Cascading oxygen loss shoreward in the oceans: Insights from the Cambrian SPICE event

Marine euxinia can amplify phosphorous-limited marine productivity by recycling phosphorous from sediments, creating a feedback loop that increases marine oxygen consumption and ultimately leads to widespread oceanic anoxia. This phenomenon is potentially more dangerous when oxygen loss arises in coastal zones. Here, we present empirical evidence and show that this cascade was set off in the Cambrian Earth system. Carbon isotopes and Mo enrichments in well-dated sediment records from the Steptoean Positive Carbon Isotope Excursion (SPICE) event reveal a rapid decline over 130 ± 30 ka to persistently low Mo levels for 1.0 ± 0.2 Ma, followed by a slower recovery. Using dynamic models for the global biogeochemical cycles, we demonstrate that marine anoxia expanded globally through a self-cascading feedback mechanism. Importantly, we find that the benthic phosphorous flux likely scaled with sedimentation, and that chemocline shoaling into coastal areas likely triggered the SPICE event. We evaluate the risk of passing the tipping point for global-scale anoxia today.

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