Nitrogen isotope evidence for stepwise oxygenation of the ocean during the Great Oxidation Event.

24 pagesInternational audienceThe Earth’s oxygenation represents one of the most important environmental drivers of life’s evolution, with the first rise, known as ‘the Great Oxidation Event’ (GOE), corresponding to unpreceded accumulation of atmospheric O2, changes in the flux of marine nutrients and possibly global glaciations. However, the detailed evolution of the GOE is still debated, as for instance the accumulation trends of oceanic versus atmospheric oxygen and the nature of biogeochemical responses to oxygenation. Here, we combine organic carbon and bulk nitrogen isotope compositions with major element concentrations and iron speciation data of sedimentary rocks recovered from two drill cores (T2 and T3) in the early Paleoproterozoic Turee Creek Group, Western Australia, to track the redox evolution of marine conditions during the GOE. T2 core samples of the Kungarra Formation, which consists of clastic sedimentary rocks overlaid by the glaciogenic Meteorite Bore Member, were deposited ∼2.31 Ga ago. T3 core intercepts, from bottom to top, quartzite of the Koolbye Formation, and shales and stromatolitic carbonates of the Kazput Formation, which were deposited around ∼2.25 Ga. Samples from T2 show minor variations of δ13Corg (avg. −34.5 ± 1.7‰, n = 30), with no significant difference between siliciclastic and glaciogenic sedimentary rocks. In contrast, T3 samples display an increase in δ13Corg from −32.0 to −24.8‰ (n = 54) from shales to carbonates. In both T2 and T3 cores, δ13Corg values are inversely correlated with Al2O3, suggesting a strong petrological control on δ13Corg values, inferred here as resulting from variable contributions of detrital organic matter. Bulk N contents are low, from 13.5 to 56.7 ppm and 15.7 to 53.4 ppm in T2 and T3 samples, respectively. The δ15N values show a bimodal distribution, with one mode at +2.6‰ in T2 and another at +8.8‰ in T3, independent from lithological variations. This δ15N values shift between T2 and T3 is interpreted as reflecting a change from dominating N2-fixers to NO3-assimilating organisms. This implies an increase of NO3− availability, and thus of O2 concentration, during the time interval separating the deposition of T2 and T3 sediments. Dissolved NO3− and O2 concentrations of the Turee Creek marine basin are estimated from two models using N isotope data. The dissolved NO3− concentration has an upper limit ranging from 1.91 to 3.04 µM, about one order of magnitude below the average value of modern oceans. The lower limit for dissolved oxygen concentration ranges from 1.8 to 4.4 µM, which is two orders of magnitude lower than modern oceans. Together with previous studies, the present data place quantitative constraints on the redox changes associated with the Great Oxidation Event and illustrate a stepwise increase of NO3− bioavailability between 2.31 to 2.25 Ga, in relation with increasing O2 level