黑色页岩与大洋缺氧事件的Re-Os同位素示踪与定年研究

地质历史中发育多次大规模的大洋缺氧事件并伴随有巨量的黑色页岩沉积, 对这些大洋缺氧事件发生时限及成因机制的研究已成为当前国际古海洋学研究的一个前沿领域。开展对大洋缺氧事件的研究,有助于了解古海洋演化、地球系统变化和地球其他圈层对生物圈的影响,以及金属成矿、油气形成及生烃环境。因此近年来人们从古海洋、古气候、古地理、古生物、大地构造和地质地球化学等学科,积极开展对大洋缺氧事件的研究。对大洋缺氧事件进行准确定年则是研究工作开展的首要任务之一;而如果用于定年的同位素体系能同时提供相关的环境变迁信息,则更加有利于我们对缺氧事件成因机制的认识。Re Os同位素作为近年来发展起来的一种新的同位素技术方法,恰好适用于对富有机质沉积岩和黑色页岩的定年,可以获得精确的地层沉积年龄,同时利用这些海洋沉积物的Os同位素比值还可获得古海水的Os同位素组成及随时间的演化规律。在中国华南地区新元古代—早古生代发育有多幕次的大洋缺氧事件和黑色页岩的巨厚沉积和广泛分布,利用Re Os同位素研究,不但可以精确厘定这些事件发生的时限,而且可以有效示踪当时的古海洋环境。

Re-Os isotope tracing and dating of black shales and oceanic anoxic events

Oceanic anoxic events took place many times in Earth's history. These events usually produced large amounts of black shale deposition. The study on the origin and precise age of these oceanic anoxic events has become one of the frontier research areas in palaeo-oceanography. The study of oceanic anoxic events can help us better understand the palaeo-ocean environmental changes, the evolution of the earth system and the effect of other Earth's spheres to the biosphere, as well as ore genesis, and hydrocarbon formation. In recent years the study of oceanic anoxic events has focused on various aspects such as palaeo-oceanography, palaeoclimatology, palaeogeography, palaeontology, tectonics, geology and geochemistry. Among these studies, the precise dating of the oceanic anoxic events is one of the key tasks. If the applied isotopic systems can, at the same time, provide information on palaeo-oceanic environmental change, these will enhance our understanding of the genesis of the oceanic anoxic events. The Re-Os isotope system is one of recently developed new geochemical technique, which can serve these purposes. The organic-rich black shales are very suitable for Re-Os isotope dating, and precise ages can be obtained. Furthermore, using Os isotope data of marine sediments, we can also obtain information on paleo-seawater Os isotopic compositions and their evolution during geological times. In South China, several-periods of oceanic anoxic events occurred from the Neoproterozoic to the Cambrian, and huge thickness of black shales deposited. Therefore, using Re-Os isotope method, we can not only estimate the time of such events, but also effectively trace the paleo-oceanic environment.