Unradiogenic strontium and moderate-amplitude carbon isotope variations in early Tonian seawater after the assembly of Rodinia and before the Bitter Springs Excursion

Abstract

Neoproterozoic sedimentary records reveal a poorly documented, ∼150m.y. time interval, between ∼1000 and 850Ma, that limit our understanding of C and Sr isotope variations in seawater and thus operation of the biogeochemical carbon cycle, changes in surface redox state, and fluctuations in continental vs. hydrothermal fluxes to the oceans following assembly of the supercontinent Rodinia. Carbon and Sr isotope data for carbonates from the Karatau Group of the Southern Ural Mountains in Russia provides a record partially covering the younger portion of this time interval. The lower part of the Karatau Group (LKG) contains well-preserved carbonate strata of the Katav, Inzer, and Min’yar formations that are focus of this study. Pb-Pb isochron ages for carbonates from the Inzer and Min’yar formations are 844±24 and 820±77Ma, respectively, establishing an early Tonian age. Carbon isotope data for unaltered carbonates in the LKG show a range of moderately positive to negative values from −2.8 to +5.9‰, with the majority below +3.0‰. 87Sr/86Sr values range from 0.70522 to 0.70534 in the Lower Inzer Member, increasing in the overlying Upper Inzer Member and Min’yar Formation, from 0.70555 to 0.70600. The Sr isotope range of the Lower Inzer Member is similar to that typical for the 1.03–0.95Ga seawater (0.70519–0.70554), suggesting that the Sr isotope composition of seawater in the aftermath of Rodinian assembly was unradiogenic for 150m.y. This pattern indicates that supercontinent-scale orogenic events do not result in an enhanced, long-term flux of radiogenic Sr from continents once the supercontinent assembled. To account for the prolonged period characterized by unradiogenic Sr seawater composition and moderate-amplitude C isotope variations, we suggest a supercontinent configuration ringed by continental arcs and with predominantly internal runoff. In this model, accompanying supercontinent blanketing resulted in thermal perturbation in the mantle, emplacement of multiple Large Igneous Provinces, and high continental freeboard. Weathering of juvenile arcs would have provided unradiogenic Sr and a high flux of sediments to the oceans, enhancing organic carbon burial and progressive oxygenation of surface environments and contributing to instability in the biogeochemical carbon cycle. Mass anomalies induced by protracted mantle plume activity led to True Polar Wander and brought the supercontinent, covered with juvenile mafic volcanics and segmented by failed rift systems, to low latitudes thus enhancing chemical weathering, atmospheric CO2 consumption, and eventually ushering in Snowball Earth conditions. Comparison with the records for early Paleoproterozoic strata indicates that a similar sequence of events bracketed both ends of the Proterozoic, highlighting fundamental relationships among plate tectonics, mantle dynamics, biogeochemical carbon cycling, surface oxygenation, and climate change.