Low-latitude glaciation in the Palaeoproterozoic era
Low-latitude glaciation in the Palaeoproterozoic era
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DOI：
10.1038/386262a0
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发表时间：
1997-03-20
期刊：
影响因子：
64.8
通讯作者：
Kirschvink, JL
中科院分区：
文献类型：
--
作者：
Evans, DA;Beukes, NJ;Kirschvink, JL
前寒武纪冰川的时空分布是地球古气候最基本的谜团之一。在40亿年的前寒武纪历史中，只有在古元古代和新元古代的记录中发现了明确的冰川沉积物（1）。尽管如此，其中一些矿床与热带-而不仅仅是极地-古纬度指标密切相关，如碳酸盐岩，红层和碳酸盐岩（1，2）。这些观测结果得到了古地磁结果的定量支持，表明澳大利亚新元古代冰川成因岩石的纬度与5度相似（3-5）。同样可靠的古纬度的老，古元古代冰川成因的岩石还没有得到，因为这样的存款通常遭受不良的保存和二次磁叠加。然而，南非Kaapvaal克拉通上的太古代-古元古代“德兰士瓦超群”保存得非常好，因此适合于沉积古纬度的古地磁测定。在这一超群中，类似于22亿年前的Ongeluk熔岩是一个区域性广泛的、基本上未变形和未变质的喷出火山序列（6），整合地覆盖在冰川沉积物（Makganyene杂岩）之上。在这里，我们报告了11 +/- 5度的熔岩沉积纬度的古地磁估计，因此为底层同生冰川岩石。古气候之谜因此加深;一个基本上无冰的前寒武纪世界显然被两个漫长的冰河时代打断，两个冰河时代都在热带纬度范围内产生冰川沉积物。
One of the most fundamental enigmas of the Earth's palaeo-climate concerns the temporal and spatial distributions of Precambrian glaciations. Through four billion years of Precambrian history, unequivocally glacial deposits have been found only in the Palaeoproterozoic and Neoproterozoic record(1). Nonetheless, some of these deposits are closely associated with tropical- rather than just polar-palaeolatitudinal indicators such as carbonate rocks, red beds, and evaporites(1,2). These observations are quantitatively supported by palaeomagnetic results indicating a similar to 5 degrees latitude for Neoproterozoic glaciogenic rocks in Australia(3-5). Similarly reliable palaeolatitudes for the older, Palaeoproterozoic glaciogenic rocks have not yet been obtained, as such deposits commonly suffer from poor preservation and secondary magnetic overprinting. The Archaean-Palaeoproterozoic 'Transvaal Supergroup' on the Kaapvaal craton in South Africa is, however, exceptionally well preserved, and is thus amenable to the palaeomagnetic determination of depositional palaeolatitudes. Within this supergroup the similar to 2.2 billion-year old Ongeluk lavas are a regionally extensive, largely undeformed and unmetamorphosed, extrusive volcanic succession(6), which conformably overlies glaciogenic deposits (the Makganyene diamictite). Here we report a palaeomagnetic estimate of 11 +/- 5 degrees depositional latitude for the lavas, and hence for the underlying contemporaneous glacial rocks. The palaeoclimate enigma is thus deepened; a largely ice-free Precambrian world was apparently punctuated by two long ice ages, both yielding glacial deposits well within tropical latitudes.