Scanning micro-X-ray fluorescence elemental mapping : a new tool for the study of laminated sediment records

Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 9 (2008): Q02016, doi:10.1029/2007GC001800.The utility of elemental mapping by scanning X-ray fluorescence (XRF) in the study of annual laminated sedimentary records was investigated on eight annually laminated sediment types. The examples were chosen to illustrate the potential of this approach in environments dominated by terrigenous, biological and chemical deposition. Individual laminae were identifiable in elemental maps of all sediment types and were enhanced through the use of data reduction techniques (e.g., principal components transformation). Laminae were least apparent in clastic dominated systems with no seasonal changes in sediment sources. In biologically dominated systems, element maps provided insights into the composition of the varve subcomponents, related to alternating terrigenous and biologically dominated seasonal periods of deposition. Chemically precipitated structures were more prevalent than expected from visual investigations alone and may provide an underutilized paleoenvironmental signature of changing limnological conditions. Elemental mapping offers a valuable tool for the study of laminated records that complements existing techniques (e.g., SEM, digital image analysis).Funding was provided through NSF Earth System History grants and an NSF Instrumentation grant awarded to J.T.O

Documentation of the physical properties of annually laminated (varved) sediments at a sub-annual to decadal resolution for environmental interpretation

This paper describes a method to digitize high-resolution physical properties of annually laminated lake sediments. We modified a water-acetone epoxy exchange method in order to preserve all the fine structural details that varved sequences provide. By applying sediment microstratigraphical investigations (X-ray radiography, SEM) and semi-automatic image analysis, we were able to establish detailed information about varve composition and structure. Mineral magnetic measurements were also applied for comparison. By comparing all available physical data at high resolution, it will be possible to identify environmental changes that has occurred in the lake drainage basin

Dissolution experiments of Na- and Ca-montmorillonite in groundwater simulants under anaerobic conditions

AbstractThe effects of simulant groundwater composition, pH and temperature on the dissolution and alteration of Na- and Ca-montmorillonite have been studied. Prior to the experiments, Wyoming type Na-montmorillonite, Swy-2, was purified to decrease the amount of accessory minerals. For Ca-montmorillonite experiments, the interlayer cation Na+ of purified Swy-2 was exchanged with Ca2+. The batch experiments were conducted with the purified montmorillonites in simulated fresh and saline waters at 25°C and 60°C under anaerobic conditions in an Ar atmosphere. The concentrations of Si, Al, Fe and Mg were analysed from ultra-filtered solution samples with High Resolution Inductively Coupled Plasma Mass Spectrometry (HR-ICP-MS) as a function of dissolution time. The pH evolution was also measured. The solid smectite phases were analysed with X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). XRD analyses indicated that the nature of the smectite mineral did not change over 140 days. However, the experimental conditions, more or less, modified the structure (e.g. the layer stacking of montmorilllonite; the partial dissolution of the smectite), which cannot be detected by XRD but was evidenced by chemical data, and can be considered as a possible contributor to the stacking faults of the montmorillonite. The log rates (mol g–1 s–1), based on the dissolved amount of Si, varied between –10.64 and –12.13 depending on the experimental conditions.</jats:p