中国科学院南京地质古生物研究所机构知识库

The majority of the deep ocean was likely under ferruginous conditions during the first four billion years of Earth's history. As the atmosphere was gradually oxygenated, the sources, sinks, redox cycling, and reservoir size of dissolved iron in the deep ocean are likely to have changed dramatically. Whether deep water was thoroughly oxygenated by the time of the Ediacaran-Cambrian transition, and the relationship of this oxygenation to the Cambrian explosion, remains debated. To explore the degree of oceanic oxygenation and its effect on Cambrian explosion, we measured the iron isotopic composition (delta Fe-56) of bulk rock (i.e., cherts and mudstones/shales) through the Piyuancun and Hetang formations, using samples collected from the Chunye-1 core, on the Lower Yangtze Block in western Zhejiang. The limited variation in delta Fe-56 values (< 0.7 parts per thousand) and low Fe-T/Al ratios (< 0.77) in euxinic samples show that the deep-water Fe2+ reservoir was quite limited, and likely similar to that of the modern ocean, during the latest Ediacaran and Cambrian Stages 1-3. Iron isotope results, combined with published data from sections on the Middle and Upper Yangtze Block, record a general decline in seawater delta Fe-56 values from > 0.55 parts per thousand during the end-Ediacaran and Cambrian Stages 1-3 to < 0 parts per thousand during Cambrian Stage 4. Seawater delta Fe-56 values in the lower and middle Hetang Formation range between 0 and 0.2 parts per thousand, suggesting that the riverine dissolved and suspended flux and/or aeolian dust was the predominant source of highly reactive iron to the deep basin. Positive deep-water delta Fe-56 values, above 0.55 parts per thousand during the terminal Ediacaran and Cambrian Stages 1-3, likely reflect a basin where pyritization, rather than oxidation, was the predominant sink for deep-water ferrous Fe. Thus, we infer that only the shallow water was sufficiently oxygenated to support complex metazoans and the evolutions of skeletons, and that atmospheric oxygen levels were not high enough to directly oxygenate deep water environments during the Cambrian explosion.