The stratigraphic expression of a large negative carbon isotope excursion from the Ediacaran Johnnie Formation, Death Valley

Abstract

The Rainstorm Member of the Ediacaran Johnnie Formation of the southern Basin and Range, US, records a large negative excursion in carbon isotope ratios of carbonate strata (δ 13C VPDB > −6‰). The character of the excursion raises fundamental questions about whether this isotopic pattern is accurately capturing the time-series behavior of marine dissolved inorganic carbon (DIC) or is a product of diagenesis. To explore this issue, we examined the expression of this isotopic excursion within the Johnnie oolite, a ∼2 m thick marker bed, which records the highest rate of change in δ 13C. Sedimentologically, the oolite unit is thought to be time-transgressive; if the isotopic excursion reflects time-series behavior of the carbon cycle, its expression in the oolite across the basin should systematically align the sections according to their slight diachroneity. Detailed carbon and oxygen 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 align along 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 between the onset of carbonate deposition and top of the oolite. 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 throughout the onset of the isotopic excursion. If correct, this implies that the stratigraphic δ 13C pattern reflects time series behavior of marine DIC.