Recent advances in understanding the terminal Ediacaran Earth-life system in South China and Arctic Siberia

The terminal Ediacaran Period witnessed the first appearance of macroscopic organisms including the earliest biomineralizing animals in Earth history (Narbonne et al., 2012; Xiao et al., 2016). However, the biogeochemical context for this evolutionary milestone remains uncertain due to the absence of radiometric constraints for key events recorded in paleontological and chemostratigraphic datasets, and to uncertainties in the correlation of these biogeochemical events worldwide. Moreover, soft-bodied Ediacara biotas are typically preserved as impressions in fine-grained siliciclastic rocks (Gehling, 1999), which offer limited opportunities for paleoenvironmental analysis using geochemical tools. In contrast, two rare soft-bodied Ediacara assemblages are preserved in well-exposed marine carbonate successions of the Dengying Formation in South China (Chen et al., 2014) and the Khatyspyt Formation in Arctic Siberia (Grazhdankin et al., 2008), where comprehensive chemostratigraphic studies have recently been completed (Gamper, 2014; Schiffbauer et al., 2014; Cui et al., 2016a; Cui et al., 2016b; Bykova et al., 2017; Zhang et al., 2018; Cui et al., 2019). In the Gaojiashan Member of the Dengying Formation, integrated bio- and chemo-stratigraphic profiles (Cui et al., 2016b; Cui et al., 2019) show that the first appearance of biomineralizing animal fossil Cloudina is closely associated with positive anomalies of δ13Ccarb, δ34Spyrite, and Sr/Ca values (Fig.1). In contrast, S isotope values of carbonate‐associated sulfate (δ34SCAS) remain steady throughout the succession, resulting in anomalously large (>70‰) sulfur isotope fractionations (Δ34SCAS-pyrite) in the lower half of the member that decline to ~30‰ in the upper half. This fractionation trend likely relates to changes in microbial communities, with sulfur disproportionation dominating in the lower interval, whereas microbial sulfate reduction was the principal metabolic pathway in the upper. We propose that the coupled paleontological and biogeochemical transition may have coincided with an increase in terrestrial weathering fluxes of sulfate (evidenced by gypsum pseudomorphs), alkalinity (evidenced by Sr/Ca positive anomaly), and nutrients to the depositional basin, which stimulated primary productivity (evidenced by high δ13Ccarb values) and the spread of an oxygen minimum zone (evidenced by high δ15N and low δ238U values). Enhanced production and burial of organic matter is thus directly connected to the δ13Ccarb anomaly, and through bottom water anoxia likely promoting microbial sulfate reduction and pyritization as the main taphonomic pathway for Conotubus and other soft‐bodied Ediacara biota. These results suggest that environmental and physiological pressures from an increase in seawater alkalinity (Cui et al., 2016b; Cui et al., 2019) – likely coupled with ecological pressures from predation (Hua et al., 2003; Hua et al., 2007) – set the stage for the evolutionary novelty of animal biomineralization. In the Khatyspyt Formation, the profound sulfur isotope fractionation trend seen in the Dengying is preserved, with δ34Spyrite values rising from ca. – 20‰ to ca. 50‰ through an interval of general invariance in δ34SCAS (Fig.2). The step function shift in sulfur isotope compositions is closely associated with the first occurrence of soft-bodied Ediacara biotas preserved in thinly bedded limestones, which suggests a possible link between seawater redox conditions and the distribution of these enigmatic macroscopic organisms (Cui et al., 2016a). Based on multiple lines of sedimentological and geochemical evidence, we propose that the development of oceanic euxinia — which could be widespread in the continental margins due to enhanced oxidative weathering (delivering both nutrients and sulfate to ocean margins) in the terminal Ediacaran Period — may have locally prohibited the colonization of Ediacara-type organisms. The 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 in the upper part of the studied section of the Khatyspyt Formation. In summary, our results reveal dramatic changes in biogeochemical cycles during the terminal Ediacaran Period, many of which are closely coupled with evolutionary transitions in the corresponding fossil records. Dynamic redox conditions may have caused a profound impact on early animal evolution. Our work highlights the significance of integrated bio-, litho-, and chemo-stratigraphy in geobiology research of the deep time. Keywords: chemostratigraphy; redox condition; early macroorganisms; biogeochemical cycles; geobiology, animal biomineralization, alkalinity, Cloudina

Jaar van publicatie:2019

Trefwoorden:chemostratigraphy, redox condition, early macroorganisms, biogeochemical cycles, geobiology, animal biomineralization, alkalinity, Cloudina

CSS-citation score:1

Toegankelijkheid:Open