SULFUR CYCLE IMBALANCE AND ENVIRONMENTAL CHANGE DURING THE NEOPROTEROZOIC ERA

Extremely negative carbon isotope anomalies are characteristic features of the Neoproterozoic Era, commonly attributed to diagenetic alteration, authigenic cementation or regional effects. However, such arguments appeal to an inexplicable sampling bias, whereby globally correlative isotopic signatures are presumed to be unrepresentative of the global carbonate sink. Instead we view these in terms of a linked carbon-sulfur-oxygen system, whereby changes in oxidant dynamics due to surplus pyrite burial caused an excess of organic carbon oxidation over burial, resulting in a smaller dissolved organic matter (DOM) reservoir. Although it is generally assumed that the oxygen released by pyrite burial is approximately matched by oxygen consumed during pyrite weathering, this is unlikely always to be the case on timescales over which the long-term carbon and oxygen cycles must be in balance. The interval of major carbon cycle disruption from c.770 Ma until c.550 Ma was a time of little or no basin-scale evaporite deposition, suggesting that, as during much of the Cenozoic, the sulfur weathering-deposition cycle was not in steady-state. However, unlike the Cenozoic, the low atmospheric oxygen and anoxic deep ocean of the Neoproterozoic allowed evaporite-derived oxidizing power to be effectively transmitted into a negative C-isotope signal. Such 'sulfur cycle imbalance', potentially driven by weathering events and related positive feedbacks, may be particularly relevant to the ending and immediate aftermath of ‘Snowball Earth’ glaciations, which are marked by euxinia, pyrite deposition and negative anomalies. The existence of a series of excursions indicates that the DOM pool served as a buffer against oxygenation and climate change throughout the Neoproterozoic, but only when the pool was sufficiently large. There are tantalising hints that this buffer was largely removed during the late Ediacaran Shuram anomaly, after which widespread evaporite deposition may even have reversed the sulfur cycle imbalance. We propose that the expansion of bilaterian fauna after c.550 Ma was an opportunistic radiation in response to a transient oxidant surplus, followed by episodic redox fluctuations, which persisted throughout the subsequent Cambrian Period.