Fertilization of the northwestern Tethys (Vocontian basin, SE France) during the Valanginian carbon isotope perturbation: Evidence from calcareous nannofossils and trace element data
Fertilization of the northwestern Tethys (Vocontian basin, SE France) during the Valanginian carbon isotope perturbation: Evidence from calcareous nannofossils and trace element data
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DOI：
10.1016/j.palaeo.2006.07.010
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发表时间：
2007-01
期刊：
影响因子：
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通讯作者：
Stéphanie Duchamp-Alphonse;S. Gardin;N. Fiet;A. Bartolini;D. Blamart;M. Pagel
中科院分区：
文献类型：
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作者：
Stéphanie Duchamp-Alphonse;S. Gardin;N. Fiet;A. Bartolini;D. Blamart;M. Pagel
在Valanginian正碳同位素偏移期间，在Angles剖面（Vocontian盆地的半远洋环境，法国东南部）进行了与地球化学分析（主要元素、微量元素和碳氧同位素地层学）相关的高分辨率钙质超微化石研究。研究了钙质超微化石分类群在肥力条件下的行为，以阐述这种环境中新的营养指数：基于Biscutum spp.的高营养指数，Discorhabdus rotatorius、Zeugrhabdotus fissus（高生育力指标）和Watznaueria barnesae（低生育力指标）;以及基于Lithraphidites carniolensis（中等生育力指标）和W. barnesae（低生育率指标）。这两个指标显示了从Stephanophorus菊石带到Trinodosum菊石带的主要施肥作用，在正碳同位素漂移期间达到最大值。由于营养指数的高值与陆源物质相关的化学元素的高值和颗石总丰度的低值相一致，因此建议碎屑输入到盆地中的脉冲触发了营养化，从而导致钙质超微化石的生物钙化危机。营养也是负责在周围的平台，如增加的Sr/Ca海水比在当时的珊瑚礁消亡。Paranà-Etendeka火山活动的加剧，引发大气中的CO2过剩，可能是水文循环加速，风化作用增加以及随后从大陆向Vocontian盆地的陆源和养分转移增加的原因。在这种情况下，营养化是控制浅海和半远洋生物钙化的主导因素。然而，不能排除全球大气CO2的增加可能会引起海面沃茨的化学变化，与营养作用一起改变碳酸盐生产者的生物钙化作用。
A high resolution calcareous nannofossil study associated with a geochemical analysis (major, trace elements, and carbon and oxygen isotope stratigraphies) was carried out in the Angles section (hemipelagic setting of the Vocontian basin, SE France) during the Valanginian positive carbon isotope excursion. The behaviour of calcareous nannofossil taxa in relation to fertility conditions was studied to elaborate new nutrient indices in this environment: a high nutrient index based on Biscutum spp., Discorhabdus rotatorius, Zeugrhabdotus fissus, (high fertility indicators) and Watznaueria barnesae (low fertility indicator); and a medium nutrient index based on Lithraphidites carniolensis (medium fertility indicator) and W. barnesae (low fertility indicator). These two indices show a major fertilization from the Stephanophorus ammonite Zone to the Trinodosum ammonite Zone, with a maximum during the positive carbon isotope excursion. Since high values of the nutrient indices are in phase with high values of chemical elements related to terrigenous material and low values of the coccolith total abundance, it is proposed that pulses of detrital inputs into the basin triggered the nutrification which, in turn, caused a biocalcification crisis of the calcareous nannofossils. Nutrification is also responsible for the reef demise in the surrounded platforms, as indicated by the increased Sr/Ca seawater ratio at that time. The intensification of the Paranà–Etendeka volcanic activity, triggering CO2excess in the atmosphere, is probably responsible for an acceleration of the hydrological cycle, the increased weathering, and the subsequent higher terrigenous and nutrient transfer from continents to the Vocontian basin. In such a scenario, nutrification is a dominant factor controlling neritic and hemipelagic biocalcification. However, one cannot exclude that the global increase of atmospheric CO2could generate chemical changes of the sea-surface waters, acting with the nutrification, to modify the biocalcification of the carbonate producers.