REY-P cycles recorded by the Ediacaran phosphorite on the Yangtze Platform (South China)

Abstract

Rare earth elements and yttrium (REY) composition of paleo-seawater and redox conditions of localized water masses during deposition are documented by the REY preserved in modern pristine chemical sedimentary phosphorites. However, contrary to modern phosphorites, some older ones (e.g., Cambrian and Proterozoic phosphorites) display anomalous REY patterns that deviate from modern seawater, and limit the use of their parameters (e.g., Ce anomalies) as a reliable proxy for redox conditions in this case. In this work, elements of bulk-rocks and apatite grains, SrNd isotopes, and phosphate oxygen isotopes (δ¹⁸O_P) from Jingxiang (JX), Yichang (YC), and Xingshen (XS) phosphorites of the Ediacaran Doushantuo Formation in South China were investigated to address this quandary. The REY indexes and patterns of bulk-rocks and apatite, as well as Nd isotopes of bulk-rocks, suggest that the phosphorites deposited in the lower, middle, and upper members of the Ediacaran Doushantuo Formation record the sources of REY as being of terrigenous, mixed, and seawater origins, respectively. During the deposition of the Ediacaran Doushantuo Formation, high δ¹⁸O_P values (18.61 ± 1.04) of all bulk-rocks, close to contemporary marine authigenic apatite, indicate that phosphorus has been involved in strong marine bio-cycling, assuming a final terrestrial source. However, REY composition of the seawater column recorded by authigenic apatite in the lower member of Doushantuo Formation exhibited a right-leaning REY pattern, representing a terrestrial REY end-member. The ⁸⁷Sr/⁸⁶Sr ratios of bulk-rocks (0.71031 ± 0.00719) and apatite grains (0.71098 ± 0.00099) also remained consistent throughout the Ediacaran Doushantuo Formation, except for the high ⁸⁷Sr/⁸⁶Sr values found in bulk-rocks (0.72200 ± 0.00085) and apatite (0.72271 ± 0.00064) in the lower member of JX, which are presumed to result from source rocks with high radioactive Sr. Combined with REY indexes (Ce anomalies, Y/Ho ratios, and the differentiation of REY) and constant ⁸⁷Sr/⁸⁶Sr values of apatite, varying REY patterns from the lower to upper members of the Doushantuo Formation indicate that the scavenging rate of terrestrial REY inputs to the ocean by suspended particles (e.g., FeMn oxyhydroxides) in the water column is controlled by the degree of seawater oxidation. Our study highlights that REY inputs to seawater could undergo seawater cycling (i.e., the removal of REY by particulates of seawater under oxygen-containing conditions), and only REY that undergo sufficient seawater cycling can reflect the true redox conditions of the water column. Therefore, the sources and cycling of REY in seawater should be carefully identified when applying Ce anomalies and Y/Ho ratios of phosphorites to trace the redox conditions of ancient seawater.