A LIBS hyperspectral imaging methodology for high-resolution element profiling of speleothems: applications within the LEAP project

Speleothems (stalagmites, stalactites, flowstones, …) are currently one of the best datable terrestrial archives valuably recording past environmental and climatic variability. Their geochemical composition reflects complex interactions between host rock, rainwater infiltration and soil processes dependent on climate (Fairchild & Baker, 2012). In particular, Mg, Sr and Ba are commonly linked to prior calcite precipitation related to water availability and therefore indirectly to rainwater amount (Fairchild et al., 2000), while other elements such as P or S, may reflect organic matter cycling or anthropogenic and marine aerosol inputs (Borsato et al., 2007). Despite their importance, high-resolution spatial profiling of trace elements in speleothems remain analytically demanding, often requiring destructive sample preparation and time-consuming laboratory workflows.

Within the framework of the LEAP project (Learning from the Past - https://www.leap-belgium.be/) funded by BELSPO, we developed and implemented a Laser-Induced Breakdown Spectroscopy (LIBS) hyperspectral imaging methodology to obtain rapid, minimally destructive trace-element profiles along speleothem growth axes. The approach combines automated raster scanning with synchronized multi-spectrometer acquisition, producing two-dimensional LIBS spectral images over scan widths of 15–20 mm at 100 µm spatial resolution. Elemental ratio maps (Mg/Ca, Sr/Ca, Ba/Ca) are generated from the hyperspectral data cube and converted into one-dimensional profiles by buffered averaging along growth-parallel transects. A robust filtering and masking strategy based on Ca signal thresholds and calculated plasma parameters allows efficient exclusion of spectra affected by surface defects, detrital inclusions or existing sampling holes.

The method was first validated through comparison with LA-ICP-MS elemental mapping on a reference speleothem section, showing consistent relative variations and stratigraphic coherence in Mg/Ca, Sr/Ca and Ba/Ca profiles. Following validation, multiple trace-element profiles were extracted from speleothems from Hotton, Père Noël and Remouchamps caves (Belgium). In the Père Noël cave for example, the approach enabled the extraction of a continuous >1 m long profile at 0.1 mm spatial resolution, demonstrating the capability of LIBS hyperspectral imaging to generate high-resolution geochemical records over large stratigraphic distances.

Applied to a flood-impacted speleothem (calcite floor) from the Hotton Cave, the LIBS-derived profiles also revealed distinct elemental profiles associated with thin detrital layers incorporated within the calcite. This allows a more precise and objective assessment of past extreme flooding events at that location that can be compared to population migration information and changes in funerary practices. This contributes to the investigation of the link between climatic and environmental changes and human behaviour in the LEAP project.

 

References:

Borsato, A., Frisia, S., Fairchild, I.J.,, Somogyi, A.,, and Susini,J. 2007. Trace Element Distribution in Annual Stalagmite Laminae Mapped by Micrometer-Resolution X-Ray Fluorescence: Implications for Incorporation of Environmentally Significant Species. Geochimica et Cosmochimica Acta 71 (6): 1494–1512.

Fairchild, I. J., & Baker, A. (2012). Speleothem science: From process to past environments. Wiley-Blackwell.

Fairchild, I. J., Borsato, A., Tooth, A. F., Frisia, S., Hawkesworth, C. J., Huang, Y., McDermott, F., & Spiro, B. (2000). Controls on trace element (Sr–Mg) compositions of carbonate cave waters: Implications for speleothem climatic records. Chemical Geology, 166(3–4), 255–269.