Constraining pathways of microbial mediation for carbonate concretions of the Miocene Monterey Formation using carbonate-associated sulfate

Details Constraining pathways of microbial mediation for carbonate concretions of the Miocene Monterey Formation using carbonate-associated sulfate Constraining pathways of microbial mediation for carbonate concretions of the Miocene Monterey Formation using carbonate-associated sulfate

Feb 2012

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2012gecoa..78...77l&link_type=abstract

Geochimica et Cosmochimica Acta, Volume 78, p. 77-98.

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Carbonate concretions can form as a result of organic matter degradation within sediments. However, the ability to determine specific processes and timing relationships to particular concretions has remained elusive. Previously employed proxies (e.g., carbon and oxygen isotopes) cannot uniquely distinguish among diagenetic alkalinity sources generated by microbial oxidation of organic matter using oxygen, nitrate, metal oxides, and sulfate as electron acceptors, in addition to degradation by thermal decarboxylation. Here, we employ concentrations of carbonate-associated sulfate (CAS) and δ34SCAS (along with more traditional approaches) to determine the specific nature of concretion authigenesis within the Miocene Monterey Formation.Integrated geochemical analyses reveal that at least three specific organo-diagenetic reaction pathways can be tied to concretion formation and that these reactions are largely sample-site specific. One calcitic concretion from the Phosphatic Shale Member at Naples Beach yields δ34SCAS values near Miocene seawater sulfate (˜+22‰ VCDT), abundant CAS (ca. 1000 ppm), depleted δ13Ccarb (˜-11‰ VPDB), and very low concentrations of Fe (ca. 700 ppm) and Mn (ca. 15 ppm)—characteristics most consistent with shallow formation in association with organic matter degradation by nitrate, iron-oxides and/or minor sulfate reduction. Cemented concretionary layers of the Phosphatic Shale Member at Shell Beach display elevated δ34SCAS (up to ˜+37‰), CAS concentrations of ˜600 ppm, mildly depleted δ13Ccarb (˜-6‰), moderate amounts of Mn (ca. 250 ppm), and relatively low Fe (ca. 1700 ppm), indicative of 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 δ13C values, and the highest concentrations of Fe (ca. 11,300 ppm) and Mn (ca. 440 ppm), consistent with formation in sediments experiencing methanogenesis in a highly reducing environment. This study highlights the promise in combining CAS analysis with more traditional techniques to differentiate among diagenetic reactions as preserved in the geologic record and shows potential for unraveling subsurface biospheric processes in ancient samples with a high degree of specificity.

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