Multi-stage Ediacaran ocean oxidation and its impact on evolutionary radiation

Abstract

High-resolution paired carbon (δ^(13)C_(carb) and δ^(13)C_(org)) and sulfur (δ^(34)S_(SO4) and δ^(34)S_(pyrite)) isotope records of the Ediacaran strata of the Huqf Supergroup, Sultanate of Oman record a three-stage oxidation of the Ediacaran ocean. Following the Marinoan glaciation, stage I oxidation lasts from ∼635 to 610 million years ago (Myr) and records an increase in marine sulfate concentrations above ∼200 μM, likely due to a rise in atmospheric oxygen. Stage II oxidation, from ∼575 to 550 Myr, coincides with the Shuram excursion, ɑ > 13‰ negative excursion in δ^(13)C_(carb)Burns and Matter, 1993 created by the oxidation of a large reservoir of deep ocean dissolved organic carbon (DOC). The onset of stage II oxidation coincides with the evolution of acanthomorph acritarchs in Australia and China Grey, 2005, and the appearance of the first Ediacaran metazoa, Charnia-type segmented fronds Narbonne, 2005, while the first motile macroscopic metazoa (e.g., Kimberella) appear at ∼555 Myr Narbonne, 2005 as the Shuram excursion draws to a close. Stage III oxidation, from ∼550 to 547 Myr, is marked by presence of sulfur disproportionating metabolisms and the onset of co-variation in δ^(13)C_(carb) and δ^(13)C_(org), which is absent in older strata. Coincident with the onset of stage III oxidation, we see the evolution of macroscopic multicellular algae Xiao, 2004, an increase in the diversity of the acanthomorph acritarchs Burns and Matter, 1993, and the appearance of the weakly calcifying metazoa Cloudina and Namacalathus Narbonne, 2005. Following stage III oxidation, there is a period of quiescence characterized by little perturbation in the carbon cycle and an absence of significant evolutionary events. At ∼542 Myr, anoxia at the Ediacaran–Cambrian boundary causes the extinction of many Ediacaran organisms, setting the stage for the Cambrian radiation.

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