Triple Oxygen Isotope Investigation of Phanerozoic and Precambrian Shales

Abstract:

We have earlier demonstrated a step wise decrease in Δ’¹⁷O values of shales and granites at ~2.3Ga related to rapid continental emergence and appearance if crust susceptible to weathering (1,2). We continue this investigation of evolution of weathering during major subsequent events in Earth history, by providing new data for Archean and Proterozoic formations targeting the Mesoproterozoic “boring billion”, Neoproterozoic glaciations, Ediacaran evolution of multicellular life, and the Devonian emergence of forest soils. We used a variety of proxies such as boron content, highly-reactive versus total iron and stable isotopic correlations to distinguish lacustrine from marine shales. To estimate effect of diagenesis on transformation of loose sediment into shales, we investigated in detail bulk Δ’¹⁷O and δ¹⁸O values in a 5km deep Texas coast drillholes through Oligocene Frio Formation and overlying Miocene Formations (2). We found limited bulk silicate oxygen-isotopic change from shallow smectite-rich unconsolidated sediments, to deep illite-rich shales. This suggests that shales inherit and preserve weathering-derived low-Δ’¹⁷O and high-δ¹⁸O signatures, thus reflecting surface water isotopic values and temperature of weathering, and not diagenetic transformations. Reconstructed T =+15° and δ¹⁸Ow= -7.5° based on shales correspond to Mississippi basin weathering conditions during the Neogene. Quartz-illite inter-mineral fractionation maintain slope of 0.524 at 190°C. These data provide a baseline for Δ’¹⁷O and δ¹⁸O shale trends with growth of supercontinents affecting the hydrologic cycle and δ¹⁸O and Δ’¹⁷O value of precipitation, glacial episodes, and proportion of continents near tropical latitudes at each latitude. Also of interest is isotopic fractionation of oxygen by rubisco and stomatal conductance as microbes and plants gained more effective control over continental weathering and increased the Dole Effect through time. We discuss the reason for difference between Archean low-Δ’¹⁷O shales and granites and post Archean records with a step-wise change broadly coincident with the Great Oxidation Event.

¹Bindeman et al. (2018) Nature 557: 545; ²Bindeman (2020) RiMG 86, 10.2138/rmg.2020.86.8; 3 Retallack G. (2020) P-cubed, 540: 109536