Shallow water redox conditions of the mid-Proterozoic Muskwa Assemblage, British Columbia, Canada

Abstract

The mid-Proterozoic was a time of apparent prolonged biological and geochemical stability, preceeding the environmental turmoil and rapid biological innovations that characterized the Neoproterozoic. Despite an upswing in work on the mid-Proterozoic over the past decade, basic aspects of the carbon cycle and Earth's surface redox state during this time period remain poorly understood. To provide a new window into the mid-Proterozoic environmental evolution, we have investigated carbonates in the well-exposed Muskwa Assemblage located in NE British Columbia, Canada, for a combined stratigraphic and geochemical study. Rare Earth Elements and Yttrium (REE+Y) geochemistry was applied to carbonate rocks deposited over a marine paleoenvironmental depth gradient in order to characterize prevailing water column redox conditions. These data provide evidence for an extremely shallow chemocline with oxic surface waters being restricted to within storm-wave base, which is consistent with globally low oxygen in the ocean-atmosphere system. Furthermore, there is evidence for facies-dependent carbonate δ¹³C variability within the Muskwa Assemblage comparable in scale to typical stratigraphic variability recorded in carbonate strata of this age, highlighting the problems of using small carbon isotope excursions for chemostratigraphy.