Contours of simulated marine dimethyl sulfide distributions under variation in a Gabric mechanism
Biogeochemical tracer bins and transformations from an established reduced sulfur cycle mechanism were introduced into the oceanic component of the Community Climate System Model. The resulting global dimethyl sulfide simulation framework was then subjected to variation in plant cell precursor content and the kinetic form of removal terms. Chi square type merit minima were computed analytically along a release rate axis over global, low latitude and localized domains. A band of width 60 degrees centered on the equator proved to be the most effective optimization area because it greatly exceeded resolution of the validation data set but avoided fronts where the driver ecodynamics module overpredicts chlorophyll. Loss terms involving bacterial consumption formulated as bimolecular kinetics and photochemical decay sensitized by dissolved organic matter independently provided superior agreement with data by modulating peaks along the equatorial divergence. Factor of ten reductions in the sulfur precursor content of diatoms or small noncalcite secretors respectively flattened and exaggerated the same features, but intermediate compositions were not tested. The parameter suite of constant intracellular sulfur, second order osmotrophy (microbial uptake) and photochemistry set proportional to photosynthetic radiation is recommended and packaged as a startup mechanism. This particular combination optimizes large-scale reduced sulfur fields across well-understood ecosystems while simultaneously maintaining parsimony. Visualizations from the inverse procedure are offered for multiple mechanisms, as mappings of both normalized deviation and concentration. The value of the rate constant for injection from plant and animal material was often determined to be of the order weeks, consistent with emission via grazing or aging/mortality. Refinement of the model will require linkage to the ecological flows associated with cell disruption, accounting of elemental metabolic stresses which in part govern reduced sulfur storage, addition of true microbial ecology, further studies of open ocean sulfur photochemistry and longer duration/more detailed optimization exercises. A stepwise strategy is outlined for moving through this task list. In next-generation experiments it may prove expedient to fix sulfur content ratios within taxonomic classes while varying the maximum cell content.
dimethyl sulfide ocean community climate system model optimization bimolecular kinetics photochemistry cell disruption microbial ecology sulfur package