Construction of high-resolution trace element time-series in slow growth speleothems by LA-ICP-MS: Importance of parameter optimization and oriented band fabric imagery

Establishing high-resolution trace element time series in speleothems requires analytical techniques capable of representative sampling at sub-annual spatial resolution, but also possessing sufficient signal-to-noise to reliably discern potential season-to-season concentration variations. Growth rate is a major factor affecting both of these analytical challenges. To date, LA-ICP-MS, LA-MC-ICP-MS, SIMS, and μXRF techniques have been successfully applied to speleothem records, but nearly all studies have focused on speleothems with relatively fast growth rates of ≥ 100 μm/yr and which display well-defined banding. U-series dating of central Texas speleothems of the Edwards aquifer karst system demonstrate that calcite growth followed glacial-interglacial climate transitions spanning the past 70 ky. In contrast to previous high-resolution studies, central Texas speleothem growth rates seldom exceeded 25-50μm/yr and thus reside within a "slow-growth" (< 100 μm/yr) regime. Furthermore, seasonal banding is seldom revealed by conventional petrographic methods, thus complicating temporal/spatial sampling. To meet the analytical challenges posed by slow growth speleothems, we present an approach using LA-ICP-MS that integrates ablation and ionization parameters customized for speleothem calcite with oriented UV-fluorescence imagery. Ablation aerosol generation, transport, and ionization efficiency in the ICP are major interrelated factors affecting resolution of micro-scale, chemically-banded materials. To enhance chemical variations in finely banded materials, the aperture diameter must: (1) not exceed the critical sampling limit defined by the Nyquist frequency of the effective chemical waveform, whether sinusoidal or otherwise skewed with a higher frequency limb; and (2) must be capable of generating signals in excess of natural lateral heterogeneity and analytical noise components of measurement. Fabric-oriented, slow line scans, using narrow (5μm) rectangular slit apertures, offer substantially improved spatial resolution over spot apertures. The addition of nitrogen (5 mL/min) to the carrier gas following the ablation cell generates a three- to five-fold increase in sensitivity for Mg, Sr and Ba. The technique involves establishing a grid of laser spots over prospective sample areas in order to provide precise correlation points for overlaying of UV-fluorescence imagery. The georeferenced imagery, which reveals the banded growth fabric, is then used to orient line scans so that the long-axis of the slit aperture is held parallel to banding throughout the length of the scan. For optimization of ablation and ionization parameters, the grid is rotated 90° so that line scans are performed parallel to banding (for which natural lateral heterogeneity can also be evaluated). Through the application of these techniques we are able to construct trace element signals that closely mimic the UV-fluorescence band spacing and are consistent with U-series growth-rate predictions, suggesting the possibility of subannual resolution at growth rates as slow as 23 μm/yr.