Radiogenic isotope chemostratigraphy reveals marine and nonmarine depositional environments in the late Mesoproterozoic Borden Basin, Arctic Canada

Abstract

The ca. 1050 Ma Bylot Supergroup in Arctic Canada is one of the best-preserved archives of late Mesoproterozoic geochemistry and biology and offers evidence that this period of Earth history may have been more biogeochemically dynamic than previously appreciated. The Bylot Supergroup was deposited in the Borden Basin and is the most thoroughly studied stratigraphic succession from a series of broadly contemporaneous late Meso protero zoic intracratonic basins known as the Bylot basins. This ~6-km-thick mixed carbonate-siliciclastic succession has undergone mini mal postdepositional deformation and is now exposed on Baffin and Bylot Islands, Nunavut, Canada. Deep-water and tidally influenced carbonate facies, traditionally interpreted as marine, have yielded important insights into the evolution of Proterozoic seawater chemistry; however, more recent studies indicate that the Borden Basin was restricted marine or lacustrine for portions of its depositional history. Here, we present new multiproxy radiogenic isotope chemostratigraphic data spanning the Bylot Supergroup. A comparison of stratigraphic trends in radiogenic isotope data from hydrogenous (black shale 187Os/188Os and limestone 87Sr/86Sr) and detrital (whole-rock mudstone 143Nd/144Nd) sedimentary phases elucidates the complex hydrologic history of the Borden Basin and reconciles these disparate interpretations. Episodic coupling and decoupling between the composition of basin waters (from Os and Sr isotopes) and the local weathering input to the basin (from Nd isotopes) indicate that depositional environments within the Borden Basin fluctuated between marine and nonmarine (i.e., lacustrine). Variations in basin hydrology controlled secular sedimentation patterns through changes in basin water chemistry. These interpretations help to characterize the environment in which the early red algal fossil Bangiomorpha pubescens evolved. Episodically restricted epeiric seaways, such as within the Borden Basin, were relatively widespread within Rodinia and may have exerted unique selective pressures on eukaryotic evolution in the Mesoproterozoic Era. Hydrogenous and detrital radiogenic isotope chemostratigraphy, as implemented in this study, may provide a useful paleoenvironmental framework for future paleontological studies aimed at testing the role of freshwater environments in eukaryotic evolution. In addition, 87Sr/86Sr compositions from 81 new middle Bylot Supergroup marine limestone samples, calibrated by recent Re-Os geochronology, contribute to the terminal Mesoproterozoic marine 87Sr/86Sr curve. These data display a rise from ~0.705 to 0.706 that reflects weathering of the active Grenville orogenic belt and demonstrates a global increase in chemical weathering during the amalgamation of Rodinia.