MULTI-PROXY STUDY OF SHALLOW-WATER EDIACARAN CARBONATES (XIAOFENGHE, YANGTZE PLATFORM, SOUTH CHINA)

Ediacaran carbonates from the Yangtze Platform, S-China provide insights into post Marinoan ocean oxidation and metazoan evolution. We studied major and trace elements and Sr-Nd-C-O isotopic compositions in acetic acid leachates of carbonates from the shallow-water Xiaofenghe section (Hubei Province), including Doushantuo (D1-4) and overlying Dengying strata. With the exception of the cap carbonates (⁸⁷Sr/⁸⁶Sr=0.7088 to 0.713), most samples show low Mn/Sr and ⁸⁷Sr/⁸⁶Sr similar to seawater values (0.7078 in D2-D4 and 0.709 in Dengying) inferred from Ediacaran carbonates elsewhere. The absence of a correlation of ⁸⁷Sr/⁸⁶Sr with δ¹⁸O_carb indicates minor influence of diagenetic fluid overprinting. D1 samples display negative δ¹³C_carb of -2.0 to -3.6‰ and δ¹⁸O_carb ranging from -6.5 to -7.3 ‰, whereas the overlying strata show positive δ¹³C_carb (D2 ~6‰ at varying TOC). The D3 and D4 samples show a total δ¹³C_carb variation from -1.4 to 9.4 ‰, with few having the highest δ¹³C_carb (+7 to +9 ‰) but lowest TOC. The Δδ¹³C_carb-org shows varying values between 23 ‰ and 38 ‰, indicating a decoupled DIC and DOC reservoir evolution. Most carbonate leachates display variable HREE enrichment (Pr/Yb <1) indicating the presence of a seawater signature and show fractionated Y/Ho (1.3-1.5 in D1, increasing up section (>2.5 in D2)). However lower Y/Ho in saponite and smectite rich D4 (clay content up to 15%) correlate with flat REEY patterns, indicating freshwater-seawater mixing and suppression of HREE/LREE fractionation by the formation of presumably authigenic clay phases. Whereas D1 shows small negative Ce anomalies, they are more pronounced in D2 to D4 and Dengying (Ce/Ce* down to 0.64), indicating carbonate precipitation from oxidized waters. Carbonates of D4 with the highest Y/Ho and lowest Ce/Ce* may be explained, in conjunction with high δ ¹⁸O_carb values, by extreme shallowing and salinity rise due to higher evaporation rates. The εNd (t) values of carbonate leachates range from -4.3 to -7.5 (with an ‘outlier’ at +3.4) with the lowest and highest values in D1. The εNd (t) values up section show only minor variations and overlap with published data for Doushantuo black shales and Proterozoic granites in S-China arguing for a relatively homogenous source of Nd input to the shallow-water at Xiaofenghe section during most of the Ediacaran.