Fe Isotope Features Of Cap Carbonates And Black Shales In Doushantuo Formation: Implications For Paleo-oceanography

The Doushantuo Formation is the first sedimentary strata after the Neoproterozoic snowball earth, recording the information of the environmental and biological evolution during this critical transition. In Yangtze platform of southern China, the lower Doushantuo strata are mainly composed of carbonate rocks and black shales. Five sections of different sedimentary facies were selected for this study, which are Xiaofenghe, Jiulongwan, Zhongling Sections of the platform facies, Sidouping Section of the shelf facies, and Huaihua-Yuanjia Section of the deep-sea basin facies.Through combination of Fe isotope investigation to some details and elemental characterization on cap carbonates and black shales from these profiles significant advances have been made:1. Ratios of trace elements and enrichment coefficient have been studied. The sedimentary environment is oxidation state in the Jiulongwan area. Following the melt of glaciers and rising of the sea-level, Ancient ocean became deeper. The sedimentary environment changes to weak oxidation, reflecting a transformation of paleo-ocean condition from oxidation to reduction right after the termination of snowball earth event. The Sidouping area lay in a relatively deep water system. Its sedimentary environment is reductive.2. Fe isotope of Doushantuo Formation carbonate rocks and black shales is for the first time measured in this study. Theδ56Fe values of analyzed black shales varies between -0.03%o and 0.69%o with an average of 0.24%o. A range of -0.67%o<δ56Fe<0.07%o is defined by carbonate samples, with an average of -0.34%o. Relative to the IRMM-014 standard, black shales samples show a tendency towards enrichment in heavy isotope (except for the black shale of Huaihua-Yuanjia), whereas carbonate samples are enriched in light isotope.3. Carbonate samples from different sedimentary facies display distinct differences in composition of Fe isotopes even at a single sidementary facies, indicating the iron isotope of seawater is influenced by iron source and sedimentary environment shows a spatial heterogeneity. Fe isotopes of dolomite and limestone samples from Sidouping were analyzed and consistent Fe isotope values were found in these samples. Take Sidouping profile into accout, iron isotope in ocean shows temporal homogeneity.4. The black shales samples from Doushantuo formation (except Huaihua-Yuanjia section) are enriched in heavy isotope. This demonstrates that iron from seawater is mainly controlled by the Fe3+ minerals. Combined with the information reflected by trace elements that Jiulongwan profile is in weak oxide environment. It's suggested the ocean is stratification. The redox interface should be located between the platform and the slope. The 8'6Fe value of black shales from Huaihua-yujia profile of the deep-sea basin facies is mainly controlled by the sulfide minerals. The environment in the deep-sea basin is in Euxinic. Stratification of ocean is strengthened from platform facies, slope facies to deep-sea basin facies gradually, which transformed from oxide water at surface to Eusintic water in deep-sea gradually.