The influence of fluid inclusions, organics, and calcite fabric on trace element distributions in stalagmites

Abstract

Growing stalagmites entrap water and organic molecules while incorporating trace elements in diverse crystallographic fabrics. However, the interrelationships between crystal defects, fluid inclusions, organic carbon incorporation, and trace element incorporation in stalagmites remain poorly constrained. Here we use for the first time Fourier Transformation Infrared Focal-Plane Array (FTIR-FPA) imaging to show the distribution of molecular water (3400 cm-1 absorbance) and organic matter (2983 cm-1) in calcite from recently grown stalagmites with known drip water hydrochemistry and one fossil stalagmite. To test if entrapped water in speleothem calcite affects the abundance or distribution of trace elements, the element distribution (Na, Mg, Sr, U, Al, Y and Zn) is mapped by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). Transmitted Light (TL) Microscope imaging constrains crystal fabrics and displays visible features such as crystallites, crystal defects like fractures, pores or at small scale fluid inclusions, as well as detrital clays. We find that stalagmite areas with strong Organic Matter fluorescence (FOM) via Confocal Laser Scanning Microscopy (CLSM) coincide with areas of high FTIR absorbance at 2983 cm-1, suggesting common co-location of multiple organic matter types in the stalagmite. While microscopic fluid inclusions visible in transmitted light are also detected by FTIR 3400 cm-1 absorbance, the FTIR additionally detects in some samples abundant water in submicroscopic fluid inclusions detected by FTIR 3400 cm-1 absorbance, but not evident in transmitted light microscopy. In most samples, Na concentration is elevated in regions of high fluid inclusion density, suggesting that a significant portion of Na may be hosted in entrapped water rather than in the calcite, consistent with the very low partitioning coefficient of Na in calcite. Only samples forming under extremely rapid drips, with 100-fold higher Y than other samples, feature Na concentrations which appear dominated by coupled Y-Na substitution and little influenced by FI Na. Our analysis provides new evidence of the influence of fluid inclusions on trace element content of stalagmites and illustrates the utility of several non- destructive imaging techniques to improve interpretations of proxy signals based on trace elements.