Microbially-mediated fossil-bearing carbonate concretions and their significance for palaeoenvironmental reconstructions: A multi-proxy organic and inorganic geochemical appraisal
Microbially-mediated fossil-bearing carbonate concretions and their significance for palaeoenvironmental reconstructions: A multi-proxy organic and inorganic geochemical appraisal
复制标题
DOI：
10.1016/j.chemgeo.2016.01.026
复制
发表时间：
2016-05
期刊：
影响因子：
3.9
通讯作者：
C. Plet;K. Grice;A. Pagès;A. Pagès;W. Ruebsam;M. Coolen;L. Schwark
中科院分区：
文献类型：
--
作者：
C. Plet;K. Grice;A. Pagès;A. Pagès;W. Ruebsam;M. Coolen;L. Schwark
碳酸盐结核在地质记录中广泛存在。然而，缺乏最近已知的类似物创建了一个新的方法来解开参与结核形成的主要微生物的球员，并建立其适合作为古环境记录器的需要。在这里，我们使用了地球化学和地质技术相结合，研究两个黄铁矿方解石结核，并比较其结果与其主机沉积物（Toarcian“Posidonia Shale”，183 Ma，德国西南部）。结核体的13 C亏损性质，δ 13 Ccarb平均值δ 13 Corg（− 32.4‰）和δ 13 C正构烷烃（− 34.9‰）表明硫酸盐还原菌（SRB）通过快速分解有机质（OM）在结核生长和细胞核保存中发挥了重要作用。然而，岩石评估分析从两个结核显示氢指数（HI）升高的身体和低HI值的边缘。这些值表明，大部分的微生物活动没有发生在结核体，而是在边缘和表面的核，这通常支持的特殊保存OM碳酸盐结核。此外，在自形黄铁矿的结核轮辋的富集表明，它们是通过增加铁还原（FeR）细菌的活动，再加上SRB活动的减少，导致更有利于黄铁矿的直接沉淀的条件。尽管δ 13 C值较低，但已知的脂质生物标志物，如无环扩展类异戊二烯或3β-甲基-藿烷，并未显示活跃甲烷循环的证据。本研究强调了碳酸盐结核在OM保存中的关键作用，并突出了其作为古环境记录器的巨大潜力。
Carbonate concretions are widespread within the geological record. However, the lack of recent known analogues creates a need for novel approaches to unravel the major microbial players involved in concretion formation and establish their suitability as palaeoenvironmental recorders. Here, we used a combination of geochemical and geological techniques to study two pyritiferous calcite concretions and compared the results with their host sediment (Toarcian “Posidonia Shale”, 183 Ma, SW-Germany).The13C-depleted nature of the concretion bodies, with average values of δ13Ccarb(− 14.8‰), δ13Corg(− 32.4‰), and δ13Cn-alkanes(− 34.9‰), indicates that sulphate-reducing bacteria (SRB), played a major role in the concretion growth and preservation of the nucleus via the rapid decomposition of organic matter (OM). However, Rock-Eval analyses from both concretions revealed elevated hydrogen indices (HI) in the body and low HI values at the rim. These values suggest that most of the microbial activity did not occur in the concretion body but rather at the rim and at the surface of the nuclei, which generally supports the exceptional preservation of OM in carbonate concretions. Furthermore, enrichment in euhedral pyrite in the concretion rims suggests they were formed through increased activities of iron reducing (FeR) bacteria coupled to a decrease of SRB activity leading towards conditions more favourable to the direct precipitation of pyrite. Despite low δ13C values, the known lipid biomarkers such as acyclic extended isoprenoids or 3β-methyl-hopanes did not reveal evidence of an active methane cycling. The present study emphasises the crucial role of carbonate concretion in OM preservation and highlights their great potential as palaeoenvironmental recorders.