INTEGRATED SR ISOTOPE VARIATIONS AND SEA-LEVEL HISTORY OF MIDDLE TO UPPER CAMBRIAN PLATFORM CARBONATES - IMPLICATIONS FOR THE EVOLUTION OF CAMBRIAN SEAWATER SR-87 SR-86/

A high-resolution Sr isotope study of Middle to Upper Cambrian platfor m carbonates of the southern Great Basin significantly refines the str ucture of the existing seawater Sr isotope curve. Samples were selecte d using rigorous stratigraphic, petrographic, and geochemical criteria in order to minimize the effects of diagenetic alteration and contami nation from noncarbonate components, Highest seawater Sr-87/Sr-86 valu es over the past 2 b.y. are constrained to <0.7093 and occurred during the latest Middle Cambrian to earliest Late Cambrian, Integrated with published Cambrian seawater Sr-87/Sr-86 values, these new data record the culmination of an increasing trend in seawater Sr-87/Sr-86 beginn ing in Early Cambrian time. This trend is interpreted to reflect incre asing flux and/or isotopic ratio of riverine Sr delivered to Cambrian oceans in response to Pan-African orogenesis and attendant enhanced ch emical weathering. High-resolution changes in seawater Sr-87/Sr-86 val ues are superimposed on the longer-term trend, Correlation between Sr- 87/Sr-86 values and sea level leads us to propose a model that links c hanges in weathering rates and riverine Sr flux with variations in exp osed continental surface area related to short-term (1-5 m.y.) sea-lev el events during greenhouse times.