Sedimentary pyrite framboid size-frequency distributions: A meta-analysis
Sedimentary pyrite framboid size-frequency distributions: A meta-analysis
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DOI：
10.1016/j.palaeo.2019.03.010
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发表时间：
2019-05
期刊：
影响因子：
--
通讯作者：
D. Rickard
中科院分区：
文献类型：
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作者：
D. Rickard
Framboid是一种黄铁矿微晶的微细球状集合体，广泛存在于各年代的沉积物中，其大小-频率分布被广泛用于确定古水的氧化状态。由于概率论中的乘性中心极限定理，沉积框体种群呈现单峰对数正态大小分布。对104个沉积物和沉积岩地点的48,063个黄铁矿框架体直径的377组测量数据的荟萃分析表明，沉积框架体的几何平均直径为6.2 μm，几何标准差为1.5。95%的沉积框体的几何平均直径在2.9~13.9 μm之间，在晚新世水柱中形成的现代同生框体的几何平均直径为4.7 μm，在沉积物中形成的成岩框体的几何平均直径为6.7 μm，估计测量误差为±10%。至少需要30次测量，理想情况下建议使用≥100测量。文献中报道的关于应用框体大小-频率测量的相互矛盾的证据主要是由于统计分析的内在本质：特定的框体大小-频率分布总是有限的机会是同生或成岩过程的结果。包括体视学测量的系统误差，同生Framboid的几何平均尺寸范围为2.9~10.9 μm，成岩Framboid的几何平均尺寸范围为3.1~20.9 μm，这表明在~3~~11 μm范围内，正、非正交环境中Framboid的几何平均直径存在显著重叠，在此范围内的Framboid几何平均直径不是古水氧化条件的可靠指标。与成岩框架体相比，同生框架体尺寸较小的原因似乎与水柱内框架体晶体生长的时间有限有关。将特定的框体大小-频率分布分配给确定的充氧条件的统计不确定性主要是由于(A)在有限的时间内形成了一些成岩的框体，从而也产生了较小的框体，以及(B)通过不可避免地对样本进行时间平均，形成了同生和成岩的框体种群的混合。现代和古代框体的大小没有任何显著差异，这表明框体不会随着时间的延长而继续增长：一旦形成，它们在地质时期保持不变，只有通过充填、过度生长和重结晶才会发生变化。其结果是，古框体可能对发现它们的沉积体系中的当代古环境进行采样。
Framboids, microscopic sub-spheroidal aggregates of pyrite microcrystals, are found in sediments of all ages and framboid size-frequency distributions are widely used to determine the oxygenation states of paleo-waters. Sedimentary framboid populations display unimodal log-normal size distributions as a consequence of the multiplicative central limit theorem in probability theory. The application of additive statistics to framboid populations and their consequent characterization in terms of arithmetic means and standard deviations is wrong because it predicts a subset of framboids with negative diameters.A meta–analysis of 377 sets of measurements of the diameters of 48,063 pyrite framboids from 104 sediment and sedimentary rock locations shows that the geometric mean diameter of sedimentary framboids is 6.2 μm and the geometric standard deviation is 1.5. Ninety-five percent of all sedimentary framboids have geometric mean diameters between 2.9 and 13.9 μm. The geometric mean diameter of modern syngenetic framboids formed within euxinic water columns is 4.7 μm and that of diagenetic framboids formed within sediments is 6.7 μm. The estimated measurement error is ±10%.Framboid mean diameters can be used to help discriminate the oxygenation state of paleo-waters. A minimum number of 30 measurements is required and ideally ≥100 measurements are recommended. The conflicting evidence reported in the literature for the application of framboid size-frequency measurements results mainly from the intrinsic nature of statistical analyses: there is always a finite chance that a particular framboid size-frequency distribution is a result of either syngenetic or diagenetic processes. Including the systematic stereological error in framboid size measurements, the geometric mean size range for syngenetic framboids is 2.9–10.9 μm and that for diagenetic framboids 3.1–20.9 μm. This suggests a significant overlap in geometric mean diameters of framboids from euxinic and non-euxinic environments between ~3 and ~11 μm. Geometric mean framboid diameters within this size range are not robust proxies for paleo-water oxygenation conditions. The origin of the smaller sizes of syngenetic compared with diagenetic framboids appears to be related to the limited time available for framboid crystal growth within the water column. The statistical uncertainty in assigning particular framboid size-frequency distributions to defined oxygenation conditions results mainly from (a) some diagenetic framboids forming over a limited time, thereby also producing smaller framboids and (b) mixtures of syngenetic and diagenetic framboid populations through unavoidable time-averaging of samples. The absence of any significant differences in the sizes of modern and ancient framboids indicates that framboids do not continue to grow over extended time periods: once formed they remain the same over geologic time periods and only change through infilling, overgrowth and recrystallization. The result is that ancient framboids may sample the contemporary paleoenvironment in the sedimentary system in which they are found.