WIDESPREAD MARINE ANOXIA DURING GENERATION OF THE EDIACARAN SHURAM EXCURSION

Reconstructing the oxygenation history of Earth’s deep oceans during the Ediacaran period has been challenging, and this has led to a polarizing debate about the environmental conditions that played host to the rise of animals at this time. One focal point of this debate is the largest negative carbonate C-isotope excursion so far recognized in the geologic record, the Shuram excursion, and whether or not this geochemical relic tracks the global-scale oxygenation of Earth’s deep oceans.

To help reconcile this debate, we applied three complementary paleoredox proxies to two siliciclastic-dominated successions from Oman deposited across the Shuram Formation: (1) Fe speciation, (2) thallium (Tl) isotopes, and (3) redox-sensitive elements (RSEs). Iron speciation is a powerful technique for estimating local redox conditions, whereas Tl isotopes and RSEs are capable of tracking more widespread, global-scale marine redox conditions.

Iron speciation data reveal that samples from both sedimentary successions were deposited under anoxic conditions. Locally anoxic conditions are ideal for reconstructing past seawater Tl isotope compositions (ε²⁰⁵Tl), and according to our new data seawater ε²⁰⁵Tl was invariant and within error of that of the bulk upper continental crust during the generation of the Shuram excursion (average ε²⁰⁵Tl = –2.4 ± 1.0; 2SD). A near-crustal seawater ε²⁰⁵Tl value implies no significant seafloor Mn oxide burial, and by inference little-to-no accumulation of O₂ in global marine bottom waters. Especially low RSE enrichments in the same samples require small seawater RSE reservoirs, and by inference a large area of anoxic seafloor for widespread RSE scavenging.

We consider alternative explanations of the data – such as changes in local redox conditions, changes in lithology, basinal restriction, a heterogenous seawater ε²⁰⁵Tl value, early diagenesis, and late-stage alteration – but find none to be more compelling. According to our data and interpretations, the generation of the Shuram excursion, and any coeval animal evolutionary events, took place in an ocean that experienced little to no deep marine oxygenation.