Sr Isotopic Variations in Upper Proterozoic carbonates from Svalbard and East Greenland

Abstract

We report initial Sr(87)/Sr(86) values from an Upper Proterozoic carbonate succession from Svalbard and East Greenland. This succession, now tectonically separated into three sequences, is thick, relatively continuous, and well preserved. The relative ages of the samples from within the basin are well constrained by litho-, bio-, and chemostratigraphic techniques. The data from this study and related data from the literature are used to construct a curve of Sr(87)/Sr(86) for Upper Proterozoic seawater. The new data reported in this study substantially improve the isotopic record of Sr in sea water for the period between 650 and 800 Ma. The data indicate that delta Sr(87) values of seawater were variable but low (delta Sr(87) ~ -500 to -250) between 900 and 650 Ma, and rose rapidly to ~ +30 by 600 Ma. The range of variation of delta Sr(*7) in seawater during the Riphean-Vendian exceeds the entire range of delta Sr(87) in seawater during the Phanerozoic. While variation in the average isotopic composition of Sr delivered to the oceans by rivers can account for some of the observed range, changes in the ratio of submarine hydrothermal flux to river water (continental) flux are responsible for the large variation in seawater Sr isotopic composition. Changes in the continental flux of Sr to the oceans can be related to tectonic factors. Large changes in the hydrothermal flux to river water flux ratio indicated by the data could have significant consequences for the chemistry of the ocean-atmosphere system.