Wide but not ubiquitous distribution of glendonite in the Doushantuo Formation, South China: Implications for Ediacaran climate

Glendonites are pseudomorphs of syndepositional or early authigenic ikaite (CaCO3 center dot 6H(2)O) that often forms at near-freezing temperatures. Silicified glendonite has been reported from inner-shelf deposits of the lower Ediacaran Doushantuo Formation at a single stratigraphic section in South China, where they are stratigraphically associated with the positive delta C-13 excursion EP1 but predate the negative delta C-13 excursion EN3 (=Shuram excursion), indicating a period of cool climate somewhere between similar to 609 Ma and similar to 551 Ma. This interpretation predicts a wider geographic distribution of Ediacaran glendonites in equivalent strata in the Yangtze Block of South China. To test this prediction, we conducted a regional survey of Doushantuo Formation and found that, although glendonites are not universally present, they occur in expected stratigraphic intervals and in association with EP1 at two new sections representing inner-shelf and intrashelf basin facies. The wide but not ubiquitous distribution of Doushantuo glendonites indicates that ikaite precipitation and glendonite formation was controlled by both regional climatic and local geochemical factors. Glendonites at the new localities are stellate clusters pseudomorphed by calcite spar and sometimes are rimmed with silica. The calcite spar is characterized by highly variable and mostly negative delta C-13 values as low as -37 parts per thousand, indicating that diagenetic transformation of precursor ikaite to calcitic glendonite may be related to anaerobic oxidation of organic matter or methane in sediment. The new data suggest an early Ediacaran cold period prior to EN3 or the Shuram excursion and facilitation of glendonite/ikaite formation by both climatic and local geochemical conditions such as redox conditions and phosphate concentrations.