Nitrogen Isotope Evidence for Redox Variations at the Ediacaran-Cambrian Transition in South China

Nitrogen isotope (N-15) analyses of the early Cambrian (ca. 542--520 Ma) strata in the Yangtze Gorges area, south China, reveal a prominent positive N-15 excursion (+8%), concurrent with a negative shift in organic carbon isotopes (C-13(org)) at the early Nemakit-Daldynian or early Fortunian stage. This positive N-15 excursion, in combination with the previously reported paired C-13(carb)-C-13(org) at this interval, can be interpreted as resulting from denitrification with large isotopic fractionation. Subsequent N-15 from early Nemakit-Daldynian to early Tommotian (stage 2) are dominated by stable values fluctuating between --2 parts per thousand parts per thousand and +2 parts per thousand parts per thousand, close to that of atmosphere N-2 (approximate to approximate to 0 parts per thousand parts per thousand) accompanied by the low and stable C-13(org) (approximate to approximate to--33 parts per thousand parts per thousand) and decoupled C-13(carb)-C-13(org). We argue that the relatively low and invariant N-15 values from this interval likely resulted from enhanced N fixation with limited isotopic fractionation. Enhanced N fixation is generally associated with significant nutrient N loss mainly through denitrification and anammox, indicating the expansion of an anoxic environment. The N-15 records of the study sections show large difference from that of potential time-equivalent Zhujiaqing Formation in the Xiaotan section. This discrepancy can be plausibly attributed to different redox conditions in a stratified ocean inherited from the Ediacaran. Anoxic ocean conditions may have lasted for millions of years until the early Atdabanian (stage 3), when trilobites started to dominate the Cambrian ocean.