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 stratigraphic, petrographic, and geochemical criteria in order to minimize the effects of diagenetic alteration and contamination from noncarbonate components. Highest seawater 87 Sr/ 86 Sr 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 87 Sr/ 86 Sr values, these new data record the culmination of an increasing trend in seawater 87 Sr/ 86 Sr 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 87 Sr/ 86 Sr values are superimposed on the longer-term trend. Correlation between 87 Sr/ 86 Sr 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.

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in coordinated 02. No activation barrier is found for rotation of the 0-0 group from the side-bonded to the linear position. The most stable bond angle in O-Fe-O is calculated to be 180°. For dissociation of the 0-0 bond in side-bonded Fe02 to form O-Fe-O, an activation energy of 100 kcal/mol is calculated. Negative charge on the complex decreases the activation energy while positive charge increases it. Thus single iron atoms are a poor catalyst for dissociation of 02, which suggests that dissociative adsorption of 02 on iron surfaces most likely occurs on a site where the 02 interacts with several iron atoms.Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research and to the Venezuelan Government for a fellowship to support F.R. AppendixIn order to demonstrate that the procedure can handle a wide range of compounds, we have done calculations for FeH, FeH4, Fe2, Fe6, Fe(CO)s, FeO, Fe02, OFeO, NiH, NiH2, Ni2H2, NiO, Ni2, Ni6, and Ni(CO)4. While some of these calculations are still in preliminary stages, it has been possible to find satisfactory parameters in all cases even though the final optimized parameters have not been established for all these compounds. Parameters selected to give FeH a bond energy of 60 kcal/mol at an equilibrium bond length of 1.50 Á produce a calculation for FeH4 in good agreement with an ab initio calculation by Hood, Pitzer, and Schaefer;37 i.e., both calculations give a moderately stable 12345T2(t22e2) ground state with a 4p orbital occupancy around 1. Parameters selected to give Fe2 a bond energy of 30 kcal/mol38 at an equilibrium bond length of 2.3 Á when used in a Fe6 cluster calculation indicate a stable cluster with a "d" bandwidth of 3.5 eV, which is similar to 4.0 eV given by our Xa calculation. The Fe(CO)5 calculation gives orbital energies that differ by less than 0.5 eV from the principal bands in the experimental39 photoelectron spectrum of Fe(CO)5. Having verified that the calculational procedure gives reasonable results for a wide variety of transition-metal compounds, this paper has concentrated on iron-oxygen interactions. More details about the procedure are being published.40

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