Carbon and sulfur isotopic constraints on Ediacaran biogeochemical processes, Huqf supergroups, Sultanate of Oman

Abstract

The link between environmental and evolutionary change is investigated in the Ediacaran Period (- 635 - 541 million years ago (Ma), an interval where we see the emergence of macroscopic animals along with large perturbations to the carbon and sulfur cycles. Paleoenvironmental reconstruction used the carbon and sulfur isotopic composition of sedimentary minerals and organic matter, supplemented by geochemical and molecular biomarker proxies, of strata from the Huqf Supergroup, Sultanate of Oman. Within Ediacaran strata, three successive stages of geochemical oxidation are observed and correlated to episodes of biological evolution. The second stage corresponds to the large Shuram 613Ccarb excursion and is identified with the oxidation of an organic carbon reservoir. An extreme enrichment in sulfur isotopes occurs in the overlying strata spanning the Ediacaran-Cambrian (E-C) boundary. This interval is characterized using paired sulfate and pyrite sulfur isotopes to quantify pyrite burial and the isotopic composition of sulfur entering the ocean, which leads to a reinterpretation of existing Phanerozoic data. These strata contain sulfates in two forms (carbonate-associated sulfate: CAS, and anhydrite), providing an opportunity to investigate isotopic offsets reported between these seawater sulfate proxies. The results indicate that changing basin restriction can significantly alter the isotopic composition of gypsum/anhydrite and suggest that CAS is the most reliable proxy for the reconstruction of the marine sulfur cycle. A combination of geochemical and biomarker proxies were used to investigate the biological and environmental changes across the Ediacaran- Cambrian boundary.

(cont.) In the Huqf strata, a crash in the photic zone primary production is observed immediately before the geochemical evidence for anoxia that is typically associated with the extinction of the Ediacaran organisms at the E-C boundary. All of these studies rely on interpretations made from geochemical data, which depend on how representative of depositional conditions (local, regional, or global) a given dataset is. Carbon and sulfur isotopes are characterized from replicate sections of the Huqf Supergroup strata to assess the basin-scale (-1,000km) reproducibility of these signals. The trends in the sulfur isotopic record were reproducible across the basin and serve to correlate sections lacking distinctive carbon isotopes or lithologies.