How carbon is pulsed into the ocean, not just how much, controls whether the marine carbon cycle fires large repeated responses.

Summary. The manuscript studies how a conceptual, excitable ODE model of the marine carbon cycle responds to atmospheric carbon injection. For single pulses it reports that the threshold carbon amount needed to trigger a large response depends on pulse duration but not on the detailed pulse shape, while shape still affects the number and duration of large transients; these differences are linked to the geometry of an increasingly winding heteroclinic orbit as excitability rises. For random multi-pulse sequences with fixed total carbon and overall duration (motivated by Large Igneous Provinces), the response varies strongly with mean pulse characteristics, and an intermediate “Goldilocks” range of pulse durations or arrival frequencies maximises the number of repeated large transients. Illustrative scenarios inspired by the Siberian Traps and Columbia River Basalt Group are used to test the framework.

Significance. If the reported thresholds, shape-independence, heteroclinic geometric mechanism, and multi-pulse Goldilocks zone survive scrutiny of the full model and numerics, the work would supply a clean dynamical-systems account of how the temporal structure of carbon injection—not only its total mass—controls marine carbon-cycle excursions. That framing is potentially useful for interpreting LIP-related paleoclimate and extinction records and for designing more realistic forcing protocols in intermediate-complexity models. The abstract’s emphasis on geometric phase-space explanation and on concrete geological scenarios is a genuine strength, provided the reduced model’s excitability and heteroclinic structure are shown to be robust rather than artefacts of a particular parameterisation.