READING THE TRACE ELEMENT AND ISOTOPIC CODE RECORDED IN SLOPE FACIES EDIACARAN CARBONATES FROM THE YANGTZE PLATFORM (SOUTH CHINA)

Resolving primary chemical signatures in ancient marine carbonates from effects produced by later processes, remains a first order challenge, which may be overcome only by multi proxy studies. Such data are particular important for the Precambrian, where different seawater composition and depositional environments may not permit direct comparison with modern environments. Here, we present data on acetic acid leachates of lower Ediacaran carbonate rocks from the slope facies Panmen section (Songtao, Guizhou) of the Yangtze Platform. Although parts of this section are condensed (most of Doushantuo 2 and the base of D. 3 are missing) and affected by minor slumping, it represents one of the few sections on the Yangtze platform that represent deep water deposition. d¹³C_carb data are similar to data from platform settings, variable and negative in D. 1 (cap carbonates), mostly constant at values of -9 in D.3, and a recovery to -1 during D.4 or the base of the upper Ediacaran. ⁸⁷Sr/⁸⁶Sr displays a systematic decrease from radiogenic values in cap carbonates (0.723-0.717) to lower values during D. 4 (0.710), indicating considerable exchange with radiogenic Sr from basin fluids. Marinoan diamictites at the base of the section and older rocks below may be a plausible source of fluids and the radiogenic Sr. Negative d¹⁸O (-5 to -14) appear to be affected by exchange with meteoric fluids, but only rarely does it show a systematic variation with other tracers such as d¹³C_carb, ⁸⁷Sr/⁸⁶Sr and Ce/Ce*. Abundances of Th, REE, Y and Mn are considerably higher than in modern marine carbonates, but mostly show no systematic correlation with other proxies. Panmen section also displays a drop in Ce/Ce* from near 1 to 0.5, which coincides with the recovery of d¹³C_carb near the boundary between the lower and upper Ediacaran. Concentrations of U and S_carb display a systematic increase up section by a factor of 3 and 10, respectively, whereas Fe displays the opposite trend (decrease in abundances by a factor of 100). Like the major changes in d¹³C_carb and Ce/Ce*, these variations may be interpreted as primary signals, possibly indicating increasing oxygenation of the water column with time. If this interpretation can be confirmed, this would indicate strong decoupling of redox sensitive proxies from proxies easily disturbed by interaction with basin fluids.