Isotopic evidence for oceanic barium cycling in the initial stage of the mesoproterozoic

Abstract

Recent studies have revealed the possibility of fluctuations of oxygen in the atmosphere and oceans across the Mesoproterozoic Era. Such fluctuations may have provided the necessary foundation for the emergence and possible early diversification of eukaryotic organisms at this time. However, the current understanding of this apparent coevolution requires further constraints on the operation of the biosphere. Here we analyzed barium isotopic compositions (δ¹³⁸Ba) together with other geochemical proxies within marine carbonates from the Gaoyuzhuang Formation (∼1570 Ma) of the Yanliao Basin, North China Craton. Our δ¹³⁸Ba values are similar to those of modern marine pelagic barite, but are lower and less scattered compared to modern seawater δ¹³⁸Ba compositions. We interpret these new δ¹³⁸Ba data to reflect the δ¹³⁸Ba composition of early Mesoproterozoic seawater, at least at the sediment-water interface. Moreover, we do not observe any clear fluctuations in δ¹³⁸Ba values coincident with carbonate δ¹³C excursions. We interpret these observations as a response to the accumulation of dissolved barium in the low-sulfate Mesoproterozoic ocean and thus, a buffering of δ¹³⁸Ba against significant variation in response to local factors such as primary productivity which influence modern marine δ¹³⁸Ba profiles. Moreover, our calculations estimate a ratio of ≈ 4:1 of continental weathering to hydrothermal activity based on a steady-state box model. Furthermore, our simulations suggest that the residence time of barium was 10–100-fold longer in the early Mesoproterozoic ocean compared to the modern, implying a much more conservative behavior of Ba and its isotopes.