Lecture Notes (PDF) - Aqueous and Environmental Geochemistry

Marine Chemistry IIEnvironmental GeochemistryDM Sherman, University of BristolCarbonate Equilibriaand the pH of Seawater1

Effect of CaCO 3 (Calcite/Aragonite) If the system is saturated in calcite, weneed the additional equilibrium:CaCO 3 (s) = Ca 2+ + (CO 3 ) 2- pK = 8.48pCa 2+ = pK 4 - p(CO 3 ) 2- + 2pH= -13.14 + 2pHCharge-balance requires that2[Ca] = 2[CO 3-2] + [HCO 3- ] ≈ [HCO 3- ]or pCa = pHCO 3-+ 0.301pH of Seawater (Surface, pP CO2 = 3.5)When pCa = pHCO 3 + 0.301, pH = 8.3 4

Actually, it’s a bit more complicated..• When we include all of the other acid-base and ionpairingequilibria, we find that surface seawatershould have a pH near 7.8.• The observed surface pH = 8.3 is a consequence ofsupersaturation of CaCO 3 (SI = 1.2).• Presumably, supersaturation is favored by calcifyingorganisms as it is easier to grow shells under suchconditions.Predicted Changes in Oceanic pH(Caldeira & Wickett 2003)5

Biological Controls on DissolvedCarbonatei) CO 2 consumption by photosynthesis:CO 2 + 2H 2 O → “CH 2 O” + O 2ii)CO 2 release (“mineralization”) by respiration“CH 2 O” + O 2 → CO 2 + 2H 2 O(Note, the arrows indicate irreversible reactions!)iii) Biomineralization of CO-23Ca +2 + CO-23 = CaCO 3 (calcite)ForaminiferaCaCO 3 -Shelled ZooplanktonPteropods6

Alkalinity and Total Dissolved CarbonAlkalinity is the capacity of a solution to neutralizeadded acid.Carbonate Alkalinity is[Alk] = [HCO 3- ] + 2[CO-23 ] + [OH - ] - [H + ]Total Dissolved Inorganic Carbon (ΣCO 2 ) is[HCO 3- ] + [CO-23 ] + [H 2 CO 3 ]Effect of PhotosynthesisPhotosynthesis will not affect alkalinity but willincrease the pH:CO 2 + H 2 O → CH 2 O + O 2H 2 CO 3 = CO 2 + H 2 OH + + HCO 3-= H 2 CO 3H + + HCO 3-→ CH 2 O + O 2The pH increase will favor pptn of CaCO 3 :Ca +2 + HCO 3-= CaCO 3 + H +8

Effect of RespirationRespiration will not affect alkalinity but will decreasepH:CH 2 O + O 2 → CO 2 + H 2 OCO 2 + H 2 O = H 2 CO 3H 2 CO 3 = HCO 3-+ H +CH 2 O + O 2 → H + + HCO 3-The pH decrease will favor dissolution of CaCO 3 :CaCO 3 + H + = Ca +2 + HCO 3-Effect of CaCO 3 precipitationHCO-3 = H + + CO-3H + + HCO-3 = H 2 CO 3H 2 CO 3 = CO 2 + H 2 OCa +2 + CO-3 → CaCO 3Ca +2 + 2HCO 3-→ CaCO 3 + CO 2 + H 2 OHence, ppt’n of CaCO 3 actually releases CO 2 ,decreases the alkalinity, and decreases pH.9

Effect of Biological ProcessesPhotosynthesisCaCO 3 pptnDistribution of Carbonate Sediments11

Carbonate Depth Profile (Mid Atlantic)Surface waters are supersaturatedwith CaCO 3 .Calcite Compensation Depth:Dissolution Rate > “Rain Rate”Deep waters are undersaturatedwith CaCO 3 .CaCO 3 saturation Depth(Feely et al., 2004)12

Change in Alkalinity/TDC in Oceans13

Absorption of Atmospheric CO 2 by theOceansAtmospheric CO 2Gas AbsorptionCO 2 + H 2 O + CO 3-2= 2HCO 3-H + + HCO 3-→ CH 2 O + O 2Ca +2 + 2HCO 3-= CaCO 3 + CO 2 + H 2 OShallow Ocean:carbon fixing by photosynthesis.UpwellingSinking of CH 2 O + CaCO 3CH 2 O + O 2 → H + + HCO 3-CaCO 3 + H + = Ca +2 + HCO 3-Deep Ocean:“mineralization by respiration.Summary• Equilibrium with atmospheric P CO2 = 10 -3.5 bar andsaturation in CaCO 3 buffers the pH of seawater to 8.3.• CO 2 is sequestered in the deep ocean by the “biologicalpump”.• Surface waters are supersaturated in calcite; deepwaters are undersaturated. The carbonatecompensation depth reflects the competition between thekinetics of dissolution and the downward flux ofparticulate CaCO 3 .14

Box Model for OceansSurface OceanIndianPacificAtlanticSouthern“Primary Productivity”15