Quantifying pH Dynamics in a Changing Ocean

Mathilde Hagens & Jack MiddelburgEarth Sciences

Utrecht University

Future Ocean

CO2 rich, Warmer and Low O2

• What are the consequences for ocean CO2 uptake?– Revelle factor: sensitivity of CO2 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=H2CO3 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

Sensitivity Unit Present 2100 CO2

2100T

2100 CO2 + T

1601 2627 1517 2480

-1751 -2705 -1674 -2567

• Positive values: addition increases pH • Negative values: addition decreases pH• Ocean acidification increases sensitivity• Ocean warming lowers sensitivity

(mol kg-1)-1

(mol kg-1)-1

Projections

pH changes and gradients will increase CO2 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 fitPredicted using sensitivities

DYFAMED

Harmonic least square fitPredicted using sensitivities

Iceland Sea

Harmonic least square fitPredicted using sensitivities

Attribution of pH changes Temperature (T)Alkalinity (TA)Inorganic carbon (DIC)

Iceland SeaALOHADYFAMED

Winter pH: + T, - DICSummer pH: - T, + DIC

Winter pH: + T, - DICSummer 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

Iceland

DyfamedBATS

Estoc

ALOHA

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

T

DIC

TA

pH a

mpl

itude

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”

• http://dspace.library.uu.nl/handle/1874/313165.