Comparisons of age estimates from Strontium-isotope values versus other Chronostratigraphy methods

Abstract

Strontium-isotope stratigraphy enables estimates of numerical dates for marine carbonate-bearing sediments by comparing their ⁸⁷Sr/⁸⁶Sr ratio to a reference curve of ⁸⁷Sr/⁸⁶Sr variations of global seawater through geologic time. That seawater reference curve is a published statistical LOWESS fit to a very large dataset (ca. 14,000) of ⁸⁷Sr/⁸⁶Sr measurements of carbonate fossils from known biozones which is converted to numerical ages using published timescales. In this study, we compute the numerical dates projected by this method for suites ⁸⁷Sr/⁸⁶Sr ratios from seven stratigraphic sections ranging from Ordovician through Oligocene. The strontium-isotope stratigraphy from the Dawangou section in Tarim Basin of NW China that hosts the Auxiliary GSSP for the base of the Late Ordovician is a new dataset; and the others are published datasets. These ⁸⁷Sr/⁸⁶Sr-derived dates are compared to those estimated for the same horizons in these sections by other chronostratigraphic methods, including cyclostratigraphy calibrated to radioisotopic dates and standard biochronostratigraphy. Except for a published study of Lower Triassic strata from Chaohu section in Anhui, China and Middle Devonian strata Jebel Mech Lrdane section of Morocco, all the ⁸⁷Sr/⁸⁶Sr-derived dates are within the estimated uncertainties of the dates from the other chronostratigraphic methods, thereby validating the usefulness of strontium-isotope stratigraphy as one tool for obtaining the ages of marine carbonate-bearing deposits. However, the datasets from all three Paleozoic sections (two Ordovician, one Devonian) did display a noticeable offset bias toward older ages, which may be partly an artifact of inadequate adjustment of the LOWESS reference curve to current age models for the biozones. The ⁸⁷Sr/⁸⁶Sr-derived dates from Chaohu section of Early Triassic conodont zones were unacceptably systematically younger by ca. 1.45 Myr; thereby indicating higher-than-expected ⁸⁷Sr/⁸⁶Sr values, which may have been partially the result of a combination of a semi-restricted basin that was slightly diluted by ⁸⁷Sr-enriched fluvial waters from the chemical weathering of adjacent landmasses and of the laser ablation (LA-ICP-MS) analytical method applied to those samples that seems to yield higher ⁸⁷Sr/⁸⁶Sr ratios than the solution-mode ICP-MS method for ⁸⁷Sr/⁸⁶Sr measurements. In summary, with rare exceptions, the method of strontium-isotope stratigraphy and the statistical ⁸⁷Sr/⁸⁶Sr seawater curve is an important method to obtain semi-precise numerical dates from marine carbonate sediments.