赫南特冰期古海洋环境转变及其成因机制研究现状

The Genesis of Hirnantian Glaciation and Paleo-Ocean Environment During Ordovician-Silurian Transition

奥陶纪—志留纪转折期是地质历史时期一个重要的时间节点,形成了广为人知的赫南特冰期。该冰期的形成不仅诱发了显生宙第二大的两幕生物绝灭事件,同样使海水中的地球化学元素循环受到强烈的影响,如海洋碳酸盐与大气CO_2的循环受到扰动,全球海平面的降低促使水体逐渐富氧化,进而导致海洋水体Mo、S、U等同位素值出现波动,而冈瓦纳大陆被冰雪大面积覆盖则削弱了铝硅酸盐岩的风化作用,并限制了陆壳放射性^(187)Os、^(87)Sr同位素向海水的运移。赫南特冰期的成因一直是学界探讨的热点,被归结于风化作用、火山活动的增强或有机碳的大量埋藏等因素,但对冰期的成因机制与持续时间仍具有较多的争论,其原因很多,比如缺乏独立的生物地层单元同时能够控制浅水碳酸盐岩及深水泥页岩区,缺少转折界限处高精度的地球化学信息与全球等时地层格架下的对比,以及未能排除风化作用、成岩作用和构造热事件对古海洋与古气候重建时的扰动等。

The Ordovician-Silurian transition was an interval which revealed major changes in the Earth's biotic,climatic,and environmental systems,triggering the Hirnantian glaciation and end-Ordovician mass extinctions.The paleo-ocean environment experienced intense shift and exerted a crucial impact on the global cycles of carbon,molybdenum and sulfur during O-S transition.In addition,the weakness of continental weathering reduced 187 Os and 87 Sr flux,resulting in positiveδ187 Os andδ87 Sr excursions.This first ice age,since the late Neoproterozoic,was the culmination of a cooling trend that had begun in the early or middle Ordovician,and was linked to some combination of reduced volcanic arc outgassing,enhanced silicate weathering,increased organic carbon burial and so on.There exist many problems about the genesis of Hirnantian glaciation,such as lacking in individual biostratigraphy which can control the carbonate platform in shallow water and mudstone in deep water simultaneously,as well as the high-precision analysis based on geochemical proxies and isochronous stratigraphic correlation.Meanwhile diagenesis,weathering and tectonic-thermal events also exerted an important impact on reconstruction of Hirnantian glaciation,resulting in inaccurate interpretation.