deposits, associated with coal coking operations from 1918-1987.  This study examined the principal environmental parameters controlling modern pedogenic siderite precipitation; microbial ecology and groundwater chemistry.  Carbon and oxygen isotopes were analyzed from the siderites. The δ¹⁸O values for siderite range from -5.1 to -4.1 (VPDB; Driese et al.,2010, JSR 80:943-954).  Seasonal groundwater isotopes values range from -8.80 to -3.36 (VSMOW) and seasonal soil temperature measurements range from 9°C to 23°C.  The outcomes of this study will have significant impact on the use of the widely applied Carothers et al. (1988, GCA 52:2445-2450) siderite-water fractionation equation.  Our data indicates that at low sedimentary temperatures, the Carothers (1988) fractionation equation estimates water δ¹⁸O values lower than is actually the case.  The ¹⁸O fractionation equation from microbial laboratory synthesis experiments of Zhang et al. (2001, GCA 65:2257-2271) provides a much better fit to our field data.  This critical, actualistic evaluation is important for interpretations of past climates, and unraveling these fractionation parameters will markedly improve  accuracy of future paleoclimate studies.