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Carbon and Oxygen Isotopes and Formation Conditions of the Upper Famennian Shale-Bearing Rocks in the Pripyat Trough, Southern Belarus
Lithology and Mineral Resources ( IF 0.8 ) Pub Date : 2020-01-31 , DOI: 10.1134/s0024490220010046
A. A. Makhnach , B. G. Pokrovsky , O. V. Murashko , O. L. Petrov

Abstract

The behavior of stable carbon and oxygen isotopes in carbonates during the deposition and diagenesis of sediments in the bioproductive Upper Famennian Pripyat Trough (southern Belarus) is discussed. Limestones and clayey limestones (Corg 0.92 ± 0.11%) are characterized by very low δ13C values (–9.6 ± 0.3‰). Parental sediments of these rocks were deposited in the shallow-water zone during slow downwarping episodes of the seafloor. Lithification of the sediments took place in oxidative conditions of the diagenesis zone. Organic matter was actively oxidized by free oxygen. Carbon dioxide with isotopically light organic carbon formed in this process was used during the crystallization of diagenetic carbonates that are visually indiscernible from the sedimentational variety. Marls, clayey marls, and carbonate-bearing clays (Corg 6.02 ± 0.80%) are characterized by δ13C values as high as –3.5 ± 0.6‰. In combustible shales (Corg >10%), δ13C value is ‒1.2 ± 0.6‰). The clayey rocks mark the episodes of sedimentation in relatively deep-water conditions that appear during the uncompensated sagging of the basin floor. The diagenetic zone with free oxygen was significantly decreased or absent at all. Here, oxygen of marine sulfates was the main or single oxidizer of organic matter (sulfate reduction). The sulfate oxygen is a weaker oxidizing agent than free oxygen. Therefore, much more organic matter was retained and fossilized in clayey rocks than in carbonate rocks. Organic carbon released during the sulfate reduction and mobilized later for the diagenetic carbonate formation was insufficient for the significant decrease of δ13C values relative to the marine carbonate standard. Isotopic composition of carbonate oxygen in the studied rocks is invariable and does not depend on the content of clay and organic matter. In these rocks, δ18O values are at the level (approximately –5‰) shown for the Famennian in the Global Chemostratigraphic Chart. This fact is consistent with the supposition that oxygen isotope composition of atmospheric precipitates, which influenced the rock formation via continental flow, is close to that of sea water in low latitudes where the territory of Belarus was located in the Late Devonian.


中文翻译:

白俄罗斯南部普里皮亚季槽中的上法门南页岩含碳,氧同位素和形成条件

摘要

讨论了生物生产性上法美尼亚普里皮亚季槽(白俄罗斯南部)中沉积物的沉积和成岩过程中碳酸盐中稳定的碳和氧同位素的行为。灰岩和粘质灰岩(C有机0.92±0.11%)的特征在于非常低的δ 13C值(–9.6±0.3‰)。这些岩石的母体沉积物是在海底缓慢向下翘曲期间沉积在浅水区的。沉积物的锂化发生在成岩带的氧化条件下。有机物被游离氧主动氧化。在成岩碳酸盐的结晶过程中使用了在此过程中形成的具有同位素轻有机碳的二氧化碳,这些成岩的碳酸盐在视觉上无法从沉积变化中分辨出来。泥灰岩,粘质泥灰岩和碳酸轴承粘土(C有机6.02±0.80%)的特征在于δ 13 C值高达-3.5±0.6‰。在可燃页岩(C有机> 10%),δ 13C值为‒1.2±0.6‰)。黏土岩石标志着在相对深水条件下的沉积现象,这些现象出现在盆地底板无补偿下垂的过程中。游离氧的成岩区明显减少或完全消失。在这里,海洋硫酸盐的氧气是有机物的主要或单一氧化剂(硫酸盐还原)。硫酸氧是一种比游离氧更弱的氧化剂。因此,与碳酸盐岩相比,粘土质岩中保留和化石的有机物更多。硫酸盐还原过程中的有机碳和释放以后动员成岩碳酸盐形成是不足以δ的显著降低13C值相对于海洋碳酸盐标准。被研究岩石中碳酸盐氧的同位素组成是不变的,并且与粘土和有机物的含量无关。在这些岩石,δ 18个O值是在全球化学地层图表示出了用于法门水平(约-5‰)。这一事实与这样的假设是一致的:大气沉积物的氧同位素组成通过大陆流影响了岩石的形成,与白俄罗斯领土位于泥盆纪晚期的低纬度地区的海水接近。
更新日期:2020-01-31
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