Redox-sensitive trace element distribution in the Loma Negra Formation in Argentina: The record of an Ediacaran oxygenation event

Abstract

The limestones of the Loma Negra Formation provide an excellent opportunity to examine the behaviour of rare earth elements plus yttrium (REY) due to the near-zero diagenetic alteration of the carbonate system. The terrigenous supply to this unit was minimal or scarce in relation to the depth of seawater in which the limestones accumulated. The interpreted palaeoenvironment suggests that limestones could have originated by suspension fall-out in a shallow, open marine ramp. The diagenetic features, composition, and original microtextures are considered good criteria for investigating their REY composition. Burial diagenesis products were limited and limestones largely preserved their original texture and organic matter, as well as their isotope and trace element composition. The study of REY distribution in the Loma Negra Formation in different settings of the Tandilia System revealed systematic chemostratigraphic patterns related to uniform palaeo-redox conditions. The general light rare-earth element depletion, marked positive La, Y, and Gd anomalies, as well as the clear negative Ce anomalies, are further features typically ascribed to oxidizing seawater conditions. From these results, it can be concluded that the Loma Negra Formation documents an oxygenation event that stands out in the overall anoxic Ediacaran ocean. This record may be correlated with the mid-Ediacaran oxygenation event (ca. 580 Ma). This may support the hypothesis that the Clymene Ocean was well oxygenated and connected during the Ediacaran, at least in the southwestern border of Gondwana.