Informace o publikaci

Fine-scale LA-ICP-MS study of redox oscillations and REEY cycling during the latest Devonian Hangenberg Crisis (Moravian Karst, Czech Republic)



Rok publikování 2018
Druh Článek v odborném periodiku
Časopis / Zdroj Palaeogeography, Palaeoclimatology, Palaeoecology
Fakulta / Pracoviště MU

Přírodovědecká fakulta

www odkaz
Klíčová slova Trace elements;REE;Devonian-Carboniferous boundary;Microbialites;Anachronistic facies
Popis Trace elements and REEY distribution of thinly laminated limestone (laminite) and underlying pre-event calciturbidite (Siphonodella praesulcata conodont Zone) deposited in association with the Hangenberg Black Shale Event were analyzed using laser ablation-inductively coupled plasma-mass spectrometry. These deposits were also analyzed by carbonate petrography and scanning electron microscopy. Our study documents the origin of authigenic microbial limestones during the latest Devonian Hangenberg extinction event, when unusual oceanic conditions appear to have enhanced precipitation of calcite within microbial carbonates. Detrital contamination of the calciturbidite and laminated limestone is very low, as demonstrated by low contents of Al, Zr, Ti and Th. These deposits display only limited degree of early diagenetic overprint of limestones, and some of them fall within or very close to the seawater field. Elevated U and Mo enrichments and covariation patterns reflect the episodic establishment of oxic to euxinic bottom condition during deposition of the laminite. The oxic conditions that existed during accumulation of the underlying calciturbidite were likely the result of turbidity current introduction of oxygen into the bottom environment. REE contents of both the laminite and calciturbidite are nearly ten times higher than those of most modern carbonates and similar to Devonian microbialites from the Canning Basin. Calciturbidite display a prominent MREEN bulge. Shale-normalized REE patterns of the overlying laminite are similar to those of modern seawater and Devonian microbialites from the Canning Basin (LREE depletion and progressive enrichments in heavier REE) reflecting the dominance of authigenic carbonate phases. Minor contamination is bound to apatites, siliciclastic minerals, pyrites and organic matter. The element content of authigenic phases benefited from Fe and Mn oxyhydroxide redox cycling. Negative Ce and positive Eu anomalies, low Y/Ho ratios, and very low Al/(Al + Fe + Mn) ratios suggest an influence of diluted high- temperature hydrothermal fluids perhaps related to latest Devonian rift magmatism in the eastern part of the Rhenohercynian zone. The proliferation of anachronistic facies following the Devonian-Carboniferous (D-C) extinction event recognized in Moravia and traced worldwide seems to share some features in common with anachronistic carbonate sequences documented in the aftermath of the end-Permian mass extinction.
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