Coincidence of photic zone euxinia and impoverishment of arthropods in the aftermath of the Frasnian-Famennian biotic crisis

Krzysztof Broda,Leszek Marynowski,Michał Zatoń,Michał Rakociński

doi:10.1038/s41598-019-52784-4

Krzysztof Broda, Leszek Marynowski + Show 2 more

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https://doi.org/10.1038/s41598-019-52784-4

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Journal: Scientific Reports	Publication Date: Nov 18, 2019
Citations: 7	License type: open-access

Affiliation: University of Silesia

Abstract

The lowermost Famennian deposits of the Kowala quarry (Holy Cross Mountains, Poland) are becoming famous for their rich fossil content such as their abundant phosphatized arthropod remains (mostly thylacocephalans). Here, for the first time, palaeontological and geochemical data were integrated to document abundance and diversity patterns in the context of palaeoenvironmental changes. During deposition, the generally oxic to suboxic conditions were interrupted at least twice by the onset of photic zone euxinia (PZE). Previously, PZE was considered as essential in preserving phosphatised fossils from, e.g., the famous Gogo Formation, Australia. Here, we show, however, that during PZE, the abundance of arthropods drastically dropped. The phosphorous content during PZE was also very low in comparison to that from oxic-suboxic intervals where arthropods are the most abundant. As phosphorous is essential for phosphatisation but also tends to flux off the sediment during bottom water anoxia, we propose that the PZE in such a case does not promote the fossilisation of the arthropods but instead leads to their impoverishment and non-preservation. Thus, the PZE conditions with anoxic bottom waters cannot be presumed as universal for exceptional fossil preservation by phosphatisation, and caution must be paid when interpreting the fossil abundance on the background of redox conditions.

Highlights

Euxinic conditions in aquatic environments are defined as the presence of H2S and absence of oxygen[1]
We show that photic zone euxinia (PZE) did not lead to mass mortality events of arthropods, which abundantly inhabited the basin and, importantly, have not promoted their fossilisation as was considered previously
The total abundance of some body fossils within each bed was difficult to establish because their skeletons tend to disarticulate post-mortem or were extremely rare; these fossils were marked on diagrams but not evaluated quantitatively

Summary

Introduction

Euxinic conditions in aquatic environments are defined as the presence of H2S and absence of oxygen[1]. We document the recurrent PZE conditions during sedimentation of the fossiliferous lower Famennian (Upper Devonian) deposits at the Kowala quarry (Holy Cross Mountains, Poland). Because of the great abundance and excellent preservation of phosphatic fossils of thylacocephalan, phyllocarid and angustidontid crustaceans[9,10,11,12], articulated coelacanth fish[13], conulariids[14], coprolites[15], as well as carbonaceous non-biomineralised algae[16], these deposits were coined as the Kowala Lagerstätte[9] These fossiliferous deposits originated during the earliest Famennian, so in the aftermath of the famous Frasnian-Famennian (F-F) biotic crisis, during which many marine groups suffered extinction and, especially, reef ecosystems collapsed[17,18]. The depositional environment is generally interpreted as deep shelf, below storm wave-base, but at least episodically within the limits of the photic zone[33,36,37,38]

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