Clay Mineralogy, Organic Carbon Burial, and Redox Evolution in Proterozoic Oceans

Tosca, Nicholas J. and Johnston, David T. and Mushegian, Alexandra and Rothman, Daniel H. and Summons, Roger E. and Knoll, Andrew H. (2010) Clay Mineralogy, Organic Carbon Burial, and Redox Evolution in Proterozoic Oceans. Geochimica et Cosmochimica Acta, 74 (5). pp. 1574-1592. DOI

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Clay minerals formed through chemical weathering have long been implicated in the burial of organic matter (OM), but because diagenesis and metamorphism commonly obscure the signature of weathering-derived clays in Precambrian rocks, clay mineralogy and its role in OM burial through much of geologic time remains incompletely understood. Here we have analyzed the mineralogy, geochemistry and total organic carbon (TOC) of organic rich shales deposited in late Archean to early Cambrian sedimentary basins. Across all samples we have quantified the contribution of 1M and 1Md illite polytypes, clay minerals formed by diagenetic transformation of smectite and/or kaolinite-rich weathering products. This mineralogical signal, together with corrected paleo-weathering indices, indicates that late Archean and Mesoproterozoic samples were moderately to intensely weathered. However, in late Neoproterozoic basins, 2M1 illite/mica dominates clay mineralogy and paleo-weathering indices sharply decrease, consistent with an influx of chemically immature and relatively unweathered sediment. A late Neoproterozoic switch to micaceous clays is inconsistent with hypotheses for oxygen history that require an increased flux of weathering-derived clays (i.e., smectite or kaolinite) across the Precambrian-Cambrian boundary. Compared to previous studies, our XRD data display the same variation in Schultz Ratio across the late Neoproterozoic, but we show the cause to be micaceous clay and not pedogenic clay - paleo-weathering signals cannot be recovered from bulk mineralogy without this distinction. We find little evidence to support a link between these mineralogical variations and organic carbon in our samples and conclude that modal clay mineralogy cannot by itself explain an Ediacaran increase in atmospheric oxygen driven by enhanced OM burial.

Item Type: Article
Uncontrolled Keywords: 09AREP
Subjects: 01 - Climate Change and Earth-Ocean Atmosphere Systems
Divisions: 01 - Climate Change and Earth-Ocean Atmosphere Systems
Journal or Publication Title: Geochimica et Cosmochimica Acta
Volume: 74
Page Range: pp. 1574-1592
Identification Number:
Depositing User: Sarah Humbert
Date Deposited: 18 Dec 2009 15:59
Last Modified: 23 Jul 2013 09:56

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