Evaluation of paired carbon isotopic signals from the Ediacaran Doushantuo phosphorites: Diagenetic or primary?

Abstract

This study reports high-resolution C-isotopic data of carbonate carbon (δ13Ccarb) and organic carbon (δ13Corg) from the Ediacaran Doushantuo carbonate and phosphorite as well as from the underlying tillite of the Nantuo Formation from the Datang mine and Chuanyuandong sections. For the Nantuo tillite in the Datang section, the striking negative δ13Ccarb values ranging mostly from −10‰ to −16‰ with negative δ18O values down to −27.2‰ and clear covariance between δ13Ccarb and δ18O, suggest that nearly all of the primary C and O-isotopic compositions have been reset by meteoric diagenesis. The near complete modification of the primary isotopic signals may be due to frequent exposure to fresh water conditions during diagenesis and the lithification of porous tillite sediment. The extremely negative δ13Ccarb values down to −16.8‰ suggest the contribution of 12C-enriched dissolved inorganic carbonate which originated from the remineralization of organic carbon during burial diagenesis. These observations suggest that caution is needed when using tillite samples for paleoenvironmental reconstruction.The correlative lower phosphorite deposits from the Datang and Chuanyuandong sections show clear covariance between δ13Ccarb and δ18O, although the Chuanyuandong phosphorites have more negative values of δ13Ccarb and δ18O than the Datang section. Although most of the δ18O values of the lower phosphorites may have been altered by secondary diagenetic processes, the δ13Ccarb values of >−3.5‰ and the δ13Corg values of <−28.0‰ may represent the primary isotopic compositions of contemporaneous phosphate-rich seawater. In contrast, the upper phosphorites, characterized by δ13Ccarb values ranging from −0.9‰ to 1.7‰ with the δ18O values from −0.2‰ to −4.5‰ and the absence of covariance between δ13Ccarb and δ18O, may have recorded the primary isotopic signals of seawater from which the phosphorites were formed. It appears that the chemical composition of phosphate-rich seawater may have changed during the two phosphogenic events of the Ediacaran Doushantuo Formation.