Comparison of laser-ablation and solution-mode ICP-MS techniques for measuring speleothem 87Sr/86Sr values
Abstract:Reconstructing past changes in precipitation amount is critical to delineating controls on paleovegetation dynamics in South America. Although speleothem, lake, and ice core δ18O records from the region serve as a proxy of monsoon intensity, δ18O values do not serve as a direct proxy of local precipitation amount. To address this, we are developing a reconstruction of past moisture conditions using 87Sr/86Sr values measured in a fast growing speleothem (~2 mm/yr) from the central Brazilian savanna for which a late Holocene (0-1.8 ka) δ18O record has previously been developed. Speleothem 87Sr/86Sr values reflect the degree of water-rock interaction, as dictated by water residence time, and are used to interpret relative moisture conditions. In this study, we explore the potential of developing a high-resolution speleothem 87Sr/86Sr record using laser-ablation multi-collector ICP-MS by comparing analyses generated using both laser- and solution-mode techniques.
Laser-mode techniques allow for sampling of individual speleothem lamina, which provides a higher resolution record than solution-mode techniques and eliminates the potential of contamination from conventional drilling. Preliminary results from a younger portion of the speleothem yield a laser-mode (averaged) value of 0.72294 ± 0.00046 (reproduced with parallel scans) and a solution mode value of 0.72338 ± 0.00002 suggesting that these methods provide analogous 87Sr/86Sr values. However, the preliminary results on an older portion of this speleothem have solution- mode derived values of 0.72239 ± 0.00001 and 0.72166 ± 0.00001, with corresponding laser-mode values of 0.72188 ± 0.00029 and 0.72102 ± 0.00037, respectively. Suggesting that in the older portion of the speleothem, laser-mode techniques do not yield analogous solution-mode 87Sr/86Sr values. These differences in different areas of the speleothem highlight the need for method development and further testing.