Jurassic Manganese Carbonates of Central Europe and the Early Toarcian Anoxic Event
Jurassic Manganese Carbonates of Central Europe and the Early Toarcian Anoxic Event
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DOI：
10.1086/629481
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发表时间：
1991-03
期刊：
影响因子：
--
通讯作者：
H. Jenkyns;B. Géczy;J. Marshall
中科院分区：
文献类型：
--
作者：
H. Jenkyns;B. Géczy;J. Marshall
The intimate stratigraphical association between Lower Toarcian manganese carbonates and carbon-rich shales in Austria, Czechoslovakia, Germany, Hungary, Italy, and Switzerland is suggestive of a genetic connection between the two, given the similar anoxic to euxinic depositional environments in which both these distinctive sediments form today. Ammonite biostratigraphy and carbon-isotope stratigraphy suggest that these metalliferous deposits are everywhere of identical age, namely tenuicostatum to early falciferum Zones. Recognition of an early Toarcian anoxic event in the Alpine-Mediterranean domain, characterized by the development of a well-developed oxygen-minimum zone across the rifted proto-continental margins of the Tethys, indicates that these metalliferous deposits can be related to mid-water transport of manganese, possibly of continental derivation. The fact that, in the majority of localities, formation of the manganese carbonates immediately pre-dated deposition of the carbon-rich shales suggests that their deposition was characteristic of conditions immediately preceding the anoxic event itself: carbon-isotope evidence suggests that regional carbon-burial rates, presumably linked with concomitant deoxygenation of water masses, were beginning to increase during this interval. In some areas fluctuating redox conditions at the sediment-water interface during the anoxic event itself led to the precipitation of manganese carbonates. Although the early Toarcian event affected many parts of the Alpine-Mediterranean region, including Greece, it is only in central Europe, particularly Hungary, that economic manganese deposits were formed, and hydrothermal sources cannot be excluded as a factor in their genesis.
