The Evolution Of Shallow Ocean Redox On Edicaran Yangtze Platform

The Ediacaran?635⁵51 Ma?is a critical period in Earth history and shows a transition feature from Precambrian to Phanerozoic,during which rapid diversification of eukaryotes including earliest metazoan,significant ocean redox changes,and remarkable negative carbon isotope excursions?Shuram excursion?occurred.These biological and geochemical events are commonly thought to be linked,For the ambiguous with redox landscape,their casual relationships remain controversial.Ratios of iodine-to-calcium-magnesium[I/?Ca+Mg?]from carbonate can track the oxidation state of the shallow oceans in semi-quantitative calculation.This dissertation reports paired carbonate carbon isotopes(?¹³C_carb)and I/?Ca+Mg?ratios of the Doushantuo Formation from a drillcore section at Daotuo,which was paleogeographically located at the shelf margin of Edicaran Yangtze Platform.There is three?¹³C_carbarb excursions in the Daotuo core section?EN1,EN2,EN3?,which can be correlated with the Siduping section in slope environment.By comparing I/?Ca+Mg?of the two sections,we reconstructed the Ediacaran redox model of surface waters in open ocean.For analyzing the spatial difference of redox state during the Shuram excursion,additional samples from the upper Shuram-?¹³C_carb excursion?EN3?were collected from the Jiulongwan and Xiangerwan sections,which were plaeogeographically located in the inner shelf of the Yangtze Platform.The I/?Ca+Mg?ratios of the Doushantuo Formation in Daotuo are dominated by low but non-zero?<1?mol/mol?values.In the cap carbonate,the I/?Ca+Mg?both in Daotuo and Siduping show a minor increase?<2.6?mol/mol?,indicating the enhancement of primary production after the Marinoan glacial,which expanded the extent of oxic waters.In upper part of the cap carbonate,I/?Ca+Mg?in Daotuo reduced to 0.1?mol/mol,while I/?Ca+Mg?in Siduping can not be detected.The oxic-anoxic interface was likely loated at the shelf margin.There exists another minor increase of I/?Ca+Mg?in the middle part of Daotuo section,which may record a slight increase of O₂ in shllow-water environments.A prominent increase in Daotuo and Siduping occurs at the beginning of EN3.This suggests a significant ocean oxygenation event,during which the oxic-anoxic interface was pushed down below the slope,it is likely that the atmosphere O₂ at this time may have rised to at least 10%PAL.In contrast to the Daotuo and Siduping sections,I/?Ca+Mg?values in intra-shelf sections show persistently low values.This suggests a redox heterogeneity across the Ediacaran Yangtze Platform during the Shuram excursion.The increase of I/?Ca+Mg?between EN3 shows Shuram recorded a primary signal.The apparently varying magnitude of EN3 among sections of the Yangtze Platform likely suggests that DOC was not the only source of C¹² for the Shuram excursion.The oxidation of ocean and rise of atmosphere O₂ have facilitated the evolution of early metazoan.The stable sub-oxic in slope might stimulated the evolution of Miaohe and Wenghui Biota in Yangtze Platform.