Dados da Submissão

Título

PAIRED CARBON ISOTOPES AND REY SUGGEST THAT THE DEVELOPMENT OF THE FIRST ANIMALS WITH EXOSKELETONS OCCURRED IN OXIC REFUGIA DURING THE EDIACARAN-CAMBRIAN TRANSITION

Texto do resumo

The Ediacaran-Cambrian transition is characterized by important changes in our planet, such as the consolidation of Gondwana, major geochemical anomalies, and the emergence of bilaterian animals. Here, two columnar sections (PL-I and PL-III) of the Tagatiya Guazu Formation (Itapucumi Group, Paraguay), a carbonate unit deposited during the Ediacaran-Cambrian transition, were studied using major, trace, and rare earth elements + yttrium (REY) and oxygen (δ18O), organic (δ13Corg), and inorganic carbon (δ13Ccarb) isotopes. The sedimentary facies constitute a typical peritidal association in a partially restricted lagoon environment, arranged in shallowing-upward cycles formed by grainstones, thrombolites, planar stromatolites, marls, and mudstones. The facies in the upper portion of the cycles are interpreted as deposited in shallower waters (inter/supratidal) and have a diverse and abundant fossiliferous assemblage (mainly in the PL-I section), dominated by autochthonous and parautochthonous specimens of Cloudina, Corumbella, and Namacalathus, in addition to treptichnid burrows. In the grainstone facies, deposited in subtidal channels and predominant in the PL-III section, fossil specimens occur dispersed and reworked. After screening, the total REY concentrations (∑REY) vary between 4.7 and 29.4 ppm (mean = 23.1), with a relatively planar PAAS-normalized distribution pattern and small enrichments in light or heavy REY, evidenced by Nd/Yb(SN) ratios between 0.49 and 1.42. The absence of correlation between ∑REY vs. Al, Si, and Zr and between Nd/Yb(SN) vs. TOC, Al, and insoluble residue suggests that REY patterns were not significantly affected by detrital contamination. Y/Ho ratios range from 31.8 to 55.1 (mean = 39.6), with Ce/Ce* between 0.80 and 1.28 (mean = 1.03). δ13Ccarb values between +1 and +5 ‰ are consistent with the global pattern of Cloudina-bearing successions, while δ13Corg values range between -29.7 and -27.4 ‰, with ∆13C between 29.1 and 33.6 (mean = 31.4). The absence of correlation between the Mn/Sr and Mg/Ca ratios vs. δ13Ccarb and δ18O data confirms the preservation of pristine isotopic values. The δ13Corg values are generally invariant and decoupled from δ13Ccarb, except for a thick and fossiliferous interval in the shallower PL-I section, where coupling and greater variation occur. The ∆13C values, in turn, have a marked correlation with δ13Ccarb. These data, reinforced by the presence of negative Ce anomalies, the coupling between δ13Ccarb and δ13Corg in the PL-I section, as well as the facies-restricted body and trace fossil assemblages suggest an environment characterized by a thin oxygenated surface layer. Below, and separated by a shallow chemocline, the presence of a thick anoxic zone is then inferred based on the 1) high overall Ce/Ce* values; 2) low Y/Ho values, suggesting restriction; 3) allochtonous character of the deeper PL-III section fossils; 4) decoupled and invariable behavior of δ13Corg in the PL-III section; and 5) correlation between δ13Ccarb and ∆13C. Items 4 and 5 indicate buffering of the δ13Corg system and control of δ13Ccarb over ∆13C, suggesting the existence of a dissolved organic carbon reservoir much larger than the inorganic and, therefore, anoxic conditions. Thus, the obtained results may indicate that the benthic communities of the Ediacaran-Cambrian transition probably inhabited oxic refuges amid anoxic and stressful environments, which were probably not conducive to their development.

Palavras Chave

Ediacaran-Cambrian; carbonate geochemistry; redox conditions; paleoenvironmental reconstruction