Middle-Late Ordovician (Darriwilian-Sandbian) decoupling of global sulfur and carbon cycles: Isotopic evidence from eastern and southern Laurentia

作者：Young Seth A^*; Gill Benjamin C; Edwards Cole T; Saltzman Matthew R; Leslie Stephen A

来源：Palaeogeography, Palaeoclimatology, Palaeoecology, 2016, 458: 118-132.

DOI：10.1016/j.palaeo.2015.09.040

• Sulfur isotopes
• Carbon isotopes
• Pyrite burial
• Oxygenation
• Weathering
• Ordovician

Middle-Late Ordovician sequences from the Appalachian Basin and Arbuckle Mountain regions of North America were analyzed for carbonate-associated sulfate (delta S-34(CAS)) and pyrite (delta S-34(pyr)) paired with carbonate (delta C-3(carb)) and organic matter (delta C-13(org)) chemostratigraphy. Two major negative drops in delta S-34(CAS) (12%o excursions) are recognized: the older decline in delta S-34(CAS) occurs within the Histiodella holodentata-Phragmodus polonicus Conodont Zones and the younger drop is within the Cahabagnathus sweeti-Ammphognathus tvaerensis (Baltoniodus gerdae subzone) Zones. These overall these negative shifts in delta S-34(CAS) have an antithetical relationship with positive shifts in delta S-34(pyr) (similar to+10 parts per thousand) and delta C-13(carb) (similar to+2 parts per thousand) recorded in the same successions. The older negative delta S-34(CAS) shift is coincident with the widely documented mid-Darriwilian delta C-13 excursion (MDICE), and the younger negative delta S-34(CAS) shift is coincident with another positive delta C-13(carb) shift in the early Sandbian. Geochemical modeling of these sulfur isotope shifts suggests that a decrease in the global rate of pyrite burial or isotope fractionation between seawater sulfate and sedimentary pyrite could account for these negative delta S-13(CAS) trends. Additionally, a substantial increase in the weathering flux of pyrite to the global oceans could also explain these secular sulfur isotope trends. While increased crustal weathering is broadly consistent with a sea-level lowstand, and the seawater Sr-87/Sr-86 isotope record of change in continental weathering in the late Darriwilian Stage of the Ordovician, geologic and geochemical proxy evidence do not support distinct pulses of continental weathering required to generate two separate negative shifts in delta S-34(CAS). These antithetical isotope trends may be best explained by changes in the marine redox state that significantly reduced microbially mediated pyrite burial and organic matter remineralization rates. Pulses of oceanic ventilation would have expanded habitable environments for marine organisms, and thus is broadly consistent with major increases in biodiversification during this period of the Ordovician.

• 出版日期2016-9-15