The Early Mississippian (Tournaisian) positive δ13C excursion (or TICE) is one of the largest recorded in the Phanerozoic and the organic carbon (OC) burial associated with its development is hypothesized to have driven global cooling and increased glaciation. We are testing the hypothesis that expanded ocean euxinia/anoxia drove widespread OC burial and the TICE and we are testing this hypothesis using uranium isotopes (δ238U) of Lower Mississippian marine limestones from southern Nevada as a global seawater redox proxy.
δ238U trends record a prominent mid-Tournaisian negative excursion (~0.30‰ magnitude) lasting ~1 My. The lack of correlation among δ238U values and water-depth dependent facies changes, terrestrial influx proxies (Al, Th, wt% carbonate), redox-sensitive metals (U, V, Mo, Re) and diagenetic proxies (Mg/Ca, Mn/Sr) suggests that the δ238U curve represents a global seawater redox signal. The negative δ238U excursion (indicating increased sediment deposition under oxygen-depleted conditions) is coincident with the onset and peak of the first TICE positive excursion supporting the v hypothesis that expanded ocean euxinia/anoxia controlled OC burial, we term this the Tournaisian oceanic anoxic event or TOAE. These results provide the first evidence from a global redox proxy that widespread ocean euxinia/anoxia controlled Tournaisian OC burial and enhanced long-term global cooling/glaciation. U modeling results indicate that during the TOAE, the area of euxinic/anoxic seafloor increased by 6x and that the negative δ238U excursion was initially driven by euxinic conditions which waned and was replaced by anoxic/suboxic conditions where OC burial continued, but there was low U fractionation/sequestration. Comparisons between modeled OC burial amounts of the TICE versus the Late Ordovician (Hirnantian) positive δ13C excursion (HICE), which occurred during peak Gondwanan glaciation, indicates that substantially more OC was buried during the TICE and adds strong support that the TOAE ultimately enhanced Tournaisian global cooling and increased glaciation.
In contrast to most Paleozoic and Mesozoic OAEs, the TOAE developed during (and further enhanced) long-term late Paleozoic global cooling. We interpret that the TOAE developed in response to this long-term cooling, which intensified atmospheric and ocean circulation, enhanced upwelling- and eolian-derived nutrient flux, increased productivity and dissolved O2 consumption, which lead to ocean euxinia/anoxia expansion.
Earth and Planetary Sciences
Level of Degree
Department of Earth and Planetary Sciences
First Committee Member (Chair)
Second Committee Member
Third Committee Member
uranium isotopes, seawater redox, ocean anoxia, Early Mississippian, oragnic carbon burial, limestone, Late Paleozoic Ice Age
Cheng, Keyi. "Early Mississippian ocean anoxia triggering organic carbon burial and enhancing Late Paleozoic Ice Age onset: evidence from uranium isotopes of marine limestones." (2019). https://digitalrepository.unm.edu/eps_etds/261