Earth's earliest global glaciation? Carbonate geochemistry and geochronology of the Polisarka Sedimentary Formation, Kola Peninsula, Russia
Earth's earliest global glaciation? Carbonate geochemistry and geochronology of the Polisarka Sedimentary Formation, Kola Peninsula, Russia
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DOI：
10.1016/j.precamres.2013.06.007
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发表时间：
2013-09-01
影响因子：
3.8
通讯作者：
Fallick, A. E.
中科院分区：
文献类型：
--
作者：
Brasier, A. T.;Martin, A. P.;Fallick, A. E.
作为国际大陆科学钻探计划的Fennoscandian北极-俄罗斯早期地球钻探项目(ICDP Far-Deep)的一部分，在俄罗斯西北部科拉半岛的3A孔找到了古元古代辉长岩和伴生岩石。除了辉绿岩外，还发现了碳酸盐沉积岩和火山灰层(都变质到绿片岩级)。沉积学和地球化学学表明，透辉石沉积在开阔的海相文石沉积环境中。对同一地理区域的岩芯和露头进行采样，产生了一些用于分析的锆石，其中大部分是继承的。然而，在20.01米深的凝灰岩中产生了年龄为2434+/-1.2 Ma(+/-6.6Myr，包括衰变常数不确定性)的锆石，我们将其解释为岩浆年龄。这些数据与下伏侵入岩的测年相结合，表明在大约2430-2440 Ma的一段时间内，波利萨卡沉积建造中的方英岩和下伏碳酸盐沉积。碳酸盐岩可能最初包括文石灰岩，主要沉积在深水环境中(即至少在风暴波底部以下)，赋存于透辉岩之下。它们记录了两次无机碳增量C-13漂移，从大约0%的值到大约-5.4ppm的最小值，因为接近了与上复的硅质岩的接触。年长的(地层上较低的)走滑发生在直径单元底部以下约9米处，较年轻的走动发生在低于1米的位置。在这段时间内，镁/钙比值与C-13三角洲强烈相关(n=38，r=0.85)，结合岩石学观察，这表明第一次(地层较低的)漂移受到次生蚀变的影响，第二次漂移记录在重新沉积的白云岩碎屑中。人们很容易推测，这些白云岩碎屑沉积在准同时代的浅海水域，它们的低增量C-13值可能反映了氧化的大气甲烷对海洋表面的输入(因此也是冰川的原因)；白云岩随后被重新沉积到更深的海洋环境中。然而，当进一步发现受年龄限制的同时代前冰川前碳酸盐剖面时，这必须作为一个假说来检验。(C)2013爱思唯尔B.V.保留所有权利。
As part of the International Continental Scientific Drilling Program's Fennoscandian Arctic Russia-Drilling Early Earth Project (ICDP FAR-DEEP), Palaeoproterozoic diamictic and associated rocks were targeted and recovered in Hole 3A on the Kola Peninsula of NW Russia. In addition to the diamictites, carbonate sedimentary rocks and volcanic ash layers (all metamorphosed to greenschist grade) were encountered. Sedimentology and geochemistry suggest deposition of the diamictites in an open-marine aragonite-precipitating environment. Sampling of the core and of outcrops from the same geographical area yielded a number of zircons for analyses, the majority of which were inherited. However a tuff at 20.01 m core depth yielded zircons dated at 2434 +/- 1.2 Ma (+/- 6.6 Myr including decay constant uncertainties) that we interpret as a magmatic age. These data, combined with dates from underlying intrusions, indicate deposition of the Polisarka Sedimentary Formation diamictites and underlying carbonates during an interval of time from ca. 2430 to 2440 Ma. The carbonate rocks, which likely originally included aragonitic limestones, were deposited mostly in a deep-water setting (i.e. at least below storm wave base) and occur below the diamictite. They record two inorganic carbon delta C-13 excursions, from values of ca. 0% to minima of ca. -5.4 parts per thousand, as the contact with the overlying diamictite is approached. The older (stratigraphically lower) excursion occurs about 9 m below the base of the diamictic units and the younger one at 1 m below. Throughout that interval, Mg/Ca ratios correlate strongly with delta C-13 (n = 38, r = 0.85), and combined with petrographic observations, this indicates that the first (stratigraphically lower) excursion was modified by secondary alteration and the second is recorded in resedimented dolostone clasts. It is tempting to speculate that these dolostone clasts were deposited in penecontemporaneous shallow-marine waters, and that their low delta C-13 values might reflect input of oxidised atmospheric methane to the ocean surface (and therefore the cause of the glaciation); the dolostones were subsequently resedimented into the deeper marine settings. However this must be left as a hypothesis to be tested when further age-constrained contemporaneous pre-glacial carbonate sections are found. (C) 2013 Elsevier B.V. All rights reserved.