Pedogenic Siderite as a Tool for Reconstructing Paleoprecipitation: A Decade of Research on Ancient Hydrologic Cycles and a New Research Frontier for Hydromorphic Soil Systems

Pedogenic siderite (FeCO₃), and the more conventional calcite (CaCO₃), form under different environments with distinct hydrologic characters. Siderites form in wetland soils and are zonally distributed in the equatorial humid belt and temperate latitudes in settings with P-E > 0, while calcites accumulate in semi-arid and arid environments, and are zonally distributed in tropical-subtropical latitudes in settings with P-E < 0. More poorly known, pedogenic siderite occurs in a variety of morphologies, ranging from micron-scale laths, spherulitic nodules (sphaerosiderite) ranging from hundreds of microns up to a few millimeters in diameter, and macroscopic concretions of microcrystalline aggregates. Confident recognition of siderite requires micromorphologic investigations of hydromorphic soils or paleosols. The δ¹⁸O values of pedogenic carbonates serve as proxies for the isotopic composition of soil water and ultimately paleoprecipitation. Careful analysis of pedogenic sphaerosiderite and calcite allows for the differentiation between original δ¹⁸O signatures from subsequent diagenetic overprinting. Cross-plots of mineral δ¹⁸O and δ¹³C values frequently define invariant δ¹⁸O trends, i.e. meteoric sphaerosiderite or calcite lines (MSLs or MCLs), interpreted as representing precipitation from stable groundwater systems, allowing estimation of groundwaterδ¹⁸O values. Over the last decade, our research group has utilized the δ¹⁸O and δ¹³C of sphaerosiderite and calcite from Cretaceous paleosols from North Slope Alaska to paleoequatorial Colombia to reconstruct paleolatitudinal gradients of mineral and precipitation δ¹⁸O. The Cretaceous mineral δ¹⁸O gradient is much steeper than the modern mineral gradient. Isotope mass balance modeling suggests a ramped-up Cretaceous hydrologic cycle, with wider and stronger subtropical evaporative belt,, and major precipitation rate increases in the humid belts to simulate these trends. These results are at odds with Cretaceous GCM simulations. Current research focuses on 1) reconciliation of discrepancies between GCM simulations and isotope mass balance modeling, and 2) fine tuning of the sphaerosiderite proxy by calibration with modern empirical data.