THE COMPOSITIONAL EVOLUTION OF MESOPROTEROZOIC AND NEOPROTEROZOIC CARBONATE ROCKS

Petrochemical data on Meso- and Neoproterozoic (Riphean and Vendian) c arbonates from the Urals and Siberia suggest that Upper Riphean carbon ate rocks differ from both older and younger equivalents in their lowe r dolomite/limestone and higher strontium abundance. It is suggested t hat the directional increase in the Ca-content of the Earth's sediment ary shell results from the hydrothermal interaction of seawater with o ceanic basalts, manifest in a progressive reduction of dolomite format ion throughout geological history. Superimposed cyclical variation in dolomite/limestone may reflect secular changes in the rate of CO2 intr oduction into the atmosphere. In particular, the relative low dolomite /limestone ratio seen in Upper Riphean carbonates may reflect low p(CO 2).