CADMIUM ISOTOPIC FRACTIONATION IN EDIACARAN CARBONATES FROM THE SHALLOW WATER XIAOFENGHE SECTION, YANGTZE PLATFORM, SOUTH CHINA

Cadmium isotopes are used to study the utilization of cadmium by phytoplankton in the modern ocean as Cd shows nutrient-type distribution patterns in the water column, mirroring concentration patterns of phosphate. Cadmium isotopes fractionate due to phytoplankton uptake and their substitution for Zn in the Cd enzyme carbonic anhydrase (CDCA), enriching surface water in heavy isotopes. However, the onset of this process in earth history remains unknown. We present Cd isotopic data of carbonate leachates from the Ediacaran Xiaofenghe section on the Yangtze Platform, South China using the double spike method and TIMS (ε^112/110Cd relative to NIST SRM 3108, external precision: ± 8ppm). The data show εCd of +0.06 to +1.07 (in the cap carbonates and in the lower Doushantuo (DI - II), but significantly lighter values between -3.53 and -0.79 in the overlying Doushantuo and Dengying strata. Cadmium concentrations range from 6 to 130 ng/g. Neither Cd concentrations, nor isotopic compositions correlate with P or Zn abundances. Given the experimentally derived α_{CaCO3-Seawater} of 0.99955 for the fractionation of Cd into calcium carbonate lattice, seawater in equilibrium with these carbonates should have εCd between 0.97 and 5.57, which is in the range of modern surface water. However, the shift to lower values up section argues against increasing biological fractionation. The Cd isotopic variations correlate with Y/Ho_PAAS in the carbonate leachates, indicating a possible dependence on paleo-seawater salinity, which was experimentally demonstrated to cause Cd isotopic fractionation. Carbonates with negative εCd show negative Ce/Ce* anomalies and high δ¹³C_carb, which may indicate carbonate precipitation in shallow oxic water subjected to increased evaporation rates. Our data suggests that variations in the Cd isotope composition of Ediacaran carbonates at Xiaofenghe are most likely a result of kinetic fractionation of Cd into inorganic carbonates and may be solely controlled by salinity rather than by the CDCA enzyme. Nowadays, this enzyme occurs in diatoms, which may have evolved not earlier than 200 Ma ago.