REFINEMENT OF THE CAMBRIAN SEAWATER SR ISOTOPE CURVE THROUGH LASER-ABLATION MULTICOLLETOR ICP-MS

The highest ⁸⁷Sr/⁸⁶Sr ratios of the Phanerozoic secular seawater Sr isotope curve occur in the Cambrian and are only approached by Neogene values. Here we re-evaluate and further refine the late Early through early Late Cambrian seawater Sr isotope record through laser ablation multicollector ICPMS (LA-MC-ICPMS). Samples of marine cements from Great Basin and southern Canadian Rockies successions, which were correlated through independent biostratigraphic constraints, include those previously analyzed by solution thermal ionization mass spectrometry (TIMS) and newly collected samples. LA-MC-ICPMS analysis was carried out on 200 µm thick sections with 500 to 1500 µm lines using a 40 or 55 µm beam size and 60 to 75% output. Average internal precision is better than ±50 ppm. The accuracy of measured ⁸⁷Sr/⁸⁶Sr ratios is verified by analysis of a suite of in-house carbonate standards. The newly acquired LA-MC-ICPMS ⁸⁷Sr/⁸⁶Sr values of marine cements are in good agreement with values previously obtained by microdrilling and solution TIMS. Statistically significant (> ±0.0001) intra-sample discrepancies, however, exist for some samples between the TIMS and LA-MC-ICPMS ⁸⁷Sr/⁸⁶Sr values. This heterogeneity is typically accompanied by petrographic and cathodoluminescence evidence of post-precipitation alteration of the calcite cements but at a spatial resolution below that constrained by microsampling and solution TIMS analysis. Heterogeneity in Cambrian marine cements resolved at the 10¹ to 10² µm-scale resolution of laser ablation would lead to artificial homogenization of ⁸⁷Sr/⁸⁶Sr values if microdrilled for solution-TIMS analysis. The results of this study indicate LA-MC-ICPMS analysis of well-preserved carbonate marine cements can accurately and precisely recreate TIMS and solution obtained ⁸⁷Sr/⁸⁶Sr data while elucidating previously hidden secular seawater Sr isotope curve structure. The discovery and explication of these additional complexities has important implications for estimating paleo-weathering fluxes and rates and inferring tectonic history and paleoenvironmental and paleoclimatic conditions from marine carbonate ⁸⁷Sr/⁸⁶Sr values.