Nitrogen isotope gradient on continental margins during the late Paleoproterozoic

The late Paleoproterozoic was a key period in the evolution of Earth’s surface environments and life. It is generally accepted that during the 2.43–2.06 Ga Great Oxidation Episode, the atmospheric oxygen level rose to an intermediate state (0.1 % to 10 % of the present atmospheric level) and the chemical composition of the oceans changed significantly. The paleontological record indicates that eukaryotes appeared during the late Paleoproterozoic. Nitrogen, an essential nutrient for life, is thought to have been one of the key factors in eukaryote evolution. However, nitrogen bioavailability and spatial heterogeneity in nitrogen cycle in the late Paleoproterozoic oceans remain controversial. Here, we examine carbon and nitrogen cycling in the late Paleoproterozoic ocean, via analysis of redox-sensitive element contents and organic carbon and nitrogen isotope compositions of black shales of the ca. 1.9 Ga Nuvilic Formation of the Povungnituk Group, Cape Smith Belt, Canada. Two diamond drill-holes (DDHs) 718–3333 (∼50 m-long) and 4G8069 (∼90 m-long) investigated in this study contain turbiditic greywackes and black shales deposited on the northern margin of the Archean Superior craton below the storm wave-base. Sedimentary rocks of DDH 4G8069 might have been deposited closer to the continental source area, as indicated by sandstone abundance, than those of DDH 718–3333. Redox-sensitive element (V, U, and Mo) contents in black shales of DDH 4G8069 are low and show positive correlation with Al contents, whereas those of DDH 718–3333 are irregularly elevated. Fe/Al ratios of DDH 718–3333 are low, which likely reflect Fe loss from sediments under anoxic water-column conditions. Depositional environment is inferred to be above and below the redoxcline for DDHs 4G8069 and 718–3333, respectively. Organic carbon isotope values of DDH 718–3333 are approximately 1.7 ‰ lighter than those of DDH 4G8069, which might reflect methane cycling at the redoxcline. Nitrogen isotope values are positive (> +3‰) for all the black shale samples, corresponding to aerobic nitrogen cycling and presence of bioavailable nitrate on the northern margin of the Superior craton during the late Paleoproterozoic. Furthermore, nitrogen isotope values for DDH 718–3333 are approximately 2 ‰ heavier than those for DDH 4G8069. The observed isotopic heterogeneity is similar to that in modern oxygen-minimum zones and could represent intense denitrification at the redoxcline. Although no ca. 1.9 Ga microfossils reported to date are considered unambiguously eukaryotic, the aerobic nitrogen cycling inferred for the Nuvilic Formation, as well as for the previously studied Rove, Virginia, and Menihek formations, suggests that nitrogen limitation was unlikely to have inhibited biological evolution during the late Paleoproterozoic, a finding that is consistent with molecular clock studies suggesting that the emergence of eukaryotes occurred during the Paleoproterozoic.