Global marine redox changes drove the rise and fall of the Ediacara biota

Abstract

The role of O2 in the evolution of early animals, as represented by some members of the Ediacara biota, has been heavily debated because current geochemical evidence paints a conflicting picture regarding global marine O2 levels during key intervals of the rise and fall of the Ediacara biota. Fossil evidence indicates that the diversification the Ediacara biota occurred during or shortly after the Ediacaran Shuram negative C‐isotope Excursion (SE), which is often interpreted to reflect ocean oxygenation. However, there is conflicting evidence regarding ocean oxygen levels during the SE and the middle Ediacaran Period. To help resolve this debate, we examined U isotope variations (δ238U) in three carbonate sections from South China, Siberia, and USA that record the SE. The δ238U data from all three sections are in excellent agreement and reveal the largest positive shift in δ238U ever reported in the geologic record (from ~ −0.74‰ to ~ −0.26‰). Quantitative modeling of these data suggests that the global ocean switched from a largely anoxic state (26%–100% of the seafloor overlain by anoxic waters) to near‐modern levels of ocean oxygenation during the SE. This episode of ocean oxygenation is broadly coincident with the rise of the Ediacara biota. Following this initial radiation, the Ediacara biota persisted until the terminal Ediacaran period, when recently published U isotope data indicate a return to more widespread ocean anoxia. Taken together, it appears that global marine redox changes drove the rise and fall of the Ediacara biota.

Constraining oceanic oxygenation during the Shuram excursion in South China using thallium isotopes

“…Despite the inability to use Tl isotopes to directly constrain ocean oxygenation, our interpretation of increased ocean oxygenation at the EN3a to EN3b transition is consistent with previously reported evidence for an oxygenation event during this time, which was based on globally enhanced continental weathering (Li et al, ; Sawaki et al, ), elevated marine sulfate concentrations and decreased sulfur isotope composition (Fike et al, ; Kaufman et al, ; Li et al, ; Loyd et al, ; Osburn et al, ), elevated marine iodate concentrations observed in a range of global depositional settings (Hardisty et al, ), and a positive U isotope excursion (Kendall et al, ; Zhang et al, ). Although spatially heterogeneous, paired δ 13 C and δ 34 S records show coincident negative excursions interpreted to represent an event that oxidized large dissolved organic carbon and sulfide pools in the deep oceans (Fike et al, ; Kaufman et al, ; Li et al, ; Loyd et al, ; Osburn et al, ; Shi et al, ).…”

“…The latter mechanism would be similar to the interpretation for the Phanerozoic events where the Tl isotope composition of seawater was observed to respond prior to the carbon isotope excursion (Ostrander et al, ; Them et al, ; Bowman et al, ; and reviewed in Owens, ). The decreasing U isotope (δ 235 U) composition of seawater recorded by shales from the Member IV of the Doushantuo Formation and carbonate from the Nama Group suggested a rapid and dramatic anoxic expansion from ~560–550 Ma (Kendall et al, ; Tostevin et al, ; Zhang et al, ), providing additional evidence for our proposed decrease in ocean oxygenation during the EN3c.…”

“…In addition, the Tl could be responding faster to deoxygenation (or reduction in oceanic Mn reservoir) than Mo and U, due to the difference in modern residence times for these three elements. Nevertheless, if our proposed deoxygenation event during the uppermost portion of the Shuram NCIE indeed occurred, we can argue that the oxygenation event during the EN3a‐b intervals could have stimulated the evolution of Ediacaran‐type biota (e.g., McFadden et al, ; Zhang et al, ) and the subsequent deoxygenation event during EN3c did not prevent the appearance of macroscopic mobile bilaterians (Figure ).…”

Regional nutrient decrease drove redox stabilisation and metazoan diversification in the late Ediacaran Nama Group, Namibia

“…These global and local datasets are not necessarily in conflict, as the uranium proxy data may record an integrated global increase in the spatial extent of productive mid-depth waters at ~547 Ma. In fact, recent compilations of δ 238 U carb from globally distributed late (2020) 10:2240 | https://doi.org/10.1038/s41598-020-59335-2 www.nature.com/scientificreports www.nature.com/scientificreports/ Ediacaran successions do suggest a trend towards an increasingly oxygenated global ocean between 545-540 Ma, in the wake of the highly negative values recorded in the lower Nama Group and Dengying Formation [62][63][64] . However, these global oxygenation events were transient, and their influence on the pace of animal evolution in shallow shelf ecosystems which were subject to highly heterogeneous local redox conditions demands further investigation.…”

On the co‐evolution of surface oxygen levels and animals

“…The difference between 1% and 0.1% PAL is critical for metazoans (across this range of oxygen levels in modern OMZs there is a shift from low‐diversity animal assemblages to no animals), but this difference is within error of all reports of “low” mid‐Proterozoic atmospheric oxygen (e.g., Planavsky, Cole, et al, ). Further, most marine elemental or isotopic redox proxies calculate the proportion of anoxic or euxinic seafloor with considerable uncertainty (for instance, a recent U isotope study by Zhang et al, , estimated anoxic seafloor area prior to the Shuram carbon isotope excursion to be between 25% and 100%—an enormous range). Even if these proxies had less error they would not provide a full view of the water column redox landscape.…”