A syn‐depositional age for Earth’s deepest δ13C excursion required by isotope conglomerate tests

Abstract

Terra Nova, 24, 318–325, 2012AbstractThe most negative carbon isotope excursion in Earth history is found in carbonate rocks of the Ediacaran Period (635–542 Ma). 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 δ13C of carbonates (δ13Ccarb) [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 δ13Ccarb values (−12 to +4‰) observed in the formation. In some locations, the Wonoka Fm. is 700 metres (m) of mixed shelf limestones and siliclastics that record the full 16‰δ13Ccarb excursion. In other places, the Wonoka Fm. is host to deep (∼1 km) 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 δ13Ccarb–δ18Ocarb values before brecciation and redeposition in the palaeocanyons.