Redox-dependent distribution of early macro-organisms: Evidence from the terminal Ediacaran Khatyspyt Formation in Arctic Siberia

The Ediacaran Period witnessed the first appearance of macroscopic animal life in Earth's history. However, the biogeochemical context for the stratigraphic occurrence of early metazoans is largely uncertain, in part due to the dearth of integrated paleobiological and chemostratigraphic datasets. In this study, a comprehensive geochemical analysis was conducted on the fossiliferous Khatyspyt Formation in Arctic Siberia, in order to gain insights into the Ediacaran paleoenvironments. This study was designed to specifically address the relationship between paleoredox conditions and Ediacaran fossil occurrences in the Khatyspyt Formation. Our data reveal a dramatic shift in pyrite sulfur isotope compositions (δ34Spyrite) from ca. − 20‰ to ca. 55‰, and this shift is intriguingly associated with the first occurrence of Ediacara-type macrofossils at the studied section, suggesting a possible link between seawater redox conditions and distribution of early macroscopic organisms. Based on multiple lines of sedimentological and geochemical evidence, we propose that the development of oceanic euxinia — which may be widespread in the continental margins due to enhanced oxidative weathering in the terminal Ediacaran Period — may have locally prohibited the colonization of Ediacara-type organisms and resulted in low δ34Spyrite values in the lower Khatyspyt Formation. In the middle and upper Khatyspyt Formation, progressive secular transition from euxinic to non-euxinic and more habitable conditions may have allowed for the colonization of Ediacara-type and other macro-organisms

Carbonate-hosted avalon-type fossils in arctic Siberia

Avalon-type fossils are crucial to understanding the origin of Phanerozoic ecosystems, but their usual occurrence in volcaniclastic and siliciclastic facies greatly limits their paleobiological resolution. The unique carbonate-hosted assemblage of the Khatyspyt Formation, on the Olenek uplift of north-central Siberia, promises a much enhanced anatomical and paleo-ecological view of these enigmatic organisms. Avalon-type fossils are preserved by authigenic carbonate cementation in intervals of finely laminated nodular limestones (Khatyspyt-type taphonomic window). Interbedded silicified calcareous mudstones yield diverse carbonaceous compressions, occasionally with taphonomic phantoms of Avalon-type taxa (Miaohe-type taphonomic window). Styles of moldic preservation do not appear to be taxon selective, and therefore cannot alone be responsible for the morphological distinctiveness of Ediacaran macrofossils and the profound disparity in the taxonomic composition between fossil assemblages. On the other hand, the exclusion of Avalon-type fossils from carbonaceous compressions (Miaohe-type preservational window) is a real taphonomic signal that provides an important constraint on the properties of certain Ediacaran tissues