Speleothem organic matter content imaging. The use of a Fluorescence Index to characterise the maximum emission wavelength


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January 2005


The study of palaeoenvironments, especially pedologic and biologic environments, is fundamental to a complete understanding of continental climate changes. Many types of sediment contain organic molecules (OM) that were trapped during the depositional process, with the quantity and the nature of these organic molecules being strongly influenced by climate and other local factors. The quantity of organic matter in sediment can be measured by fluorescence intensity, but its nature is more difficult to determine. For this research, the organic molecules in stalagmites were analysed using emission fluorescence spectroscopy. The analysis of carbonated karst sediments was complemented by studies of clay, soil and seepage water samples. The main objective of this paper is to describe a method for the continuous imaging of the spectroscopic features of stalagmite organic molecules. Continuous imaging provides a means of circumventing the nonlinearity, both in space (of the sediment) and in time (of the sedimentation process), of the trapping of organic matter. This methodological report presents a protocol for calculating a Fluorescence Index (FI) that can be used in palaeoenvironmental studies of sediment. A similar approach to that used for determining E4/E6 ratios was used to determine the ratio of the fluorescence intensities of a sample at 514 nm and at 456 nm. This Fluorescence Index is strongly correlated to the wavelength of the maximum intensity of the organic matter spectrum. Due to the relatively stable chemical environment of calcite growth, changes in the Fluorescence Index can be interpreted as being due to changes in the nature of the organic molecules rather than to pH or quenching effects. As an illustration of how this index can be used, we present some examples of fluorescence indices for speleothem samples that show short-term and long-term environmental changes. To allow fuller palaeoenvironmental interpretations to be made, fluorescence indices need to be calibrated to environments and samples need


Chemical Geology, Vol. 214, no. 3-4 (2005-01-25).


Stalagmite, Organic Matter, Spectroscopy, Laser, Karst, Imagery, E4/E6



Subject: topical

Stalagmite; Organic Matter; Spectroscopy; Laser; Karst; Imagery; E4/E6