古全球变化与非等时韵律沉积及前寒武纪条带状铁矿建造形成的终止

前寒武纪各种地质记录表明了太古代—元古代早期古全球环境具有显著的陆少洋多和大气富ＣＯ２、贫Ｏ２的特征。ＢＩＦｓ大气－海洋系统中Ｆｅ２＋的氧化是导致铁质发生沉淀的一个重要过程，在前寒武纪早期缺Ｏ２的环境，生物－无氧氧化和日光－紫外线为主的宇宙射线光致氧化作用产生的直接非生物－无氧氧化过程，在前寒武纪早期缺Ｏ２环境对Ｆｅ２＋的氧化可能起着重要的和主导的作用。ＢＩＦｓ条带状矿石的宏、微观ＳｉＯ２或Ｆｅ２Ｏ３／Ｆｅ３Ｏ４单层厚度非等厚性表征了硅、铁供给非等量性与交替淀积非等时性的韵律堆积。文章提出了一种与地球自转速率有关的日长年际变化（古ＥＬＮｉｎｏ事件）－硅铁临界饱和浓度模式，从统一的地球动力学条件和不同类型ＢＩＦｓ的成因联系解析了这种韵律堆积的机制。元古代早—中期大气－海洋系统发生了Ｏ２增多和ＣＯ２减少的古全球变化，导致了２．２～１．８Ｇａ之间大气圈平流层古臭氧层的形成。古臭氧层大幅度消减日光－紫外线对地球表面的强烈辐射，不仅使全球性由日光－紫外线光致氧化作用直接产生的Ｆｅ２＋非生物－缺氧氧化过程消失，而且也保证了生命演化的延续和生物进入多样性繁衍。与其伴随的全球性巨厚碳酸盐建造，导致了上壳岩下伏的地球早期富铁?

THE PAST GLOBAL ENVIRONMENT AND CHANGES IN RELATIONSHIP WITH THE UNPERIODIC RHYTHMIC DEPOSITION AND THE CEASE FROM FORMING PRECAMBRIAN BANDED IRON FORMATIONS

For a better understanding of the past global environment and changes the ore forming process and the rhythmic deposition as well as the cease from forming Precambrian Banded lron Formations (BIFs) are discussed. The geologic records in the early Precambrian show that the past global environment is characteristic of the little principal land areas and high CO 2 and low O 2 levels in the atmosphere ocean system. Biotic anoxygenic processes and abiotic anoxygenic processes as result of the direct photooxidation of UV and sunlight cosmic radiation of iron, are of importance for the deposition of iron. The unisochore thickness of asingle layer in different size in banded ore reveals itself a unperiodic rhythmic deposition of chert and hematite/magnetite. The origin of the unperiodic rhythmic deposition is explained by the model of the daily variation in years (Palaeo EL Nino event) and SiO 2/Fe(OH) 3 critical saturated concentration. This interpretation suggests that the unperiodic rhythmic deposition with the scale of time in years relates to the interaction between earth rotation rate, ocean, atmosphere and fluid action on the ocean floor during the Precambrian. The rise of paleo O 2 levels in the Early Middle Proterozoic time was accompanied by the forming of worldwide carbonate formations, and had resulted in the forming of a palaeoozone layer in the atmosphere at 2.2 1.8 Ga which leads to a strong reduction of the direct photooxidation of UV cosmic radiation of iron and to further evolution of the early life on earth. In consequence of the later piling up of the sea basin and the change of the sea floor after the Proterozoic carbonate formations, the ferro bearing rocks in the early earth were covered and the sources of SiO 2/FeO became rare. The geologic effects of the past global changes in the Precambrian age lead to the ceasing of the Banded lron Formation in the earth′s history.