Abstract Calcareous nannofossils are considered to be the most productive calcifying organisms, but the impact of their emergence on the chemistry of the ocean in the Late Triassic is not understood yet. Evolutionary details of this bio-event are missing due to the scarcity of well-preserved outcrops worldwide. The current study focuses on the calcareous nannofossil assemblage of the middle Norian to lower Rhaetian at Sommeraukogel and Steinbergkogel in the Northern Calcareous Alps (Austria). The sediments were deposited on a topographic high on a deeper shelf at a palaeolatitude around 20–30°N. The steps in the emergence of pelagic calcifiers are documented by a scanning electron microscope (SEM) study. The First Occurrence (FO) of coccolith, not identified at a species level, was recorded in the middle Norian (Alaunian 3). The oldest Crucirhabdus minutus and Archaeozygodiscus koessenensis were observed in the late Norian (Sevatian) and were followed by the FO of Crucirhabdus primulus in the early Rhaetian. These observations suggest a rather slow temporal evolution of the first coccolithophorids, with millions of years from the ancestor C. minutus to C. primulus, and in between the evolution of the new genus A. koessenensis. Diagenetic overprinting of the sedimentary succession has affected the preservation degree of the calcareous nannofossils but not the trend of their quantity in the studied samples, as proven by petrographic studies and trace element signatures of the calcareous sediments. This supports our quantitative estimates of the CaCO3 volume- and palaeo-fluxes due to the export productivity of the calcareous nannofossils, Prinsiosphaera triassica. Our results show the dominance and the increase in abundance of Prinsiosphaera triassica slightly above the Norian/Rhaetian boundary. However, calcareous nannofossils did not reach rock-forming abundances at this time and thus did not significantly influence the geochemical composition of the Western Tethys Ocean.

中文翻译：

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晚三叠世早期钙质微浮游生物的发育（奥地利北部钙质阿尔卑斯山）

摘要 钙质纳米化石被认为是生产力最高的钙化生物，但它们的出现对晚三叠世海洋化学的影响尚不清楚。由于世界范围内保存完好的露头稀少，这一生物事件的进化细节缺失。目前的研究重点是在北钙质阿尔卑斯山（奥地利）的 Sommeraukogel 和 Steinbergkogel 中诺里阶到雷蒂阶下层的钙质纳米化石组合。沉积物沉积在古纬度约 20-30°N 的较深陆架上的地形高处。扫描电子显微镜 (SEM) 研究记录了浮游钙化体出现的步骤。第一次出现 (FO) 未在物种水平上鉴定出的球状石，记录在诺里阶中部 (Alaunian 3)。最古老的 Crucirhabdus minutus 和 Archaeozygodiscus koessenensis 在晚诺里阶 (Sevatian) 被观察到，其次是在早期雷蒂亚 (Rhaetian) 的 Crucirhabdus primulus FO。这些观察结果表明，从祖先 C. minutus 到 C. primulus 有数百万年的时间，并且在新属 A. koessenensis 的进化之间，第一批球石藻的时间演化相当缓慢。岩相研究和钙质沉积物的微量元素特征证明，沉积层序的成岩叠加影响了钙质纳米化石的保存程度，但不影响其在研究样品中的数量趋势。由于钙质纳米化石 Prinsiosphaera triassica 的出口生产力，这支持了我们对 CaCO3 体积和古通量的定量估计。我们的研究结果表明，在略高于 Norian/Rhaetian 边界的地方，Prinsiosphaera triassica 的优势和丰度有所增加。然而，钙质纳米化石此时并未达到成岩丰度，因此对西特提斯洋的地球化学组成没有显着影响。