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 1 M and 1 M d 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, 2 M 1 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.
Highlights
The rise of atmospheric oxygen during the last ~2400 Ma (Ma: million years) of Earth history has driven profound and irreversible changes, including the oxidation of the world’s oceans, eukaryotic diversification, and the emergence of animal life (Cloud, 1976; Holland, 1984; Canfield, 2005; Gaidos et al, 2007)
Relatively little is known on the nature of clay mineralogy and the mechanisms behind clay-TOC interactions through much of the Precambrian, when the most crucial steps in Earth’s oxygenation took place (Weaver, 1989; Kennedy et al, 2006)
Weaver (1967; 1989) suggested that either smectitic weathering products were converted to more stable illite during burial diagenesis, or much of the illite is, itself, a primary precipitate formed under distinctly Proterozoic environmental conditions
Summary
The rise of atmospheric oxygen during the last ~2400 Ma (Ma: million years) of Earth history has driven profound and irreversible changes, including the oxidation of the world’s oceans, eukaryotic diversification, and the emergence of animal life (Cloud, 1976; Holland, 1984; Canfield, 2005; Gaidos et al, 2007). For this reason, clay mineral studies are central to understanding OM burial and O2 accumulation through time (e.g., Kennedy et al, 2002). Cambrian boundary reflects fundamental changes in continental weathering associated with the early evolution of land plants. In this view, plant-driven increases in chemical weathering and clay formation led to increases in OM burial, driving late Neoproterozoic O2 increase. Plant-driven increases in chemical weathering and clay formation led to increases in OM burial, driving late Neoproterozoic O2 increase To test this model and, more generally, to constrain better the evolution of clay mineralogy through time, we examine the quantitative relationship between clays and OM from the late Archean Eon to the early Cambrian Period. Proterozoic shales, we ask whether weathering style and magnitude of clay generation changed fundamentally through the Proterozoic and how these results relate, if at all, to changes in the carbon cycle and Neoproterozoic rise of atmospheric O2
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