Ethe Wonoka Formation (33)^ and the western United States ERainstorm

United States ERainstorm Member, Johnnie Formation (34)^ but chose to equate it with a postulated Gaskiers-related dC excursion from circa 580 Ma (35). Instead, our data indicate that this globally correlated negative dC excursion is not related to any known glaciation (36). The duration of this excursion is unconstrained; however, given that it is captured within over 100 m of section in Oman (Fig. 1) combined with approximate sediment accumulation rates calculated with our constraints, we suggest a duration of 91 and G10 My. The Doushantuo and correlative strata record a fundamental shift from an interval of large carbon isotopic anomalies corresponding to glacial episodes (750 to 580 Ma) to an interval of anomalies unrelated to obvious glacial episodes (i.e., the anomalies from circa 551 and 542 Ma), as well as subsequent large fluctuations in the lower Cambrian. These new geochronological data allow us to calibrate that shift as being synchronous with the appearance of larger and more complex metazoans; this suggests possible feedback relationships between evolutionary innovation and seawater chemistry (Fig. 2). Our ages indicate that the Doushantuo Formation spans more than 90% of the Ediacaran Period. These constraints are consistent with the upper Doushantuo/Shuram/ Kuibis excursion being broadly coincident with the first appearance of complex trace fossils and mollusk-like bilaterian Kimberella (37), dated as slightly older than 555.1 T 1.0 Ma (24). The advent of large pelagic bilaterians with unidirectional guts would have increased the flux of organic carbon to the deep ocean (38). Additionally, the radiation of algae containing resistant biopolymers in cell wall and cysts (i.e., Miaohe Biota) and the advent of biomineralization (Namacalathus and Cloudina, 9549 Ma) would have also resulted in an increased organic carbon and carbonate carbon flux (39). These changes would have resulted in a downward flux of organic carbon with a possible coupled oxidation of the organic reservoir (38, 39) driving the negative dC excursion. This feedback loop would lead to an increase in marine oxygen levels and stimulate productivity and inferentially predation. It may be no coincidence that the first reefs inhabited by abundant weakly calcified and rare fully calcified metazoans appeared at about the same time as the isotopic anomaly Ei.e., before 549 Ma in Namibia (6, 40)^.