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Th-234 measurements are widely used to estimate the downward carbon flux of particles via the oceanic Biological Pump. Carbon export is evaluated from Th-234-U-238 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 Th-234 (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-Th-234 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 similar to 15-day duration to weeks in blooms longer than similar to 30 days.

Plain Language Summary Climate change driven by anthropogenic CO2 emissions has a significant impact on mankind. An important component of Earth's carbon cycle is the ocean's biological carbon pump (BCP), which buffers the air-sea balance of CO2. It starts with production of organic matter in the surface ocean, continues with its sinking into the dark ocean, and finishes with the storage of a small fraction below 1,000m. Without the BCP, atmospheric CO2 concentration would be 50% higher than it currently is. Understanding the BCP helps to predict the future CO2 that will be stored by the oceans and how that will affect the global carbon cycle. The photosynthetically fixed carbon is mainly transferred as large particles. Among the methods currently used to estimate the downward flux of particles, sediment traps, and Th-234-U-238 radioactive pair are the most accepted ones. However, the simultaneous use of these two techniques does not always provide comparable flux estimates. This questions the consistency of our current calculations of the magnitude of the carbon flux and the accuracy of the global predictions. In this study, we assess the accuracy of the Th-234 method in relation to the traps and explain the source of the discrepancies between them.