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Organic carbon isotope gradient and ocean stratification across the late Ediacaran-Early Cambrian Yangtze Platform |
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| Authors: | XinQiang Wang XiaoYing Shi GanQing Jiang DongJie Tang |
| Institution: | 1. State Key Laboratory of Biogeology and Environmental Geology, Beijing, 100083, China 2. School of Earth Sciences and Resources, China University of Geosciences, Beijing, 100083, China 3. Department of Geoscience, University of Nevada, Las Vegas, NV, 89154-4010, USA |
| Abstract: | Organic carbon isotope (δ 13Corg) data from two well-preserved sections across a shallow-to-deep water transect of the late Ediacaran-Early Cambrian Yangtze Platform in South China show significant temporal and spatial variations. In the shallow-water Jiulongwan-Jijiapo section, δ 13Corg values of the late Ediacaran Dengying Formation range from ?29‰ to ?24‰. In the deep-water Longbizui section, δ 13Corg values from time-equivalent strata of the Dengying Formation are mostly between ?35 and ?32‰. These new data, in combination with δ 13Corg data reported from other sections in South China, reveal a 6‰-8 shallow-to-deep water δ 13Corg gradient. High δ 13Corg values (>?30‰) occur mostly in shallow-water carbonate rocks, whereas low δ 13Corg values (<?32‰) dominate the deep-water black shale and chert. The large temporal and spatial δ 13Corg variations imply limited buffering effect from a large dissolved organic carbon (DOC) reservoir that was inferred to have existed in Ediacaran-Early Cambrian oceans. Instead, δ 13Corg variations between platform and basin sections are more likely caused by differential microbial biomass contribution to total organic matter. High δ 13Corg values (>?30‰) documented from shallow-water carbonates are within the range of typical Phanerozoic δ 13Corg data and may record the isotope signature of organic matter from primary (photosynthetic) production. In contrast, low δ 13Corg values (<?32‰) from deep-water sections may have resulted from higher chemoautotrophic or methanotrophic biomass contribution to bulk organic matter in anoxic environments. The δ 13Corg data provide indirect evidence for ocean stratification and episodic chemocline fluctuations in the Ediacaran-Early Cambrian Yangtze Platform. |
| Keywords: | late Ediacaran Early Cambrian South China organic carbon isotopes ocean stratification |
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