The strontium isotopic composition of Ordovician and Silurian brachiopods and conodonts: relationships to geological events and implications for coeval seawater

Abstract

One hundred and fifty-six 87Sr/ 86Sr ratios were measured from Ordovician and Silurian brachiopod shells, marine calcite cements, and conodonts in order to establish the secular strontium isotope curve for the coeval seawater. Preservation of the brachiopod shell material has been evaluated by petrographic and geochemical criteria and only the well preserved internal secondary layer of the shells has been utilized for strontium isotope measurements. The results document a gradual decrease in 87Sr/ 86Sr, from 0.7091 to 0.7087, from Tremadoc to Llandeilo, a sharp decline to 0.7078 during the late Llandeilo-early Caradoc; little change during Caradoc and the Ashgil; and a steady rise to 0.7087 through the Silurian. These long-term (10 7 yr) variations, with magnitudes in the range of 10 −3, are interpreted to be controlled primarily by continental collisional tectonics and its associated erosion and weathering. The gradual decrease in 87Sr/ 86Sr ratio during the Early Ordovician may record the reduction in uplift and weathering rates due to waning of the Pan-African orogenies. The rapid decline near the Llandeilo/Caradoc boundary suggests a strong hydrothermal flux likely due to increased sea-floor spreading and a possible superplume event. The latter may have caused the prominent transgressive phase, the largest in the Phanerozoic, which would have muted continental flux input. Although the Caradoc was the main interval for the Taconic Orogeny, its impact on the Sr continental flux may have been delayed until the early Llandovery. This effect, complemented by reworking of glacial deposits near the Ordovician-Silurian boundary and enhanced by phases of the Silurian Salinic Orogeny, may have combined to give the progressive increase in the strontium isotope ratio through the Silurian. The scale and directionality of these changes makes the strontium isotope curve valuable for dating and correlation purposes.