CALIBRATION OF THE SIDERITE CLUMPED ISOTOPE PALEOTHERMOMETER

The clumped isotope composition of authigenic soil carbonates can be exploited to reconstruct the climate history of the continents. This proxy is particularly powerful because it provides temperature estimates that are independent of fluid isotopic compositions. To date, soil calcites have attracted most of the attention in non-marine clumped isotope work. This is not surprising considering that this mineral is commonly found in fossil soils, and calcite has long been a target for stable isotope studies. Pedogenic calcite, however, is not the only soil carbonate that can be used to reconstruct continental climate. Siderite (FeCO₃) has been widely used as a proxy record of the δ¹⁸O composition of paleoprecipitation, so it is also an appealing target for clumped isotope based climate reconstructions. Moreover, siderite offers two important important advantages over calcite. First, siderite forms in environments where pedogenic calcite is not found, so it can significantly expand the global extent of localities that are amenable to clumped isotope determinations. Second, siderite has the potential to expand the current reach of the clumped isotope proxy into older sediments and into sediments that experienced higher burial temperatures because it may be more resistant to the effects of solid-state diffusion than calcite. Here we present clumped isotope results from siderites grown under temperature-controlled conditions in the laboratory. We show that the siderite clumped isotope vs. temperature calibration is indistinguishable from calcite at current levels of analytical precision. We also present clumped isotope measurements of a natural siderite collected from Holocene sediments of the Mississippi River Delta. We find that siderites record warm season marsh water temperatures instead of mean annual temperatures as it has long been presumed.