Effects of bioturbation on carbon and sulfur cycling across the Ediacaran–Cambrian transition at the GSSP in Newfoundland, Canada

Abstract

The initiation of widespread penetrative bioturbation in the earliest Phanerozoic is regarded as such a significant geobiological event that the boundary between Ediacaran and Cambrian strata is defined by the appearance of diagnostic trace fossils. While ichnofabric analyses have yielded differing interpretations of the impact of Fortunian bioturbation, the disruption of sediments previously sealed by microbial mats is likely to have effected at least local changes in carbon and sulfur cycling. To assess the geochemical effects of penetrative bioturbation, we conducted a high resolution chemostratigraphic analysis of the siliciclastic-dominated basal Cambrian Global Stratotype Section and Point (GSSP; Chapel Island Formation, Newfoundland, Canada). A positive δ13C excursion in organic matter starts at the Ediacaran–Cambrian boundary and returns to stably depleted values near the top of member 2, while the δ13C of carbonate carbon increases from strongly depleted values toward seawater values beginning near the top of member 2. Pyrite sulfur coincidently undergoes significant 34S depletion at the Ediacaran–Cambrian boundary. These isotope anomalies most likely reflect progressive ventilation and oxygenation of shallow sediments as a consequence of bioturbation. In this interpretation, sediment ventilation in the earliest Cambrian may have spurred a temporary increase in microbial sulfate reduction and benthic sulfur cycling under low-oxygen conditions. In the late Fortunian, local carbon cycling appears to have stabilized as reductants were depleted and more oxygenated conditions predominated in the shallow substrate. Overall, these data attest to the geochemical significance of the initiation of sediment ventilation by animals at the dawn of the Phanerozoic.

Sedimentology and chemostratigraphy of the terminal Ediacaran Dengying Formation at the Gaojiashan section, South China

“…Sulfur isotope chemostratigraphy of the DYF@GJS shows overall positive δ 34 S pyrite values ranging between +20 ‰ and +30 ‰ through most of the section, except for two episodes of anomalous negative excursions down to c. -30 ‰ in the lower and middle Gaojiashan Member (Figs 12e, 13f). The occurrence of highly positive δ 34 S pyrite values in the terminal Ediacaran strata has also been reported from the roughly equivalent strata in Oman (Fike & Grotzinger, 2008), Namibia (Ries et al 2009;Tostevin et al 2017), Arctic Siberia (Cui et al 2016a) and Newfoundland (Canfield et al 2007;Hantsoo et al 2018), suggesting a global phenomenon. The origin of the low δ 34 S pyrite signals in the Gaojiashan Member has been fully discussed in our earlier paper (Cui et al 2016b); we will mainly focus on the overall high δ 34 S pyrite signals below.…”

The rise and early evolution of animals: where do we stand from a trace-fossil perspective?

“…Albeit limited in intensity and depth, bioturbation styles show a marked departure from those in the Ediacaran, most likely having played a role in triggering changes in the sediment and the water column. Biogenic activity, although relatively close to the sediment-water interface, produced some disruption of the primary fabric, deepening of the redox discontinuity surface and generating concomitant improved substrate ventilation, and flux exchange with the water column [12,190].…”

A protracted Ediacaran–Cambrian transition: an ichnologic ecospace analysis of the Fortunian in Newfoundland, Canada

“…There is a growing consensus that bioturbation may have played a key role in this evolutionary event (e.g. Hantsoo et al 2018;Kaufman, 2018;Lenton & Daines, 2018). During the Ediacaran-Cambrian transition and early Cambrian Period there exists a marked infaunalization, and a switch from an Ediacaran-style matground ecology to a Cambrian-style mixground ecology (Seilacher, 1999;Gougeon et al 2018).…”