Strontium and carbon isotope stratigraphy of the Llandovery (Early Silurian): Implications for tectonics and weathering

Abstract

A high-resolution 87Sr/ 86Sr curve and paired δ 13C carbonate-organic data set is generated for the Llandovery Series from the Ikla drill core in Estonia. A δ 13C carbonate curve is also presented from the Pancake Range in Nevada. Observed 87Sr/ 86Sr values in the Ikla drill core are at a minimum in the early Llandovery Rhuddanian Stage (∼ 0.7079 to 0.7080), and then trend to more radiogenic ratios in the basal part of the Telychian Stage. An 87Sr/ 86Sr high near ∼ 0.7084 is observed in the Telychian at the top of the studied section. The range of values is in general agreement with the data from previous sample sets of brachiopods and conodonts recovered from localities in North America and Europe that record a rising trend in the 87Sr/ 86Sr ratio throughout the Llandovery from approximately 0.7080 to 0.7084. The major increase in the 87Sr/ 86Sr ratio during the late Llandovery may be due to weathering of radiogenic source rocks that were uplifted during early Silurian continent–continent collisions. The Sr rise potentially coincides with the occurrence of an unusually thick sequence of K-bentonite beds representing large-magnitude ash falls in the early Telychian. A previously documented negative δ 13C excursion in marine carbonates in the lower Telychian interval of the Ikla core is quasi-synchronous with the increase in 87Sr/ 86Sr. Our new organic matter δ 13C data from the Ikla core confirm that this negative δ 13C carbonate excursion is not a result of diagenesis. Furthermore, a negative δ 13C excursion in carbonates from the early Telychian portion of the Pancake Range section in Nevada seems to confirm the global scope of this carbon cycle perturbation.