The most negative carbon isotope excursion in Earth history is found in carbonate rocks of the Ediacaran Period (635-542Ma). Workers have interpreted the event as the oxidation of the Ediacaran oceans [Rothman et al., Proc. Natl. Acad. Sci. USA100 (2003) 8124; Fike et al., Nature444 (2006) 744; McFadden et al., Proc. Natl. Acad. Sci. USA105 (2008) 3197], or as diagenetic alteration of the δ 13C of carbonates (δ 13C carb) [Knauth and Kennedy, Nature460 (2009) 728; Derry, Earth Planet. Sci. Lett.294 (2010) 152]. Here, we present chemo-stratigraphic data from the Ediacaran-aged Wonoka Formation (Fm.) of South Australia that require a syn-depositional age for the extraordinary range of δ 13C carb values (-12 to +4‰) observed in the formation. In some locations, the Wonoka Fm. is 700metres (m) of mixed shelf limestones and siliclastics that record the full 16‰δ 13C carb excursion. In other places, the Wonoka Fm. is host to deep (∼1km) palaeocanyons, which are partly filled by tabular-clast carbonate breccias that are sourced from eroded Wonoka canyon-shoulders. By measuring the isotopic values of 485 carbonate clasts (an isotope conglomerate test), we show that canyon-shoulder carbonates acquired their δ 13C carb-δ 18O carb values before brecciation and redeposition in the palaeocanyons.
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