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.