Chemistry that determines ph in Swedish waters Leif G. Anderson Department of Marine Sciences University of Gothenburg Sweden

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1 Chemistry that determines ph in Swedish waters Leif G. Anderson Department of Marine Sciences University of Gothenburg Sweden

2 CO 2 The ocean takes up CO 2 when the surface water partial pressure (pco 2 ) is lower than that of the atmosphere. The result is that H + conc. increases and ph decreases. CO 2 (g) CO 2 (aq) -CO 2 dissolves in seawater and reacts with water CO 2 (aq) + H 2 O H 2 CO 3 H 2 CO 3 H + + HCO 3 - HCO 3- H + + CO 3 Dominating form

3 OA historic records show variability Pacific & Atlantic Oceans Mediterranean Great Barrier Reef from boronisotopes in corals Bubbles in glacier ice Boron-isotopes in surface ocean foraminifers found in sediment Pelejero et al. 2010

4 Hence many processes impact ph and pco 2 in seawater In Swedish waters these includes: Temperature: Cold water dissolve more CO 2 than warm River runoff: ph in runoff is directly coupled to the surface water pco 2, where ph gives pco 2 Primary production: CO 2 is fixed pco 2 decreases & ph Decay of organic matter: CO 2 is released pco 2 increases & ph

5 Two chemical definitions Dissolved Inorganic Carbon, DIC = [CO 2 (aq)] + H 2 CO 3 ] + [HCO 3- ] + [CO 3 ] Total alkalinity, TA = HCO CO 3 + B(OH) 4- + HPO PO SiO(OH) 3- + OH - - H + - HSO 4- - HF - H 3 PO 4 + HS organic acids/bases Example on the effect of temperature

6 Or, if DIC and TA are constant, at 2100 and 2300 µmol/kg, respectively

7 The low saline of our waters is highly relevant S T TA pco 2 ph [CO 3 ] * This is not a direct effect of runoff, but by the decreased buffer capacity. * This is less than the TA at this S diluted by Baltic Sea water

8 Kattegat How does it look? Investigations by SMHI y = x R 2 = surface waters Bay of Bothnia ph-nbs (25 C) ph-nbs (25 C) ph-nbs (25 o C) Year Year 2008

9 Kattegat How does it look? Investigations by SMHI y = x R 2 = surface waters Bay of Bothnia ph-nbs (25 C) ph-nbs (25 o C) Year 1996 Year

10 The state of CaCO 3 (s) solubility decreases. This is even more obvious in low salinity waters. S T TA pco 2 ph [CO 3 ] Precipitation of calcium carbonate, CaCO 3 (s) Ca HCO 3- CaCO 3 (s) + CO 2 + H 2 O where the chemical solubility product equals K so = [Ca 2+ ] [CO 3 ] and we have the expression for the state of solubility = [Ca 2+ ] [CO 3 ] K so c a c Calcite a Aragonit

11 Average salinity and ph in the waters around Sweden RRO low in CaCO 3 Bottenhavet Rigabukten RRO high in CaCO 3 ph map by Adam Ulfsbo

12 Primary production in the ocean from a chemists perspective gives: 140 CO NO 3- + HPO H + + Me H 2 O (CH 2 O) 91 (CH 2 ) 16 (NHCH 2 CO) 16 (CHPO 4 Me) O 2 Carbon dioxide, nutrients and protons are consumed and oxygen is produced. Hence pco 2 decreases while ph increases.

13 If we rewrite to the dominating form of C - HCO 3- ; 140 HCO NO 3- + HPO H + + Me H 2 O + + (CH 2 O) 91 (CH 2 ) 16 (NHCH 2 CO) 16 (CHPO 4 Me) O 2 there is a strong increase of ph, i.e. consumption of protons Decay of organic matter is equal to this reaction going in the other direction if oxygen is available and ph will decrease correspondingly, i.e. a seasonal signal. H: C = 156:

14 Effect by OM decay is evident Mixing of subsurface waters into the SML increases the pco 2 values. This is evident for the data within the red oval, where high winds prevailed during the period of sampling. W 48 W 49

15 But when organic matter is decayed using other electron acceptors Denitrification (CH 2 O) 91 (CH 2 ) 16 (NHCH 2 CO) 16 C(MeHPO 4 ) NO HCO H N NH 4+ + HPO 4 + Me H 2 O [H + ]: C = 12: Manganese reduction CH 2 O(org) + 2 Mn(IV)O H + HCO H 2 O + 2 Mn(II) 2+ Iron reduction CH 2 O(org) + 4 Fe(III)OOH + 7 H + HCO H 2 O + 4 Fe(II) 2+ Sulfate reduction CH 2 O(org) SO 4 HCO H HS - [H + ]: C = -3:1 = -3 [H + ]: C = -7:1 =-7 [H + ]: C = 0.5:1 = 0.5 All these reactions produce HCO 3- but for 2 & 3 protons are consumed while for 1 & 4 they are produced. Hence the electron acceptor is essential for the change in ph.

16 Re-oxidation of the reduced chemical species Low oxygen concentrations (or anoxia) are normally only found in deep and bottom water. To impact the conditions for PP or gas exchange these waters have to be mixed back into the surface and the reduced chemical constituents are re-oxidized, bringing the ph change to the oxic conditions, if not precipitated or reacted to another oxidation state. Fe(II) 2+ + HS - H + + FeS(s) Oxidation of hydrogen sulfide to elemental sulfide HS O 2 + H + H 2 O + S 0 Also here the effect on ph is determined by the reaction.

17 Conclusions It is clear that the natural variability of ph is large in our waters Eutrofication result in a larger seasonal variability in ph Even if not obvious in all our waters it is without doubt that anthropogenic emissions of CO 2 decreases ocean ph This anthropogenic caused decrease in ph will result in that the natural variability will cover a range of lower ph A Swedish OA project would have to consider all relevant processes effecting ph when investigating the impact on the ecosystem