ABSTRACT

The precipitation of both biotic and abiotic calcium carbonate is of great importance in modern and ancient global biogeochemical cycles. In the present-day oceans, the widespread precipitation of inorganic CaCO₃ on the seafloor or in the water column is possible only under extraordinary circumstances. By contrast, in the geological record, authigenic seafloor carbonate cements were widespread in the supersaturated, anoxic oceans of the Precambrian. Widespread authigenic carbonate precipitation ceased by the end of the Neoproterozoic as a consequence of global oceanic oxygenation: in the Phanerozoic, it occurred only during major anoxic events (for example, at the Permian/Triassic boundary) or in restricted, stagnant basins. Here, we present an anomalous record of CaCO₃ precipitation from the Cambrian Alum Shale Basin of the Baltica palaeocontinent with Precambrian-like authigenic, seafloor encrusting, crystalline carbonates.

The depositional environment of this well-recognized, cool-water, stagnant anoxic basin favoured local carbonate precipitation via the surplus generation of alkalinity in anoxic bottom waters. However, the correlation of the acme of authigenic carbonate formation with the onset and peak of the Steptoean Positive Carbon Isotope Excursion (SPICE) suggests a driver−trigger relation between the two phenomena.

On a smaller scale, carbonate authigenic precipitation is manifested by a cement-supported texture of the limestone intercalations in the Alum Shale facies. Instantaneous calcification of the faunal remains, termed here the Snedronningen phenomenon, must have been of great importance in the formation of Orsten-type Konservat Lagerstätten.

摘要

生物和非生物碳酸钙的沉淀在现代和古代全球生物地球化学循环中都具有重要意义。在当今的海洋中，只有在特殊情况下才有可能在海底或水柱中广泛沉淀无机 CaCO _{3 。}相比之下，在地质记录中，自生海底碳酸盐胶结物广泛存在于前寒武纪的过饱和缺氧海洋中。由于全球海洋氧化作用，在新元古代末期，广泛的自生碳酸盐沉淀停止了：在显生宙，它仅发生在主要缺氧事件期间（例如，在二叠纪/三叠纪边界）或受限的停滞盆地。_{在这里，我们展示了 CaCO 3}的异常记录来自波罗的海古大陆寒武纪明矾页岩盆地的降水，具有类似前寒武纪的自生、海底结壳、结晶碳酸盐。

这个公认的冷水停滞缺氧盆地的沉积环境有利于通过缺氧底水碱度的过剩产生局部碳酸盐沉淀。然而，自生碳酸盐形成的顶点与 Steptoean 正碳同位素偏移 (SPICE) 的开始和峰值之间的相关性表明这两种现象之间存在驱动-触发关系。

在较小的范围内，碳酸盐自生沉淀表现为明矾页岩相中石灰岩夹层的水泥支撑结构。动物遗骸的瞬时钙化，在此称为Snedronningen现象，在 Orsten 型 Konservat Lagerstätten 的形成中一定是非常重要的。