²³⁴Th-Derived Particle Fluxes and Seasonal Variability: When Is the SS Assumption Reliable? Insights From a Novel Approach for Carbon Flux Simulation

Abstract : ²³⁴Th measurements are widely used to estimate the downward carbon flux of particles via the oceanic Biological Pump. Carbon export is evaluated from ²³⁴Th-²³⁸U disequilibrium assuming either steady state (SS) conditions, or including a non-SS (NSS) correction. We use a novel stochastic simulation to quantify the temporal variation of vertical carbon and ²³⁴Th (dissolved and particulate) concentration profiles with high temporal resolution. We calculate seasonal export as if in situ measured with sediment trap and SS- and NSS-²³⁴Th approaches and quantify the periods of validity for SS/NSS conditions defined in previous works. The SS approach is valid throughout the entire season in oligotrophic regions. In temperate regions, the SS introduces a bias in the export if sampling takes place outside specific temporal windows. Windows of validity range from days in short blooms of 15-day duration to weeks in blooms longer than 30 days.