Rare earth element geochemistry of carbonates as a proxy for deep-time environmental reconstruction

The rare earth elements and yttrium (REY) can enter carbonate lattices by replacing cations, which can be used to reconstruct the history of carbonate precipitation. The Ediacaran Doushantuo carbonates in South China and their equivalents in the world record the largest negative carbon isotope excursions (DOUNCE) in Earth history. However, their precipitating environment remains controversial. Here, the REYs of the DOUNCE carbonates were used to reconstruct the deep-time depositional environments. At the outset, we provide a detailed overview of the REY characteristics of carbonates in different environments. We classify the depositional environments of carbonates into five: seawater in the marine environment, estuarine water associated with the transition zone between continental and marine environments, river and lacustrine of continental waters, pore water of diagenetic environments and hydrothermal fluids. The REYs of seawater are constrained by both solution and surface complexation, with carbonate complexes as the major components. From the Archean to the present, the seawater shows the similar distribution patterns in marine primary carbonates, characterized by uniform LREE depletion, positive La anomalies, and high Y/Ho ratios. The REY patterns of estuarine waters are determined by complex processes, including coagulations of colloidal materials along salinity gradients, adsorption, desorption and remobilization. The continental waters are generally characterized by relatively flat REY patterns, which may be influenced by pH. The REY patterns of pore waters are complex, including flat patterns, MREE-bulge patterns, and HREE-enriched patterns, which are constrained by the adsorption and desorption processes during diagenetic alteration. The high-temperature hydrothermal fluids display LREE enrichment and positive Eu anomalies. Our study shows that the Neoprotemzoic DOUNCE carbonates are characterized by minor LREE depletion patterns, positive La anomalies, with negative Ce anomalies, absence of Eu anomalies and lower Y/Ho ratios. These REY signatures are consistent with an intrashelf lagoonal environment with freshwater input, rather than an open marine setting. The results from our study provide novel insights into the deep-time environments of the globally important successions.

Automated summary

This study used REY to reconstruct the deep-time depositional environments of carbonates, classifying them into five categories and analyzing the REY characteristics in different environments. The results showed that the DOUNCE carbonates of the Ediacaran period exhibit characteristics of an intrashelf lagoonal environment with freshwater input.