The terminal Ediacaran 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 palaeontological 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 palaeoenvironmental 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.

In the Gaojiashan Member of the Dengying Formation, integrated bio- and chemo-stratigraphic profiles 2019) show that the first appearance of the biomineralizing animal fossil Cloudina is closely associated with positive anomalies of δ¹³C_carb, δ³⁴S_pyrite, and Sr/Ca values (Fig. 1). In contrast, S isotope values of carbonate-associated sulfate (δ³⁴S_CAS) remain steady throughout the succession, resulting in anomalously large (>70‰) sulfur isotope fractionations (δ ³⁴S_CAS-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 palaeontological 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 δ¹³C_carb 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 δ¹³C_carb 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 - likely coupled with ecological pressures from predation (Hua et al., 2003, 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 δ³⁴S_pyrite values rising from ca. - 20‰ to ca. 50‰ through an interval of general invariance in δ³⁴S_CAS (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. 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 the Gaojiashan Member of the Dengying Formation, China, environmental and physiological pressures were linked to an increase in seawater alkalinity, probably coupled with ecological pressures from predation, setting the stage for the evolutionary novelty of animal biomineralization. In the Khatyspyt Formation of Arctic Siberia, 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.