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

Abstract

A high-resolution Sr isotope study of Middle to Upper Cambrian platform carbonates of the southern Great Basin significantly refines the structure of the existing seawater Sr isotope curve. Samples were selected using rigorous stratigraphie, petrographic, and geochemical criteria in order to minimize the effects of diagenetic alteration and contamination from noncarbonate components. Highest seawater 87Sr/86Sr values 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 87Sr/86Sr values, these new data record the culmination of an increasing trend in seawater 87Sr/86Sr beginning in Early Cambrian time. This trend is interpreted to reflect increasing flux and/or isotopic ratio of riverine Sr delivered to Cambrian oceans in response to Pan-African orogenesis and attendant enhanced chemical weathering. High-resolution changes in seawater 87Sr/86Sr values are superimposed on the longer-term trend. Correlation between 87Sr/86Sr values and sea level leads us to propose a model that links changes in weathering rates and riverine Sr flux with variations in exposed continental surface area related to short-term (1-5 m.y.) sea-level events during greenhouse times.