Paleoceanographic Interpretation of Variations in the Sulfur Isotopic Compositions and Mn/Fe Ratios in the Miocene Monterey Formation, Santa Maria Basin, California

Abstract

Variations in sulfur isotopic compositions of pyrite and Mn/Fe ratios for core samples from the Monterey Formation, Santa Maria Basin, correspond to sea level fluctuations associated with periods of glacial expansion in the Antarctic and changes in the oceanic thermal gradient during the Miocene. Elevated ratios of Mn/Fe occur in both the lower calcareous and middle phosphatic lithofacies during a period when sea level was relatively high. A change to more positive sulfur isotopic compositions of pyrite at or near the transition from the middle phosphatic to upper siliceous lithofacies is associated with a major drop in sea level and stabilization of cold bottom waters in the Pacific Ocean. An inverse relationship between titanium concentration and organic carbon content uggests both the elevated ratios of Mn/Fe and enriched isotopic values of pyrite occur within a condensed section. The occurrence of these geochemical signals within a condensed section may be helpful in identifying chemically reactive sedimentary strata and potential source rocks in frontier basins. In addition to fluctuations of sea level and oceanic circulation patterns, intra-basin variations, such as intensity of bottom currents, rates of bacterial sulfate reduction relative to rates of sulfate throughput in sediments, and thickness of the oxygen-minimum zone, influence manganese and sulfur reactions. These intra-basin processes resulted in differences in cycling of sulfur and sequestering of manganese within the Santa Maria basin. Specifically, the highest Mn/Fe ratios are observed near the flanks of the basin, whereas pyrite sulfur is enriched toward the center of basin. End_Page 205------------------------