Stable isotope (S, C) chemostratigraphy and hydrocarbon biomarkers in the Ediacaran upper section of Sierras Bayas Group, Argentina

Abstract

The Neoproterozoic sedimentary successions of the Sierras Bayas Group (SBG) are situated within the Tandilia Belt, Buenos Aires Province, Argentina. The Loma Negra Formation (LNF) from the upper part of the SBG has been investigated using a combination of inorganic, organic and isotopic geochemical methods in order to determine the stable isotope composition of carbonates (δ13Ccar, δ18Ocar), kerogen (δ13Cker), pyrite and carbonate associated sulfur (δ34Spy, δ34SCAS). Concentrations of major, trace and rare earth elements and hydrocarbon distributions were also investigated. This data set was used to further the understanding of the interactions between paleo-biodiversity and paleoenvironmental conditions for terminal Ediacaran (post-Gaskiers) shelf deposits. The high δ34Spy values exceeding the coeval δ34SCAS values in green micritic limestones of the lower LNF may be explained by a combination of different events such as globally low seawater sulfate concentrations, an increased rate of bacterial sulfate reduction, and a decrease in sulfate levels in the near-bottom deepest water layer inherited from glaciations. Negative Ce anomalies in the dark grey micritic limestones of the upper LNF suggest they were deposited under oxidizing conditions. However, higher concentrations of Fe, Mo, Zn and REE, and the occurrence of authigenic 34S-enriched pyrite suggest that the sediments were maintained under reducing conditions. The distributions of hydrocarbon biomarkers (e.g. hopanes maximizing at C29) are in line with this hypothesis and indicate a diverse microbial community including primary producers such as cyanobacteria (e.g. terminally-branched monomethyl alkanes, hopanoid distribution), phototrophic bacteria (e.g. acyclic isoprenoids C<21) and green bacteria (e.g. n-C18 ≫ Phytane). The unsteady Δ13Ccar-ker values reflect changes in primary biomass due to relative contributions of bacterial microorganisms using different photosynthetic carbon-fixation pathways. The lowest Δ13Ccar-ker values coincided with the latest biomarker signal interpreted as the signature of green non-sulfur bacteria using a less 13C fractionation pathway. The combined biogeochemical features of the Loma Negra Formation, indicated a well-stratified water column with oxygenated surface waters, oxygen-poor bottom waters and anoxic sediments which helped to refine a correlation with the Polanco Formation from the Arroyo del Soldado Group in Uruguay.