Lecture 8 Ocean Carbonate Chemistry Carbonate Reactions Reactions Solutions numerical graphical K versus K What can you measure Theme 1 continuation Interior Ocean Carbon Cycle Theme 2 Ocean Acidification man s alteration of the ocean Sarmiento and Gruber 2002 Sinks for Anthropogenic Carbon Physics Today August 2002 30 36 1Pg 1015g CO2 CO2 rocks HCO3 clays Gas Exchange Atm River Flux Ocn CO2 H2CO3 HCO3 CO32Upwelling Mixing H2O CH2O O2 Ca2 CaCO3 CO2 BorgC BCaCO3 Biological Pump Application Paleooceanography Controls pH of ocean Sediment diagenesis Weathering and River Flux Atmospheric CO2 is converted to HCO3 in rivers and transported to the ocean Examples Weathering of CaCO3 CaCO3 s CO2 g H2O Ca2 2 HCO31 2 Weathering of alumino silicate minerals to clay minerals silicate minerals CO2 g H2O clay minerals HCO3 2 H4SiO4 cation 1 1 2 A specific example of orthoclase to kaolinite KAlSi3O8 s CO2 g 11 2H2O 1 2 Al2Si2O5 OH 4 s K HCO3 2H4SiO4 Example Global River Flux River Flow x global average HCO 3 concentration Global River Flux 3 7 x 1016 l y 1 x 0 9 mM 33 3 x 1012 mol C y 1 x 12 g mol 1 396 x 1012 g C y 1 0 4 Pg C y 1 What happens to the CO2 that dissolves in water CO2 is taken up by ocean biology to produce a flux of organic mater to the deep sea BorgC CO2 H2O CH2O O2 Some carbon is taken up to make a particulate flux of CaCO 3 BCaCO3 Ca2 2HCO3 CaCO3 s CO2 H2O The biologically driven flux is called the Biological Pump The sediment record of BorgC and BCaCO3 are used to unravel paleoproductivity The flux of BorgC to sediments drives an extensive set of oxidation reduction reactions that are part of sediment diagenesis Carbonate chemistry controls the pH of seawater which is a master Variable for many geochemical processes Bronsted Concept of Acids Acids are compounds that can donate a proton to another substance which is a proton acceptor Acid1 Base1 proton Proton Base2 Acid 2 Acid1 Base2 Acid2 Base1 Examples HCl H2O H3O ClH2O H2O H3O OHH2CO3 H2O H3O HCO3 Free H don t exist in solution To simplify we will write acids as Arrhenius Acids where acids react to produce excess H in solution HCl H Cl Table of acids in seawater Element H2O C B Mg Si P S VI F Ca Reaction H2O H OHH2CO3 HCO3 H HCO3 CO32 H B OH 3 H2O B OH 4 H Mg2 H2O MgOH H H4SiO4 SiO OH 3 H H3PO4 H2PO4 H H2PO4 HPO 2 H HPO42 PO43 H HSO4 SO42 H HF F H Ca2 H2O CaOH H And in anoxic systems N NH4 NH3 H S II H2S HS H HS S2 H mol kg 1 pK logK logC 2 4 x 10 3 2 6 4 25 x 10 4 3 37 5 32 x 10 2 1 27 1 5 x 10 4 3 82 3 0 x 10 6 5 52 2 82 x 10 2 1 55 5 2 x 10 5 4 28 1 03 x 10 2 1 99 10 x 10 6 10 x 10 6 5 0 5 0 pK 13 9 13 9 e g K 10 6 0 9 1 8 7 12 5 9 4 1 6 6 0 8 6 1 5 2 5 13 0 9 5 7 0 13 4 Q Which is larger pK 6 0 or 9 1 CO2 reacts with H2O to make H2CO3 CO2 g H2O H2CO3 KH H2CO3 is a weak acid H2CO3 H HCO3 K1 HCO3 H CO32 K2 H2O is also a weak acid H2O H OH KW H2CO3 carbonic acid HCO3 bicarbonate CO32 carbonate H proton or hydrogen ion OH hydroxyl Equilibrium Constants on Different Scales Equilibrium constants can be defined on the infinite dilution activity scale and the ionic medium scale Equilibrium constants can be defined differently on these two scales There are both advantages and disadvantages of the infinite dilution and ionic medium approaches Consider the following reaction HA H AWe define the equilibrium constants as follows A On the infinite dilution scale K is expressed in terms of activities Calculated from Gr Activities K H A HA B On the ionic medium scale the equilibrium constant K is defined in terms of concentrations in the ionic medium of interest Measured Concentrations K H A HA infinite dilution scale K pros K can be calculated from Gr One K for all solutions cons Need to express concentration as activity using free ion activity coefficients i and free fi ionic medium scale K pros Use concentrations Don t need to assume activity coefficients When K s have been determined they are usually very well known cons K s need to have been determined experimentally as function of T P and S When pH is measured as the activity of H as it is commonly done the mixed constant is defined in terms of H K H A HA The difference between K and K is the ratio of the total activity coefficients K H A HA K H A HA K H A i fi HA i fi K H A HA x i fi i fi K K A HA Values of K and K Representative values for these constants are given below Equations are given in Millero 1995 with which you can calculate all K s for any salinity and T P conditions The values here are for S 35 T 25 C and P 1 atm Constant KH K1 K2 Kw S 0 Thermodynamic Constant K 10 1 47 10 6 35 10 10 33 10 14 0 S 35 Apparent Seawater Constant K 10 1 53 10 6 00 10 9 10 10 13 9 Example of calculating T from K and K The difference between K and K can be illustrated by this simple example K H A HA H A T A HA T HA Rearrange to get K H A T A K T A HA T HA T HA The difference in magnitude of K and K is the ratio of the total activity coefficients of the base to the acid If you know both K and K you can learn something about the activity corrections For H2CO3 HCO3 H K1 HCO3 H H2CO3 or K1 HCO3 T HCO3 H H2CO3 T H2CO3 or K1 HCO3 H T HCO3 10 6 3 from tables of Gr at 25 C and 1 atm H2CO3 T H2CO3 The value of K has been determined for the same reaction At S 35 25 C and 1 atm K1 HCO3 H 10 6 0 H2CO3 If we set K1 K1 T HCO3 T H2CO3 6 3 6 0 0 3 1 K vs K Example continued We can compare this ratio with that obtained from the Garrels and Thompson 1962 speciation model of surface seawater where T HCO3 T H2CO3 fHCO3 HCO3 f H2CO3 H2CO3 0 69 0 68 4 1 x 10 1 1 00 1 13 Not too bad Not too good These two estimates differ …