Environmental context for the terminal Ediacaran biomineralization of animals

Abstract

In terminal Ediacaran strata of South China, the onset of calcareous biomineralization is preserved in the paleontological transition from Conotubus to Cloudina in repetitious limestone facies of the Dengying Formation. Both fossils have similar size, funnel-in-funnel construction, and epibenthic lifestyle, but Cloudina is biomineralized, whereas Conotubus is not. To provide environmental context for this evolutionary milestone, we conducted a high-resolution elemental and stable isotope study of the richly fossiliferous Gaojiashan Member. Coincident with the first appearance of Cloudina is a significant positive carbonate carbon isotope excursion (up to +6‰) and an increase in the abundance and (34) S composition of pyrite. In contrast, δ(34) S values of carbonate-associated sulfate remain steady throughout the succession, resulting in anomalously large (>70‰) sulfur isotope fractionations in the lower half of the member. The fractionation trend likely relates to changes in microbial communities, with sulfur disproportionation involved in the lower interval, whereas microbial sulfate reduction was the principal metabolic pathway in the upper. We speculate that the coupled paleontological and biogeochemical anomalies may have coincided with an increase in terrestrial weathering fluxes of sulfate, alkalinity, and nutrients to the depositional basin, which stimulated primary productivity, the spread of an oxygen minimum zone, and the development of euxinic conditions in subtidal and basinal environments. Enhanced production and burial of organic matter is thus directly connected to the carbon isotope anomaly, and likely promoted pyritization as the main taphonomic pathway for Conotubus and other soft-bodied Ediacara biotas. Our studies suggest that the Ediacaran confluence of ecological pressures from predation and environmental pressures from an increase in seawater alkalinity set the stage for an unprecedented geobiological response: the evolutionary novelty of animal biomineralization.

Was the Ediacaran Shuram Excursion a globally synchronized early diagenetic event? Insights from methane-derived authigenic carbonates in the uppermost Doushantuo Formation, South China

“…5-7) were conducted. All isotope analyses were performed using routine methods (e.g., McFadden et al, 2008;Cui, 2015;Cui et al, 2015Cui et al, , 2016aCui et al, , 2016bCui et al, , 2016c, which are briefly outlined below.…”

“…With the emergence of bioturbation in the terminal Ediacaran and Cambrian periods (Bottjer et al, 2000;Carbone and Narbonne, 2014), ventilation of the deep ocean (Logan et al, 1995;Butterfield, 2009Butterfield, , 2011Lenton et al, 2014;Chen et al, 2015), and a larger seawater sulfate pool (Halverson and Hurtgen, 2007;Canfield and Farquhar, 2009;Cui et al, 2016b), the Phanerozoic SMTZ would have been progressively deeper compared with the SMTZ in Precambrian time (Fig. 12C) (see Wright and Cherns, 2016 for a Phanerozoic analogue).…”

“…Generally, the SMTZ is shallow where the methane flux dominates (e.g., above localized gas hydrate reservoirs), and is deeper where diffusion of seawater sulfate dominates the system. Insofar as marine sulfate concentration progressively increased in the Neoproterozoic (Halverson and Hurtgen, 2007;Canfield and Farquhar, 2009;Cui et al, 2016b), the position of the SMTZ in marine sediments may have been initially located near the sediment-water interface, and then moved deeper over time in response to the increasing flux of the oxidant (e.g., sulfate or iron oxide) as the result of elevated oxidative weathering or an increase in bioturbation.…”

Biosilicification Drives a Decline of Dissolved Si in the Oceans through Geologic Time

“…Ocean geochemistry underwent major upheavals around the Precambrian/Cambrian boundary (Brennan et al, 2004;Cui et al, 2016) and into the Palaeozoic. A concomitant biotic upheaval at this time was the appearance of biomineralized hard parts of materials like calcium carbonate, calcium phosphate, and silica.…”

Ediacaran pre-placozoan diploblasts in the Avalonian biota: the role of chemosynthesis in the evolution of early animal life

“…Considering that oxygen was available, at least periodically, in the overlying water (Hood & Wallace 2014;Wood et al 2015;Cui et al 2016;Sahoo et al 2016), the upper surface of Ediacaran organisms is likely to have been adapted for oxygen uptake. Oxygen transport could have occurred within a diploblastic Ediacaran animal by diffusion through the mesenchyme, which could also have stored oxygen during periods of hypoxia, as demonstrated in the mesoglea of modern scyphomedusae (Thuesen et al 2005;Pitt et al 2013).…”