Clay mineralogy, organic carbon burial, and redox evolution in Proterozoic oceans

Nicholas J. Tosca*, David T. Johnston, Alexandra Mushegian, Daniel H. Rothman, Roger E. Summons, Andrew H. Knoll

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    63 Citations (Scopus)

    Abstract

    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.

    Original languageEnglish
    Pages (from-to)1579-1592
    Number of pages14
    JournalGeochimica et Cosmochimica Acta
    Volume74
    Issue number5
    DOIs
    Publication statusPublished - 1 Mar 2010

    Fingerprint Dive into the research topics of 'Clay mineralogy, organic carbon burial, and redox evolution in Proterozoic oceans'. Together they form a unique fingerprint.

    Cite this