THE SEDIMENTARY EXPRESSION OF A LARGE NEGATIVE EDIACARAN ISOTOPE EXCURSION USING HIGH-RESOLUTION ISOTOPE STRATIGRAPHY AND BASIN RECONSTRUCTION

The Rainstorm Member of the Ediacaran Johnnie Formation of the southern Basin and Range, US, records a large negative carbon isotope anomaly. Though the precise age of this unit remains uncertain, this pattern has been correlated with similar magnitude occurrences in Oman (Shuram), S. Australia (Wonaka), and S. China (Doushantuo). Aside from its extreme magnitude, this isotopic excursion commonly has additional unusual attributes (e.g. covarying C and O isotope ratios, invariant d13Corg time series, and carbonate cements with rare primary carbonate phases) that have raised fundamental questions about whether this excursion accurately captures the time-series behavior of marine DIC. To explore this, we examined the stratigraphic expression of the isotopic excursion using high-resolution sampling of sections across the palinspastically-reconstructed Johnnie basin.

The Johnnie oolite, a ~2 m thick marker bed present in nearly all sections across the DV region, contains the highest rate of change in d13C and can be used as a pseudo-datum to examine the nature and timing of the isotopic excursion. Sedimentologically, the oolite is thought to be time-transgressive; if the isotopic excursion reflects time-series behavior of the carbon cycle, its expression across the basin should align the sections according to their slight diachroneity. Detailed carbon isotopic stratigraphy of the oolite at seven different locations indicates the magnitude of the excursion at the base of the oolite is spatially variable such that after a palinspastic reconstruction, the sections show a systematic north to south gradient in the isotopic data. The oolite preserved in the Old Dad Mountains, the most southerly section measured, is an outlier to this trend (and the most difficult to ordinate accurately in the reconstruction) showing the largest isotopic range within the oolite. The carbonate beds overlying the oolite that contain the nadir of the excursion (~ -10 permil) exhibit a similar north to south trend. Several hypotheses are congruent with these data, but the sum of observations is best explained by a scenario wherein the oolite is time-transgressive and deposited in a north to south manner during the onset of the isotopic excursion. If correct, this implies that the stratigraphic d13C pattern reflects an excursion in marine DIC.