Quantifying pH Dynamics in a Changing Ocean Mathilde Hagens & Jack Middelburg Earth Sciences Utrecht University

Future Ocean CO 2 rich, Warmer and Low O 2 What are the consequences for ocean CO 2 uptake? – Revelle factor: sensitivity of CO 2 to DIC changes How will (seasonal and long-term) pH dynamics change? – Buffer factor: sensitivity of H + to DIC and other acid/base additions

pH = f (T, S, DIC, TA ….) Total derivative of pH: How to obtain the partial derivatives? Numerically (small perturbations) Analytically Straightforward for DIC, but for TA …...

Partial derivative of TA This buffer factor is based on the assumption that all but H + remain constant

Sensitivity of TA to X Buffer factor if all total concentrations are constant Term for changes in total concentration of acid-base TotX Term for changes in concentration of species X where: and In the case of ocean acidification (X=H 2 CO 3 and TotX=DIC):

Why this math? Generalised sensitivities: – Consistent with prior work by Frankignoulle (1994) and Egleston et al. (2010) – All relevant acid-base systems Sensitivities: – project changes in pH dynamics – elucidate factors governing seasonal pH dynamics Similar approach applies to Revelle Factor

Revelle Factor extended Revelle and Suess (1957) Sundquist et al. (1979) Includes all acid-base systems via partial derivative Consistent with prior work that included only carbon dioxide, borate and water dissociation

Sensitivity now and in 2100 SensitivityUnitPresent2100 CO T 2100 CO 2 + T Positive values: addition increases pH Negative values: addition decreases pH Ocean acidification increases sensitivity Ocean warming lowers sensitivity ( mol kg -1 ) -1

Projections pH changes and gradients will increase CO 2 uptake will decrease

Eutrophication and Ocean Acidification

What governs pH seasonality? Station ALOHA in the Pacific Ocean Station DYFAMED in the Mediterranean Sea Iceland Sea station in the North Atlantic Time series on pH, DIC, TA, T, S, borate, sulphate, fluoride, nutrients..

ALOHA Harmonic least square fit Predicted using sensitivities

DYFAMED Harmonic least square fit Predicted using sensitivities

Iceland Sea Harmonic least square fit Predicted using sensitivities

Attribution of pH changes Temperature (T) Alkalinity (TA) Inorganic carbon (DIC) Iceland SeaALOHADYFAMED Winter pH: + T, - DIC Summer pH: - T, + DIC Winter pH: + T, - DIC Summer pH: - T, + DIC Complex pattern: T with DIC and TA

Seasonality in pH pH can be predicted TA, DIC and T most important T and DIC often compensate

Thank you Generalised sensitivity factors Extension to Revelle factor Projections for future pH sensitivity Understanding of pH seasonality Interested? PhD thesis “Impact of biogeochemical processes on pH dynamics in marine systems”