STRONTIUM ISOTOPE STRATIGRAPHY OF THE EARLY SILURIAN (LLANDOVERY): IMPLICATIONS FOR TECTONICS AND WEATHERING

A high resolution Sr isotope data set, generated from a fossiliferous drill core through the Llandovery in Estonia, shows that the 87Sr/86Sr ratio reaches a minimum in the early Llandovery, and then trends to more radiogenic ratios in the mid to late Llandovery. The range of values is in general agreement with previous sample sets of calcitic brachipods and conodonts recovered from localities in North America and Europe that record a rising trend in the 87Sr/86Sr ratio throughout the Silurian from approximately 0.70787 to 0.70835. Our data, however, show a decrease in the 87Sr/86Sr ratio from the end of the Ashgillian/beginning of the Rhuddanian until the early Aeronian, ranging from 0.70846 to 0.70804. During the Aeronian, 87Sr/86Sr ratios slowly trend towards more radiogenic ratios. Starting in the late Aeronian, the isotope record shows a rapid shift to radiogenic ratios, ranging from 0.70807 to 0.70841 through the remainder of the Aeronian and the Telychian. Increases in the 87Sr/86Sr ratio during the late Llandovery may be due to increased riverine flux of radiogenic Sr into the oceans due to weathering of non-volcanic continental silicate rocks that were uplifted during early Silurian continent-continent collisions. Alternatively, or in addition to non-volcanic weathering, a radiogenic Sr flux from exposed felsic volcanics in the Balto-Scandanavian region is also consistent with the presence of K-bentonites in late Aeronian and early Telychian strata.