Geochemical Fingerprints of Maximum Flooding Surfaces in the ca. 850 Ma Upper Fifteenmile Group, Yukon Reflect the Iron Shuttle

English Abstract:

We have performed a multi-proxy geochemical study (δ¹³C, δ⁵⁶Fe, major and trace elements) spanning three maximum flooding surfaces within the ca. 850 Ma, mixed carbonate-shale Reefal Assemblage in Yukon, northwestern Canada, in order to evaluate the role of relative sea level fluctuations on redox sensitive geochemical proxies. The maximum flooding (MF) intervals are characterized by very fine-grained and finely laminated mudstones with relative high %TOC and low Fe_T/Al. The transition above the MF intervals, which reflects shoaling at the beginning of the highstand systems tract (HST), shows a sharp decline in %TOC, rise in Fe_T/Al, and spikes in redox sensitive trace metal concentrations, most notably U. Iron isotope compositions are inversely correlated with Fe_T/Al across this transition, with a sharp decline in δ⁵⁶Fe corresponding to an increase in Fe_T/Al. We interpret these patterns to record the influence of the benthic iron shuttle, whereby ⁵⁶Fe-depleted iron is removed from mudstones (rendering them ⁵⁶Fe-enriched) during sea level maxima, when high relative sea level, low sedimentation rates, and high organic carbon flux result in anoxic conditions at the sediment-water interface. As these conditions retreat and dissipate during the HST, a return to oxic bottom waters results in an accumulation of ⁵⁶Fe-depleted iron and trace metals released from anoxic sediments immediately downslope. These results and our interpretation have multiple implications for the redox state of early Neoproterozoic seawater and the application of redox proxies in ancient organic-rich sediments. First, it is evident that at least during period of high sea level, anoxic conditions prevailed below the surface mixed layer on continental shelves. Second, fluctuations in relative sea level under these conditions generates large variations in redox proxies which do not directly reflect changes in the redox state of the global ocean. Finally, the iron shuttle, driven by dissimilatory iron reduction, was vigorous in the early Neoproterozoic, and while it may have resulted primarily in downslope transport of iron, this mobilized iron would have also diffused upslope, where it was removed under oxic conditions of the surface ocean.