Primary Ediacaran Ooid Formation Texture Changes as an Indicator of Shifts in Local Shallow-Sea Chemistry: Exploring Diagenetic Effects on Preservation of Carbonate Associated Nitrate in the Johnnie Formation, Death Valley, California

Abstract:

We examined a stratigraphic section of the Johnnie Oolite containing a large Ediacaran negative d13C excursion to evaluate potential effects of diagenesis on Carbonate Associated Nitrate (CAN). Previously, high concentrations of CAN have been determined in this carbonate sequence, potentially pointing to a large perturbation in the marine nitrogen cycle prior to the onset of the d13C excursion. The main source of nitrate within the ocean comes from oxidation of biologically fixed nitrogen. Nitrate enters the carbonate mineral lattice in trace quantities, and may serve as a proxy for the degree of ocean oxygenation as well as for the concentration of fixed nitrogen in the environment. Until now no work has been done to address potential diagenetic alteration of the CAN concentrations. Here we conducted a comparative study between a set of redox-sensitive trace metals, carbonate textures and CAN concentrations in order to place some initial constraints on the extent of diagenetic processes, and in doing so to determine whether the CAN signal within the Johnnie Oolite is primary. We found no clear relationship between CAN and any considered diagenetic markers, pointing to a likely primary nature of the CAN signature. Furthermore, examination of the properties of the ooids that form the rock revealed potential indications of primary environmental changes recorded within the Johnnie Oolite. Ooids are known to form by two distinct mechanisms, which produce either tangential-concentric ooids or radial. Focusing on a single ~3 m stratigraphic section, the North Springs Mountains area, we found a clear shift from tangential-concentric formation at the base to radial formation near the top, with a distinct change occurring between 2-2.5 m. The distinction between ooid formation mechanisms is often thought to be governed by changes in the turbidity of their shallow depositional environments; however, the lack of any marked changes in sedimentary sorting point to a chemical influence in this case. Observed correlations between ooid formation texture and d13C values support this conclusion, and further point towards some distinct shift in the local paleocean chemistry, apparently occurring at the time of substantial changes in CAN content recorded within the rocks.