Variation of chemical index of alteration (CIA) in the Ediacaran Doushantuo Formation and its environmental implications

The Ediacaran Period (635-541 Ma) witnessed rapid turnovers of marine organisms that eventually led to the early Cambrian radiation. The Ediacaran Doushantuo Formation (ca. 635-551 Ma) serves as one of the most important and intensively studied stratigraphic units for understanding the co-evolution of life and environment during this critical time interval. To better understand the early-middle Ediacaran climate change, we reconstruct a temporal variation curve of chemical index of alteration (CIA) from a drill core and an outcrop of the Doushantuo Formation, in combination with previously published CIA data from segments of time-equivalent strata. The results show three distinct decreases in CIA values that correlate well with the positive carbon isotope (delta C-13) excursion at the low Doushantuo Formation (EP1) and the negative delta C-13 excursion at the middle (EN2) and upper (EN3) Doushantuo Formation. The decline of CIA in EP1 is consistent with occurrence of glendonites at the same stratigraphic interval that suggests a cooling event. The low CIA values in EN2 and EN3 also suggest relatively colder periods that are consistent with evidence from strontium isotope (Sr-87/Sr-86) and oxygen isotope (delta C-13) data. With existing bio- and chemostratigraphic constraints, the cooling event associated with EN2 might be synchronous with the Gaskiers glaciation. The other two cold periods associated with EP1 and EN3, however, have not been documented from other successions globally, which needs to be tested in future studies.