Summary
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.
Relevant passage:
Applying our model to today’s atmosphere-ocean state with an enhanced anthropogenic P flux from land in a sustained 130% excess over average Phanerozoic continental P weathering rates will ultimately trigger the anoxia-P-dependent cascade again in a manner that locks the oceans into an extensively anoxic state for more than half a million years. The oceanic redox state passes a tipping point when the oceanic P/O2 ratio is ∼2.3 and continues into a eutrophic ocean state with 3.5 times higher P/O2 than today and sustained high productivity and organic C burial. The Earth system tips back into the oxic ocean state as atmospheric O2 levels rise and cause oceanic P/O2 ratio to decline below the tipping point (Figure 4). The dynamics of the event depend critically on the formulation of the anoxia function, which is assumed represented by a sigmoidal function with parameters calibrated in 3D (GENIE)64 and 1D (CANOPS)65 Earth System models. The new sedimentation-dependent formulation of the benthic P flux does make the oceans more sensitive towards runaway anoxia when anoxia develops in shallower depths, but our model still requires a long period (>104 years) of sustained P input to pass the tipping point for global marine anoxia today. With oceanic P input at a sustained 130% excess over average Phanerozoic continental P weathering rates for ∼120 ka, anthropogenic forcing will eventually trigger the anoxia-P-dependent cascade in a manner that locks the oceans into an extensively anoxic ocean state for more than 0.5 Ma until oceanic P/O2 begins to decline as a result of rising atmospheric O2 levels (Figure 4).