The nitrate-limited freshwater environment of the late Paleoproterozoic Embury Lake Formation, Flin Flon belt, Canada

Abstract

Earth's atmosphere and surface ocean were pervasively and mildly oxygenated after the ca. 2.4 Ga Great Oxidation Event (GOE), which induced dramatic environmental and biological changes. Positive nitrogen isotopic compositions of Paleoproterozoic marine deposits reveal aerobic nitrogen cycling and the widespread availability of bioavailable nitrate, but the nature of nitrogen cycling in freshwater environment remains unclear. To elucidate the redox conditions and bioavailability of nitrogen, redox-sensitive element and nitrogen isotopic compositions, as well as the depositional age, were determined for black shales of the late Paleoproterozoic Embury Lake Formation, Flin Flon belt, Canada. This formation accumulated in a sulfate-poor freshwater basin isolated from the open ocean, as indicated by its low total‑sulfur/total-organic‑carbon ratio of ∼0.05. Our U-Pb ages for detrital zircons show that the formation is younger than 1862.2 ± 2.6 Ma. Redox-sensitive elements (V and U) in the black shales show positive correlation with detrital tracers Al and Ti. Low enrichment factors for V and U indicate minor authigenic accumulations of the two elements. Moreover, the black shales are depleted in Mn, suggesting deposition under suboxic to anoxic conditions. δ15N values of the black shales are generally < +1‰, indicating a nitrate-limited environment, and are distinct from those of previously reported coeval marine deposits. It is proposed that the nitrate was exhausted by nitrogen-loss processes such as denitrification in the freshwater basin, but was available for nitrate-assimilating organisms in the ocean during the late Paleoproterozoic.