Formation mechanisms of carbonate concretions of the Monterey Formation: Analyses of clumped isotopes, iron, sulfur and carbon

Carbonate concretions can form as a result of organic matter degradation within sediments. However, the ability to determine specific processes and formation temperatures of particular concretions has remained elusive. Here, we employ concentrations of carbonate-associated sulfate (CAS), SCAS and clumped isotopes (along with more traditional approaches) to characterize the nature of concretion authigenesis within the Miocene Monterey Formation. Integrated analyses reveal that at least three specific diagenetic reaction pathways can be tied to concretion formation. One calcitic concretion from the Phosphatic Shale Member at Naples Beach yields SCAS values near Miocene seawater sulfate (~ +22‰ VCDT), abundant CAS (ca. 1000 ppm), depleted Ccarb (~ –11‰ VPDB), very low concentrations of Fe (ca. 700 ppm) and Mn (ca. 15 ppm) and clumped isotope temperatures of ~18°C—characteristics most consistent with shallow formation in association with microbially mediated, organic matter degradation by nitrate, ironoxides and/or minor sulfate reduction. Cemented concretionary layers of the Phosphatic Shale Member at Shell Beach display elevated SCAS (up to ~ +37‰), CAS concentrations of ~600 ppm, mildly depleted Ccarb (~ –6‰), moderate amounts of Mn (ca. 250 ppm), relatively low Fe (ca. 1,700 ppm) and clumped isotope temperatures of ~27-35°C, indicative of somewhat deeper formation in sediments dominated by sulfate reduction. Finally, concretions within a siliceous host at Montaña de Oro and Naples Beach show minimal CAS concentrations, positive C values (up to +16‰), the highest concentrations of Fe (ca. 11,300 ppm) and Mn (ca. 440 ppm) and clumped isotope temperatures of ~28-35°C, consistent with formation in sediments experiencing methanogenesis in a highly reducing environment. This study highlights the promise in combining CAS and clumped isotope analyses with more traditional techniques to more precisely characterize subsurface biospheric processes and particularly how they relate to Fe-S-C cycling in ancient sediments. Two contrasting Snand W-bearing granites in the Nanling Range, South China: evidence from Hf isotopes