Iodine Content of the Carbonates from the Doushantuo Formation and Shallow Ocean Redox Change on the Ediacaran Yangtze Platform, South China
Abstract
The Ediacaran Period (635–541 Ma) is characterized by remarkable carbon isotope (δ13C) excursions, significant ocean redox changes and rapid diversification of eukaryotes including early animals. These biological and geochemical events are commonly thought to be linked, but their causal relationships remain controversial. A particular point relates to the shallow-ocean redox change across the negative δ13C excursions. Most existing geochemical proxy data focused on tracking the redox conditions of deep- or bottom-water columns and provided limited information about the surface ocean. Ratios of iodine-to-calcium-magnesium [I/(Ca + Mg)] from carbonates represent one of the few redox proxies that track the oxidation state of the shallow oceans. Here we report paired carbonate carbon isotopes (δ13Ccarb) and I/(Ca + Mg) ratios of the Doushantuo Formation from a drillcore section at Daotuo, paleogeographically located at the slope of the Ediacaran Yangtze Platform, South China. In addition, we compare the I/(Ca + Mg) ratios across the Shuram-upper Doushantuo δ13Ccarb excursion in four sections of a shelf-to-slope transect. In the slope section at Daotuo, the δ13Ccarb of the Doushantuo Formation shows three negative anomalies that are correlatable with those documented from the Yangtze Gorges area (shelf) and the Siduping section (slope). The I/(Ca + Mg) ratios of the Doushantuo Formation from the drillcore are dominated by low but non-zero (<1.0 μmol/mol) values, with two minor increases (to ≤ 2.6 μmol/mol) at the basal and middle part of the Doushantuo Formation and one prominent rise (to 4.5 μmol/mol) at the top of this unit. The two minor positive shifts in I/(Ca + Mg) may record slight to moderate increase of O2 in the water column of the depositional environments. The large increase of I/(Ca + Mg) ratios in the upper Doushantuo Formation, coincident with the Shuram-upper Doushantuo δ13Ccarb excursion, suggests a significant ocean oxygenation event during which the oxic-anoxic interface was pushed down below the slope environments. The concomitant increase in I/(Ca + Mg) and decrease in δ13Ccarb in the South China sections and other sections globally support that the Shuram-upper Doushantuo δ13Ccarb excursion was likely formed by oxidation of 13C-depleted organic carbon. In contrast to the slope sections, the I/(Ca + Mg) ratios across the Shuram-upper Doushantuo δ13Ccarb excursion from the two intra-shelf sections are persistently low (≤1.3 μmol/mol), which suggests the local depositional environments at or below the redoxcline. The spatial variability of I/(Ca + Mg) ratios suggests redox heterogeneity across the Ediacaran Yangtze Platform during the Shuram-upper Doushantuo δ13Ccarb excursion and calls for integrated study of different redox proxies across the shelf-to-basin transects of this critical time interval.