A syn-depositional age for Earth's deepest δ ¹³C excursion required by isotope conglomerate tests

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.