What role does anoxia play in exceptional fossil preservation? Lessons from the taphonomy of the Posidonia Shale (Germany)
What role does anoxia play in exceptional fossil preservation? Lessons from the taphonomy of the Posidonia Shale (Germany)
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DOI：
10.1016/j.earscirev.2023.104323
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发表时间：
2023-01
影响因子：
12.1
通讯作者：
A. Muscente;Olivia Vinnes;Sinjini Sinha;J. Schiffbauer;E. Maxwell;G. Schweigert;R. Martindale
中科院分区：
文献类型：
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作者：
A. Muscente;Olivia Vinnes;Sinjini Sinha;J. Schiffbauer;E. Maxwell;G. Schweigert;R. Martindale
Konservat-Lagerstätten 是保存完好的多元素骨骼和软组织化石的矿床，提供了地质记录中古代生物和群落最完整的快照。一个典型的例子是德国西南部的波西多尼亚页岩，其中包含在~183年玛托阿尔纪海洋缺氧事件期间保存下来的一系列化石。对这一矿床的开创性研究得出了这样的假设：许多 Konservat-Lagerstätten 保存在停滞盆地中，那里的缺氧条件限制了软组织的退化。然而，迄今为止，还没有研究彻底调查推动波西多尼亚页岩化石矿化的地质微生物过程。因此，缺氧在其特殊保存中的作用仍然不确定。在这里，我们通过回顾波西多尼亚页岩的地质来解决这些问题；描述其化石的矿化情况；综合新数据和现有数据，为 Lagerstätte 的古环境和埋藏学开发新模型。尽管贝壳和碳酸盐骨骼元素以黄铁矿和碳质化石的形式保存下来，但非生物矿化组织主要通过磷酸盐化（将遗骸转化为磷酸钙矿物质）而保存下来。磷酸盐化的明确例子包括菊石壳、甲壳动物甲壳、鱼龙遗骸、粪化石和鞘藻、地幔组织和墨囊。磷化甲壳动物和胶体动物含有充满黄铁矿、闪锌矿和铝硅酸盐矿物的裂缝。这种裂缝很可能是在埋藏压实过程中产生的，它使磷化组织破裂，将其有机物暴露于集中的微生物硫酸盐还原作用下，从而导致硫化物和粘土矿物的形成和填充。这些观察结果表明，磷化作用发生在成岩作用早期，在埋藏压实和微生物硫酸盐还原之前，在（亚）含氧底层水之下，并证实了动物在盆地和/或缺氧水体边界沿线的环境中短暂的氧合脉冲期间得以保存的假设。总的来说，我们的研究结果支持这样的观点：缺氧底层水并不能直接促进特殊的保存；事实上，它可能会阻碍它。 Konservat-Lagerstätten，特别是“停滞沉积物”，往往在氧化还原梯度陡峭和/或高沉降速率的（亚）氧沉积环境中形成。在这些条件下，生物体迅速埋藏在氧化还原边界以下，其中集中的地质微生物过程促进了它们的矿化，而降解则受到周围微环境中氧化剂供应的限制。
Konservat-Lagerstätten—deposits with exceptionally preserved fossils of articulated multi-element skeletons and soft tissues—offer the most complete snapshots of ancient organisms and communities in the geological record. One classic example, the Posidonia Shale in southwestern Germany, contains a diverse array of fossils preserved during the ∼183 Ma Toarcian Oceanic Anoxic Event. Seminal work on this deposit led to the hypothesis that many Konservat-Lagerstätten were preserved in stagnant basins, where anoxic conditions limited soft tissue degradation. To date, however, no studies have thoroughly investigated the geomicrobiological processes that drove fossil mineralization in the Posidonia Shale. As a result, the role of anoxia in its exceptional preservation remains uncertain. Here, we address these issues by reviewing the geology of the Posidonia Shale; describing the mineralization of its fossils; and synthesizing novel and existing data to develop a new model for the paleoenvironment and taphonomy of the Lagerstätte. Although shells and carbonate skeletal elements were preserved as pyritized and carbonaceous fossils, non-biomineralized tissues were primarily preserved via phosphatization (transformation of remains into calcium phosphate minerals). Unambiguous examples of phosphatization include ammonite shells, crustacean carapaces, ichthyosaur remains, coprolites, and coleoid gladii, mantle tissues, and ink sacs. Phosphatized crustaceans and coleoids contain cracks filled with pyrite, sphalerite, and aluminosilicate minerals. Such cracks were likely generated during burial compaction, which fractured phosphatized tissues, exposed their organic matter to focused microbial sulfate reduction, and thereby led to formation of, and infilling by, sulfide and clay minerals. These observations indicate that phosphatization happened early in diagenesis, prior to burial compaction and microbial sulfate reduction, beneath (sub)oxic bottom water, and corroborate the hypothesis that the animals were preserved during ephemeral pulses of oxygenation in the basin and/or within environments located along boundaries of anoxic water bodies. Overall, our findings support the view that anoxic bottom water does not directly promote exceptional preservation; in fact, it may impede it. Konservat-Lagerstätten, particularly “stagnation deposits”, tend to form in (sub)oxic depositional environments with steep redox gradients and/or high sedimentation rates. Under these conditions, organisms are rapidly buried below the redox boundary, where their mineralization is promoted by focused geomicrobiological processes, and degradation is limited by the supply of oxidants in the microenvironments around them.