| Summary: | The Arctic Ocean is particularly vulnerable to ocean acidification, a process that is mainly driven by the uptake of anthropogenic carbon (C ant ) from the atmosphere. Although C ant concentrations cannot be measured directly in the ocean, they have been estimated using data-based methods such as the transient time distribution (TTD) approach, which characterizes the ventilation of water masses with inert transient tracers, such as CFC-12. Here, we evaluate the TTD approach in the Arctic Ocean using an eddying ocean model as a test bed. When the TTD approach is applied to simulated CFC-12 in that model, it underestimates the same model's directly simulated C ant concentrations by up to 12%, a bias that stems from its idealized assumption of gas equilibrium between atmosphere and surface water, both for CFC-12 and anthropogenic CO 2 . Unlike the idealized assumption, the simulated partial pressure of CFC-12 ( p CFC-12) in Arctic surface waters is undersaturated relative to that in the atmosphere in regions and times of deep-water formation, while the simulated equivalent for C ant is supersaturated. After accounting for the TTD approach's negative bias, the total amount of C ant in the Arctic Ocean in 2005 increases by 8% to 3.3 ± 0.3PgC. By combining the adjusted TTD approach with scenarios of future atmospheric CO 2 , it is estimated that all Arctic waters, from surface to depth, would become corrosive to aragonite by the middle of the next century even if atmospheric CO 2 could be stabilized at 540 ppm.
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