Investigating the potential preservation of seasonal dripwater Mg/Ca and Sr/Ca variations in a speleothem from central Texas

High resolution paleoclimate records are critical for improving understanding of past climate. Speleothems have the potential to record climate variations on a seasonal time scale. Fast growing (≥0.5 mm/year) speleothems have been shown to preserve annual chemical laminae recorded as cyclical variations in trace elements. This study investigates the potential preservation of seasonal Mg/Ca and Sr/Ca variations in a slow growing (< ~0.05 mm/year) speleothem in central Texas. The speleothem was collected from a site that underlies a drip with observed seasonal variation in Mg/Ca and Sr/Ca, making it ideal for evaluating preservation of seasonal chemical laminae. The speleothem was cored, slabbed, dated (using U-series methods) and analyzed for high resolution variations in Mg and Sr concentrations using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). Three sets of parallel transects were ablated (using a 6 x 50 μm slit aperture) parallel to the stalagmite growth axis. The laser scanned at 1 μm/second and made three measurements per second, resulting in sub-micron sampling. Parallel transects were ablated along different growth intervals to determine signal reproducibility of high frequency Mg and Sr variations. Wavelet analysis was conducted using detrended time series to determine presence and length of statistically significant cyclicity. Preliminary U-series ages (n=2) indicate the interval of interest (1.6 cm) grew from around 10 ka to 8 ka, but the chronology is not well constrained. The median growth rate estimated from U-series dating is 15 μm /year with a range from approximately 4 μm/year to >1.6 cm/year. Line scans of several standard materials (NIST 612, BHVO-2G, MACS-3) and scans perpendicular to the stalagmite growth axis were measured with the same analytical parameters used for scans parallel to the stalagmite growth axis in order to eliminate the possibility that cyclicity could be an instrumental artifact. Wavelet analysis of these scans demonstrates that cyclicity greater than 2 μm/sec cannot be attributed to the instrument. There is good correspondence between Mg and Sr variations in three sets of paired, parallel transects. Detrended data show that Mg and Sr varied concordantly on smaller length scales despite differences in Mg and Sr variations over larger length scales. Wavelet analysis results indicate statistically significant cyclicity of Mg and Sr concentrations between 30 and 60 μm in 5 of 6 transects, suggesting that this speleothem may be recording chemical laminae of annual growth. Growth rate estimates (30-60 μm/year) derived from cyclical Mg and Sr variations are consistent with growth rate estimates from the preliminary U-series chronology. In addition, growth rates are comparable in magnitude to some regional speleothems from the same time interval. These results suggest that it may be possible to observe seasonal geochemical banding in a slow growing stalagmite from central Texas.