Sea Water Chemistry Changes in the Shallow Water Ediacaran NeoProterozoic Ocean

Secular changes of water chemistry in shallow-water Ediacaran ocean: Evidence from carbonates at Xiaofenghe, Three Gorges area, Yangtze Platform, South China

Authors:

Hohl et al

Abstract:

Ediacaran carbonates from the shallow-water section near Xiaofenghe (Three Gorges area, Hubei Province) on the Yangtze Platform, South China have been studied to understand post Marinoan changes in seawater chemistry. Major and trace element abundances and Sr-Nd isotopic compositions were obtained on acetic acid leachates of carbonate sediments from both the Doushantuo Formation (Members D1-D4) and the overlying Dengying Formation. C and O isotopic compositions of these samples were analysed using the phosphoric acid method. With the exception of the D1 cap carbonates (87Sr/86Sr = 0.7088 to 0.713), most samples show low Mn/Sr and 87Sr/86Sr (0.7078 in D2-D4 and 0.709 in the Dengying Formation) similar to seawater values inferred from Ediacaran carbonates elsewhere. These data, together with the absence of a correlation between 87Sr/86Sr and δ18Ocarb, indicate minor diagenetic fluid overprinting. The initial ɛNd values of the carbonate leachates show a limited variation range from -4.3 to -7.5 with the highest values in D1. These data overlap with published data for Doushantuo sediments in South China, and possibly indicate an isotopically homogenous Nd source in the source area during most of the Ediacaran. D1 samples of cap carbonates display negative δ13Ccarb values of -2.0 to -3.6 (VPDB) and δ18Ocarb values ranging from -6.5 to -7.3 (VPDB), whereas the overlying D2 strata show positive δ13Ccarb values around 6. Carbonate rocks from D3 and D4 show a large variation of δ13Ccarb from -1.4 to 9.4, with some samples having unusually high δ13Ccarb values coupled to low TOC contents. Most carbonate leachates at the base of the Doushantuo display superchondritic Y/Ho and variable HREE enrichment (Pr/Yb less than 1), similar to modern seawater. This suggests a similar seawater chemistry as under modern open ocean conditions at the beginning of the Ediacaran, whereas lower Y/Ho in D3 correlate with flat REE + Y patterns, indicating a change to variable freshwater-seawater mixing and suppression of HREE/LREE fractionation as in modern estuaries. Carbonate rock samples from D4 are rich in clay minerals (up to 15%), of possible authigenic origin, such as nontronite and saponite. Formation of these clay minerals may only be possibly under extreme alkaline and saline conditions. The D4 carbonate leachates show the highest Y/Ho and lowest Ce/Ce* values and have unusually high δ 13Ccarb values, which may also support the conditions of extreme shallowing and perhaps temporarily restricted basins that led to a rise in salinity due to high evaporation rates. Ce anomalies in carbonate leachates show strong variation throughout the profile. A secular increase of seawater oxygenation through the Ediacaran is not obvious. However the most pronounced Ce anomalies can be found in the upper Doushantuo (Ce/Ce* as low as 0.64).