It is not known whether environmental O2 levels increased in a linear fashion or fluctuated dynamically between the evolution of oxygenic photosynthesis and the later Great Oxidation Event. New rhenium-osmium isotope data from the late Archean Mount McRae Shale, Western Australia, reveal a transient episode of oxidative continental weathering more than 50 million years before the onset of the Great Oxidation Event. A depositional age of 2495 ± 14 million years and an initial 187Os/188Os of 0.34 ± 0.19 were obtained for rhenium- and molybdenum-rich black shales. The initial 187Os/188Os is higher than the mantle/extraterrestrial value of 0.11, pointing to mild environmental oxygenation and oxidative mobilization of rhenium, molybdenum, and radiogenic osmium from the upper continental crust and to contemporaneous transport of these metals to seawater. By contrast, stratigraphically overlying black shales are rhenium- and molybdenum-poor and have a mantle-like initial 187Os/188Os of 0.06 ± 0.09, indicating a reduced continental flux of rhenium, molybdenum, and osmium to seawater because of a drop in environmental O2 levels. Transient oxygenation events, like the one captured by the Mount McRae Shale, probably separated intervals of less oxygenated conditions during the late Archean.
展开▼
机译:尚不清楚环境中的O2水平是线性变化还是在氧气光合作用的发展与随后的重大氧化事件之间动态波动。来自澳大利亚西部已故的太古宙山麦克雷页岩的新的-is同位素数据显示,在大氧化事件发生之前的五千万年前,氧化性大陆风化的短暂发作。富含rh和钼的黑色页岩的沉积年龄为2495±1400万年,初始 187 sup> Os / 188 sup> Os为0.34±0.19。最初的 187 sup> Os / 188 sup> Os高于地幔/地球外值0.11,这表明环境中的轻度氧合作用以及from,钼和放射性genic从中的氧化动员。上地壳以及将这些金属同时运输到海水中。相比之下,地层上覆的黑色页岩是rh和钼贫乏的,其地幔状初始 187 sup> Os / 188 sup> Os为0.06±0.09,表明大陆通量减少由于环境O2含量下降,导致to,钼和变成海水。短暂的充氧事件,如麦克雷页岩山捕获的事件,很可能是在太古宙时代后期,氧合条件较少的间隔期。
展开▼