The origin of Earth's deepest δ¹³C excursion


Understanding the extreme carbon isotope excursions of the Ediacaran Period (635-541 Ma), where δ13C of marine carbonates (δ13Ccarb) reach their minimum (-12‰) for Earth history, is one of the most vexing problems in Precambrian geology. Known colloquially as the 'Wonoka-Shuram' excursion, the event has been interpreted by many as a product of a profoundly different Ediacaran carbon cycle. More recently, diagenetic processes have been invoked, with the very negative δ13C values of Ediacaran carbonates explained via meteoric alteration, late-stage burial diagenesis or growth of authigenic carbonates in sediments, thus challenging models which rely upon a dramatically changing redox state of the Ediacaran oceans. As part of my Ph.D. work with Adam Maloof, Blair Schoene and Princeton undergraduate ('13) Christine Chen, I worked in South Australia to study the Edicaran-aged Wonoka Formation. In the southern portions of the basin, the Wonoka is ~700 meters of mixed shelf limestones and siliciclastics that record a 17‰ δ13C excursion (-12 to +5‰). Further north, the Wonoka is host to deep (~1 km) paleocanyons (figure right; general dip direction to the east), which are partly filled by tabular-clast carbonate breccias. This preserved canyon-shoulder to canyon-fill depositional system is rare in the rock record, and allows for unique insights into Ediacaran carbon isotope systematics.

Isotope conglomerate tests


In paleomagnetics, a conglomerate test is used to determine whether clasts in a conglomerate were magnetized prior to transport and deposition (preserving random magnetic directions) or after deposition (preserving uniform magnetic directions, despite random clast orientations). Analogously, we performed an isotope conglomerate test on the breccia units of the Wonoka paleocanyons (figure left) to assess the provenance and relative timing of acquisition of δ13C by measuring the isotopic values of limesone and dolostone clasts. The clasts are sourced from eroded and redeposited horizons from southern canyon-shoulder stratigraphy. Breccia units range from 0.2 to 11 m thick, are clast-supported in a matrix of fine sand, and are most common at the base of the canyon-fill stratigraphy. Within single flows, δ13C of individual clasts can vary between -12‰ to +5‰ (color bar on figure left), the same range observed in intact canyon-shoulder sections. These results require that the negative δ13C values were acquired in Wonoka carbonates before those carbonates were brecciated and redeposited into the paleocanyons, and could not be a result of late-stage burial diagenesis

Evidence for syn-depositional diagenesis


These isotope conglomerate tests (figure above) preclude a burial diagenesis model for the origin of the extremely negative δ13C values in South Australia, but what about about styles of diagenesis which may have occurred earlier? An excellent place to start with this question is the sedimentology, and there is indeed textual evidence that supports early diagenesis (figure right). Pictured here is a high-contrast, black and white image of a polished slab of Wonoka grainstone, oriented so that up is the paleo-up direction. At the base, there are well preserved, flat laminae, but this primary texture is lost towards the top of the bed, with replacement by a secondary texture called stylonodular bedding, with nodule size increasing towards the top. The texture is interpreted to have formed early, most likely during sedimentary hiatuses between the deposition of event beds. Attendent with this physical transformation is a 0.5‰ drop in δ13C values and rises in concentrations of manganese (Mn) and iron (Fe). Thus, while Wonoka carbonates do contain a record of syn-depositional diagenesis, the scale of the signal is small (<0.5 to 1‰) cannot be used to explain the full 17‰ excursion observed in the Ediacaran Wonoka Formation.