CARBONATE-ASSOCIATED SULFATE, δ34S AND δ13C ANALYSES OF DOLOMITE CONCRETIONS OF THE MIOCENE MONTEREY FORMATION: INSIGHTS INTO PRECIPITATION ENVIRONMENTS

Dolomite concretions are a significant component of the Miocene Monterey Formation, California. Concretion growth is commonly thought to proceed as organic matter is progressively degraded in the subsurface, increasing pore water alkalinity. Previous studies have used carbon and oxygen isotope signatures to deduce pore water conditions during concretion growth; however, these proxies remain equivocal in that they do not help distinguish diagenetic zones dominated by organic matter oxidation by oxygen, nitrate, metal oxides and sulfate, and degradation by thermal decarboxylation. Here we employ concentrations of carbonate-associated sulfate (CAS) and d³⁴S_CAS in order to more uniquely determine the nature of concretion authigenesis in the Monterey Formation.

Combining the CAS concentration, d³⁴S_CAS, and d¹³C reveals that concretions from the Monterey Formation formed above, within, and below the zone of sulfate reduction, depending on locality. One nodular concretion from the Phosphatic Shale Member at Naples Beach yields d³⁴S_CAS near Miocene seawater sulfate (~22‰), abundant CAS (ca. 1000 ppm), and depleted d¹³C, which are consistent with shallow formation in association with organic matter degradation by oxygen, nitrate, and/or metal oxides, with only minor contributions from sulfate reduction. In contrast, cemented concretionary layers of the Phosphatic Shale Member at Shell Beach display elevated d³⁴S_CAS (up to ~37‰), CAS concentrations of ca. 600 ppm, and mildly depleted d¹³C (ca. –6‰), indicative of formation in sediments influenced more strongly by sulfate reduction. Finally, concretions of the Siliceous Member of Montana de Oro and Naples Beach show depleted d³⁴S_CAS (less than Miocene seawater), minimal CAS concentrations, and positive d¹³C values, consistent with formation in sediments experiencing methanogenesis. The sub-seawater sulfur isotope values likely indicate pyrite oxidation during the CAS extraction procedure. In light of these low d³⁴S_CAS values, we emphasize that caution must be exercised when interpreting d³⁴S_CAS­­ values in samples with high pyrite/CAS ratios.

This study highlights the utility of combining CAS analysis with more traditional techniques and reveals that concretions of the Monterey Formation formed across a range of diagenetic environments.