FY 2024

Amplified subsurface signals of ocean acidification

Fassbender, A.J., B.R. Carter, J.D. Sharp, Y. Huang, M.C. Arroyo, and H. Frenzel

Global Biogeochem. Cycles, 37(12), e2023GB007843, doi: 10.1029/2023GB007843, View open access article online at AGU/Wiley (external link) (2023)

We evaluate the impact of anthropogenic carbon (C_ant) accumulation on multiple ocean acidification (OA) metrics throughout the water column and across the major ocean basins using the GLODAPv2.2016b mapped product. OA is largely considered a surface-intensified process caused by the air-to-sea transfer of C_ant; however, we find that the partial pressure of carbon dioxide gas (pCO₂), Revelle sensitivity Factor (RF), and hydrogen ion concentration ([H⁺]) exhibit their largest responses to C_ant well below the surface (>100 m). This is because subsurface seawater is usually less well-buffered than surface seawater due to the accumulation of natural carbon from organic matter remineralization. pH and aragonite saturation state (Ω_Ar) do not exhibit spatially coherent amplified subsurface responses to C_ant accumulation in the GLODAPv2.2016b mapped product, though nonlinear characteristics of the carbonate system work to amplify subsurface changes in each OA metric evaluated except Ω_Ar. Regional variability in the vertical gradients of natural and anthropogenic carbon create regional hot spots of subsurface intensified OA metric changes, with implications for vertical shifts in biologically relevant chemical thresholds. C_ant accumulation has resulted in subsurface pCO₂, RF, and [H⁺] changes that significantly exceed their respective surface change magnitudes, sometimes by >100%, throughout large expanses of the ocean. Such interior ocean pCO₂ changes are outpacing the atmospheric pCO₂ change that drives OA itself. Re-emergence of these waters at the sea surface could lead to elevated CO₂ evasion rates and reduced ocean carbon storage efficiency in high-latitude regions where waters do not have time to fully equilibrate with the atmosphere before subduction.

Plain Language Summary. The chemistry of the upper ocean is changing due to the absorption of excess carbon in the atmosphere resulting from human activities, a process commonly referred to as ocean acidification (OA). The highest concentrations of excess carbon in the ocean are generally found at the surface; however, some of the largest chemical changes resulting from this carbon buildup are occurring below the sea surface. This subsurface intensification is caused by nonlinear responses of some chemical parameters to opposing vertical gradients of natural carbon versus excess carbon. Our findings emphasize the need to study multiple metrics of OA throughout the water column to comprehensively evaluate changes in the habitability of interior ocean realms and potential connections to ocean carbon storage timescales.