Solid phase and pore water profiles of compounds containing iron and sulfur have been determined by wet chemical, magnetic, and X-ray diffraction techniques in three California continental borderland basins. The observed profiles have been fit by simple reaction-diffusion models in order to determine reaction rates and constrain budgets for iron and sulfur. More than 95% of the solid phase reduced sulfur is pyrite, and down core profiles are well fit by a model in which net sulfate reduction rates decrease exponentially with depth. Net sulfate reduction rates determined from models fit to solid phase reduced sulfur measurements and pore for modern sediments in San Pedro, Santa Catalina, and San Nicolas Basins are, 11.4, 6.3, and 6.3--8.8 (μmol cm-2 yr-1), respectively. Measurements of solid phase iron species indicate that surficial sediments are enriched in easily-reducible ferric oxyhydroxides. The enrichment is maintained by a combination of oxidation of Fe2+ diffusing upward from underlying anoxic sediments, as well as input of fresh sediment enriched in ferric oxyhydroxides. The three primary sources for iron converted to pyrite and the sequence in which they are utilized are: ferric oxyhydroxides, magnetite and other crystalline oxides, and ''exchangeable'' iron in phyllosilicates. The majority (50--80%) of the iron converted to pyrite is from the silicates, and budgetary calculations indicate the amount of iron release from San Pedro basin silicates agrees within 35% with the amount of magnesium removed from pore water to solid phases. Fe2+ is enriched in near-surface pore waters because rates of dissolved iron production by oxyhydroxide reduction exceed rates of sulfate reduction and pyrite formation.

At depth, pore waters are sulfidic because rates of sulfate reduction exceed rates of iron release from silicates. Sulfide produced at depth diffuses upward until it reaches sediments with available iron, causing a step-like increase in solid phase sulfur concentration. Over 90% of the magnetite present in surficial sediments is dissolved at depth due to reaction with H2S. A model is developed to predict the depth at which magnetite dissolution should occur, based on sulfate reduction rates and the flux of ferric oxyhydroxides. The results of this model predict the onset of dissolution at depths of 5--40 cm in different basins and agree well with the observed depths of magnetite dissolution. ¿ American Geophysical Union 1990