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 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 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.

中文翻译：

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古元古代晚期大陆边缘氮同位素梯度

古元古代晚期是地球表面环境和生命演化的关键时期。人们普遍认为，在2.43–2.06 Ga大氧化事件期间，大气氧水平上升到中间状态（目前大气水平的0.1%至10%），海洋化学成分发生显着变化。古生物学记录表明，真核生物出现于古元古代晚期。氮是生命必需的营养素，被认为是真核生物进化的关键因素之一。然而，古元古代晚期海洋的氮生物利用率和氮循环的空间异质性仍然存在争议。在这里，我们通过分析加拿大史密斯角 Povungnituk 群 1.9 Ga Nuvilic 组黑色页岩的氧化还原敏感元素含量以及有机碳和氮同位素组成，研究了古元古代晚期海洋的碳和氮循环。本研究中调查的两个金刚石钻孔 (DDH) 718–3333（约 50 m 长）和 4G8069（约 90 m 长）含有沉积在风暴下方太古代高级克拉通北缘的浊积灰砂岩和黑色页岩波基。从砂岩丰度来看，DDH 4G8069 的沉积岩可能比 DDH 718-3333 的沉积岩更靠近大陆源区。 DDH 4G8069 黑色页岩中氧化还原敏感元素（V、U、Mo）含量较低，且与 Al 含量呈正相关，而 DDH 718~3333 则不规则升高。 DDH 718-3333 的铁/铝比较低，这可能反映了缺氧水柱条件下沉积物中铁的​​损失。推测 DDH 4G8069 和 718-3333 的沉积环境分别高于和低于氧化还原线。 DDH 718–3333 的有机碳同位素值比 DDH 4G8069 的有机碳同位素值轻约 1.7‰，这可能反映了氧化还原素处的甲烷循环。所有黑色页岩样品的氮同位素值均为正值（> +3‰），这对应于古元古代晚期有氧氮循环和上克拉通北缘存在生物可利用的硝酸盐。此外，DDH 718–3333 的氮同位素值比 DDH 4G8069 的氮同位素值重约 2‰。观察到的同位素异质性与现代最低氧区域的同位素异质性相似，可能代表氧化还原素的强烈反硝化作用。尽管迄今为止报道的 1.9 Ga 微化石均不被认为是明确的真核生物，但为 Nuvilic 地层以及之前研究的 Rove、Virginia 和 Menihek 地层推断的有氧氮循环表明，氮限制不太可能抑制生物进化。古元古代晚期，这一发现与分子钟研究一致，表明真核生物的出现发生在古元古代。