Buick Roger
Department of Earth and Space Science and Astrobiology Program, University of Washington, Seattle, WA 98195-1310, USA.
Philos Trans R Soc Lond B Biol Sci. 2008 Aug 27;363(1504):2731-43. doi: 10.1098/rstb.2008.0041.
The atmosphere has apparently been oxygenated since the 'Great Oxidation Event' ca 2.4 Ga ago, but when the photosynthetic oxygen production began is debatable. However, geological and geochemical evidence from older sedimentary rocks indicates that oxygenic photosynthesis evolved well before this oxygenation event. Fluid-inclusion oils in ca 2.45 Ga sandstones contain hydrocarbon biomarkers evidently sourced from similarly ancient kerogen, preserved without subsequent contamination, and derived from organisms producing and requiring molecular oxygen. Mo and Re abundances and sulphur isotope systematics of slightly older (2.5 Ga) kerogenous shales record a transient pulse of atmospheric oxygen. As early as ca 2.7 Ga, stromatolites and biomarkers from evaporative lake sediments deficient in exogenous reducing power strongly imply that oxygen-producing cyanobacteria had already evolved. Even at ca 3.2 Ga, thick and widespread kerogenous shales are consistent with aerobic photoautrophic marine plankton, and U-Pb data from ca 3.8 Ga metasediments suggest that this metabolism could have arisen by the start of the geological record. Hence, the hypothesis that oxygenic photosynthesis evolved well before the atmosphere became permanently oxygenated seems well supported.
自约24亿年前的“大氧化事件”以来,大气明显已被氧化,但光合产氧开始的时间仍存在争议。然而,来自更古老沉积岩的地质和地球化学证据表明,产氧光合作用在这次氧化事件之前就已充分演化。约24.5亿年前砂岩中的流体包裹体油含有明显源自同样古老干酪根的烃类生物标志物,这些生物标志物未经后续污染而得以保存,且源自产生并需要分子氧的生物体。稍早(25亿年前)的含干酪根页岩中的钼和铼丰度以及硫同位素体系记录了大气氧的一次短暂脉冲。早在约27亿年前,来自外源还原能力不足的蒸发湖沉积物中的叠层石和生物标志物强烈暗示产氧蓝细菌已经演化出来。甚至在约32亿年前,广泛分布的厚层含干酪根页岩与需氧光合自养海洋浮游生物相符,来自约38亿年前变质沉积物的铀铅数据表明这种新陈代谢可能在地质记录开始时就已出现。因此,产氧光合作用在大气永久氧化之前就已充分演化这一假说似乎得到了充分支持。