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

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 similar to 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. delta N-15 values of the black shales are generally < +1 parts per thousand, 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.

Automated summary

After the Great Oxidation Event, Earth's atmosphere and surface ocean were oxygenated, resulting in significant changes in the environment and biology. Positive nitrogen isotopic compositions in marine deposits from the Paleoproterozoic era indicate aerobic nitrogen cycling and the availability of bioavailable nitrate, but the nitrogen cycling in freshwater environments remains unclear. Through analysis of black shales in the late Paleoproterozoic Embury Lake Formation, it was found that the formation accumulated in a sulfate-poor freshwater basin and had depleted nitrogen conditions. The nitrogen isotopic compositions suggest a nitrate-limited environment, which is different from coeval marine deposits. Nitrate depletion in the freshwater basin is proposed, while nitrate was available for nitrate-assimilating organisms in the ocean during the late Paleoproterozoic.