Journal Article

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

Yool, Andrew, Oliver, Sophy, Henson, Stephanie A., Martin, Adrian P.

2025

Overview

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. Plain Language Summary Up to 90% of the sinking organic carbon created in the surface ocean is broken down back into its dissolved state within the mesopelagic (approximately 100–1,000 m), therefore any changes in the governing processes have a significant impact on how much carbon is locked away in the deep ocean. Balancing mesopelagic carbon budgets, which involves assessing the relative contributions of the inputs and outputs of carbon, helps us understand the flow of organic carbon and hence the amount stored in the deep ocean. We use a simplified but perfectly understood model system to analyze the seasonal budget of the different processes acting on organic carbon in the mesopelagic. While the sources and sinks balance over 1 year, they rarely balance on shorter timescales. This is due to the complex interplay between the seasonal variability in the amount of organic carbon sinking from the surface, vertical mixing, and horizontal movement, and may be why balancing the biological carbon pump has proved difficult using observations. Understanding these interacting effects will help reduce uncertainty in the efficiency with which carbon is transported from the surface ocean and stored in the deep ocean. Key Points Assessing the different sources and sinks of mesopelagic (100–1,000 m) carbon helps us understand the flow of carbon from the surface to depth Sources and sinks rarely balance over the period for which oceanic measurements are taken, due to seasonal variations in fluxes Assuming steady state when calculating mesopelagic carbon budgets overlooks significant seasonal factors and is ill‐advised

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Balancing / biological carbon pump / Carbon / Carbon budget / Climate feedback / Efficiency / Estimates / ocean biogeochemical modeling / Oceans / Organic carbon / organic carbon budgeting / Organic matter / Seasonal variability / Seasonal variations / Sinking / Steady state / Vertical mixing