| Understanding the nature and dynamics of paleoclimate change is a central challenge in the Earth Sciences. Continued release of CO2 into the atmosphere and rising global temperatures can drive climate to closely resemble that of the Cretaceous within the next 100 years. Therefore, a look into warm periods in Earth history (i.e. Cretaceous) could provide an understanding of hydrologic cycles associated with past and future climate warming events. Here, pedogenic (soil formed) siderite (FeCO3) is applied as a paleoclimate proxy for analysis of warm periods in Earth's history, specifically to humid continental environments of the Cretaceous and Paleogene (145 to 23 Ma). Initial calibrations of this proxy were based on experimental laboratory studies, but actualistic studies of modern siderites are needed to substantiate the laboratory findings. Recently discovered modern (last 100 years) pedogenic siderite precipitation in Chattanooga Tennessee provides such an opportunity. Here, siderite forms in a wetland site contaminated with coal tar deposits, associated with coal coking operations from 1918-1987. This study examines the principal environmental parameters controlling modern pedogenic siderite precipitation: microbial ecology and groundwater chemistry. The groundwater chemistry and the presence of the microbial ecology associated with the field side indicate that at low sedimentary temperatures, the Carothers et al (1988) siderite-water fractionation equation estimates water delta 18O values that are lower than observed. The 18O fractionation equation from microbial laboratory synthesis experiments of Zhang et al. (2001) provides a much better fit to the field data. This critical, actualistic evaluation provides insight for interpretations of past climates because siderite is widely applied as a paleoclimate indicator, and unraveling these fractionation parameters will markedly improve the accuracy of future paleoclimate studies. |
