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Project Progress

In the past year we have focused on building a database of carbon-isotopic events and interpreting the results. The database now contains 31 events, roughly evenly distributed throughout the Phanerozoic. Some of these events are associated with mass extinctions, and others are not. The events are each indexed by the magnitude of the carbon-isotopic excursion and the time scale over which the carbon-13 content of carbonate rocks becomes more depleted in comparison to carbon-12. From these data, we obtain a theoretical increase in the marine reservoir of dissolved organic carbon (DIC) associated with each event.

Results show that there is a critical rate of growth of the DIC reservoir beyond which mass extinction occurs. A remarkably large fraction of the events are associated with DIC growth near the critical rate, but not faster. We hypothesize that the critical rate is associated with a marginally stable carbon cycle, such that faster rates strongly disrupt the entire earth system and slower rates are relatively benign. The clustering of events near the critical rate suggests that the system has a tendency to self-organize to a state of marginal stability. Curiously, the magnitude and duration of these marginally stable events decreases systematically with time throughout the Phanerozoic.

We interpret the marginally stable rate theoretically in terms of basic dynamics and biogeochemistry. The dynamical theory shows that the marginally stable rate is the fastest possible rate of growth consistent with steady-state (i.e., quasistatic) changes of long-duration events. Biogeochemically, the critical rate is precisely the rate of CO₂ production that would occur under the cessation of organic-carbon burial, regardless of the duration of the event.

We tentatively conclude that typical “large” carbon-cycle events are associated with episodic changes in microbial ecosystems that disrupt the “normal” sequestration of recalcitrant organic matter in sediments. Events that exceed the typical (i.e., critical) rate are associated with mass extinction. The decrease in the magnitude and duration of these disruptions suggests that the underlying cause of the perturbations is related to the evolution of microbial ecoystems in marine sediments.