The Central Paratethys during Oligocene as an ancient counterpart of the present-day Black Sea: Unique records from the coccolith limestones
The Central Paratethys during Oligocene as an ancient counterpart of the present-day Black Sea: Unique records from the coccolith limestones
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DOI：
10.1016/j.margeo.2018.06.011
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发表时间：
2018-09-01
期刊：
影响因子：
2.9
通讯作者：
Bijl, Peter K.
中科院分区：
文献类型：
--
作者：
Bojanowski, Maciej J.;Ciurej, Agnieszka;Bijl, Peter K.
对来自喀尔巴阡山脉的四个等时世球石石灰岩层进行了检查，以重建副特提斯中部的古海洋条件。小且尺寸均匀的黄铁矿藤蔓占主导地位，低多样性甲藻包囊和颗石藻的出现以及生物标记分子28,30-二去藿烷的存在表明水体分层，上部水体相对充氧良好，但底部水体缺氧。后者的证据是盆地各地细密一致的水平层理、底栖生物及其遗迹化石的稀缺。石灰岩表现出典型的海洋三角洲 C-13 值，但相对于同期海水，O-18 显着减少，Sr-87 丰富。这些同位素组成是由于河流输入增加而导致地表水盐度降低的结果。浮游有孔虫的缺乏或贫乏、大量的角虫甲藻包囊的存在以及低多样性的超小型浮游生物组合的大量出现都证实了这一点，这表明盐度降低至千分之十七，而上层水体的生产力很高。这些观测结果表明，石灰岩是在中央副特提斯海峡与全球海洋的联系有限的时期形成的，这阻碍了水交换，导致地表水低盐条件和底层水缺氧的发展。在最古老的蒂拉瓦地层沉积期间，初级生产力增强，趋化素在水体中的位置异常高。此外，该层位碳酸盐和有机质中 Delta C-13 值的下降表明，NP23 期间盆地内发生了广泛的甲烷排放。所有这些数据表明，在渐新世期间，中央副提斯海经历了与现在黑海相似的条件，可以用作现代的类比，特别是对于蒂拉瓦地平线。因此，Tylawa 地平线可以被视为未来石炭泥灰沉积后过程的潜在影响，类似于那些在黑海沉积了 2.7 kyr 的石炭泥灰岩。
Four isochronous Oligocene coccolith limestone horizons from the Carpathians were examined in order to reconstruct paleoceanographic conditions in the Central Paratethys. The dominance of small and size-uniform pyrite framboids, the occurrence of low-diversity dinoflagellate cysts and coccolithophorids and the presence of biomarker molecule 28,30-dinorhopane indicate that the water column was stratified with the upper water column being relatively well-oxygenated, but the bottom water being anoxic. The latter is confirmed by the fine and consistent horizontal laminations in various parts of the basin, scarcity of benthic organisms and their trace fossils. The limestones exhibit typical marine delta C-13 values, but are significantly depleted in O-18 and enriched in Sr-87 relative to contemporaneous ocean water. These isotopic compositions result from a decreased salinity of the surface waters caused by an increased riverine input. This is confirmed by the lack or impoverishment of planktonic foraminifers, presence to abundance of goniodomid dinoflagellate cysts and massive occurrence of low-diversity nannoplankton assemblages, which indicates decreased salinities as low as 17 parts per thousand and high productivity in the upper water column. These observations indicate that the limestones were formed during periods when connection of the Central Paratethys with the global ocean was limited, which impeded water exchange causing the development of low-salinity conditions of surface water and bottom-water anoxia. During the deposition of the oldest Tylawa horizon, primary productivity was enhanced and chemocline was positioned exceptionally high in the water column. Moreover, decreased delta C-13 values in both carbonates and organic matter of this horizon suggest that widespread methane venting took place in the basin during NP23. All these data show that during the Oligocene the Central Paratethys experienced similar conditions to those of the current Black Sea, which can be used as a modem analogue, especially for the Tylawa horizon. Therefore, the Tylawa horizon can be perceived as a potential effect of future post-depositional processes of coccolith marls analogous to those having been deposited in the Black Sea for 2.7 kyr.