Paper No. 293-4
Presentation Time: 2:20 PM
WIDESPREAD OCCURRENCES OF GLENDONITE IN THE DOUSHANTUO FORMATION, SOUTH CHINA: IMPLICATION FOR EDIACARAN CLIMATE CHANGE AND STRATIGRAPHIC CORRELATION
Glendonites are pseudomorphs of syndepositional or early authigenic ikaites (CaCO3·6H2O) that form at near-freezing temperatures. Silicified glendonite was reported from inner-shelf deposits of the lower Ediacaran Doushantuo Formation at a single stratigraphic section in the Zhangcunping area of South China. At Zhangcunping, glendonite occurs in the lower Doushantuo Formation in association with a positive carbon isotope excursion known as EP1, indicating a period of cool climate somewhere between ~609 Ma and ~551 Ma based on currently available age constraints. This interpretation predicts a wider geographic distribution of Ediacaran glendonite in the lower Doushantuo Formation and equivalent strata in South China. To test this prediction, we conducted a systematic survey of Doushantuo glendonites in South China and discovered several new occurrences of Doushantuo glendonite at Huji (inner-shelf, drill core), Changyang (intrashelf basin, drill core), and Yangjiaping (shelf margin, outcrop) sections. The new data demonstrate widespread occurrences of Doushantuo glendonite across the continental shelf of the Yangtze Block in South China. In contrast to completely silicified glendonite in the Zhangcunping area, glendonites at the new localities are stellate clusters that are pseudomorphed by calcite spar and sometimes are rimmed with silica, thus allowing for carbon isotope analysis. Calcispar in the glendonites is characterized by negative carbon isotopic signatures, with δ13C values as low as –37‰, indicating that diagenetic transformation of precursor ikaites to glendonites may be related to anaerobic oxidation of organic matter or methane in sediment. In contrast, carbonate strata that host the glendonites are characterized by positive δ13C, consistent with their stratigraphic association with EP1. It is uncertain whether a single or multiple glaciations occurred in the Ediacaran Period. A resolution of this uncertainty and how Ediacaran glaciations are correlated with carbonate carbon isotope excursions are critical to understand Ediacaran Earth-life system.