Where and When the Mesopelagic Carbon Budget Balances, if at All

Abstract The ocean biological carbon pump (BCP) transports organic matter from the surface to the deep ocean. Accurately quantifying the efficiency of the BCP is essential for understanding potential climate feedbacks and entails measuring the flux of organic material in and out of the mesopelagic layer (approximately 100–1,000 m). Observational estimates are often restricted to measuring the BCP efficiency over short timescales. Here we use an ocean biogeochemical model to diagnose where, and on what timescales, the mesopelagic is sufficiently in steady state that balancing the carbon budget may be possible. For the majority of the ocean the sources and sinks of organic carbon in the mesopelagic do not balance on timescales shorter than 1 year. Assuming steady state risks falsely inferring the existence of missing processes or the magnitudes of known ones to close the budget and will lead to incorrect estimates of the strength of the BCP.