Using X-ray Fluorescence and Absorption Spectroscopy to Investigate Redox-Sensitive Metals in Late Cretaceous Pedogenic Carbonate Nodules

Abstract

Synchrotron-based X-ray absorption spectroscopy (XAS), in the context of X-ray florescence (XRF) maps, is a powerful analytical tool that can define element speciation and characterize the local atomic structure of a suitable sample. While this type of analysis is regularly used to provide advances in a variety of scientific fields, including but not limited to material sciences, molecular physics, and medical sciences, it remains an underutilized tool in the (paleo)geosciences.  Here, we show that XAS can advance our knowledge of the Critical Zone in deep time. For this study, we used the X-ray fluorescence microprobe (XFM) beamline above the uranium (U) LIII-edge to create detailed XRF elemental maps. Based on these maps, we used X-ray absorption near edge spectroscopy (XANES) to probe U and Manganese (Mn) speciation of pedogenic carbonate nodules collected from Late Cretaceous paleosols from Big Bend National Park, Texas, USA. Pedogenic carbonate nodules are derived from soil waters that are oversaturated with respect to calcite. Accumulation of pedogenic calcite primarily occurs in semi-arid environments with mean annual precipitation ranging from 500-1200 mm/yr. Nodules are commonly used for uranium/thorium (U/Th) dating, atmospheric CO2 concentration (pCO2) reconstruction, and weathering intensity reconstruction using major and trace element concentrations. As prevalent as nodule research is in these various geological applications, little is known about the compositional variation on a microscale. We present and elucidate the texture, grainsize, and redox-sensitive elemental heterogeneity found within and between carbonate nodules. Preliminary results from U LIII-edge and Mn K-edge XANES showed primarily oxidized U and Mn.  XRF mapping of the samples indicate element discrimination between different carbonate fabrics as well as element zoning. The methods utilized in this study enable us to understand not only the microscale geochemistry of pedogenic carbonate nodules, but to provide an example of how XAS can be beneficial and useful for the paleoclimate and petrography community.