Constraining pathways of microbial mediation for carbonate concretions of the Miocene Monterey Formation using carbonate-associated sulfate
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 dist...
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Veröffentlicht in: | Geochimica et cosmochimica acta 2012-02, Vol.78, p.77-98 |
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Zusammenfassung: | 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 δ
34S
CAS (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 δ
34S
CAS values near Miocene seawater sulfate (∼+22‰ VCDT), abundant CAS (ca. 1000
ppm), depleted δ
13C
carb (∼−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 δ
34S
CAS (up to ∼+37‰), CAS concentrations of ∼600
ppm, mildly depleted δ
13C
carb (∼−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 δ
13C 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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ISSN: | 0016-7037 1872-9533 |
DOI: | 10.1016/j.gca.2011.11.028 |