Global Carbon Cycle - II

Transcription

1 Global Carbon Cycle - II OCN Biogeochemical Systems Reading: Schlesinger, Chapter Current status of global CO 2 2. CO 2 speciation in water 3. Atmosphere ocean interactions 4. Ocean acidification Outline 5. Saturation state and calcium carbonate dissolution 1

2 Current Status of Global CO 2 CO 2 input to atmosphere is primarily through oxidation of fossil organic matter (i.e., oil, coal and natural gas) and cement production We are currently adding considerably to the CO 2 inventory of the atmosphere, though it is not accumulating as fast as would be expected given the known sources / sinks There is 53x as much CO 2 in the ocean as in the atmosphere 2

3 CO 2 Speciation in Water CO 2 (g) has many possible transformations upon dissolution in water Major dissolved forms: CO 2(aq) (aqueous carbon dioxide a dissolved gas) H 2 CO 3 (carbonic acid trace amount) HCO - 3 (bicarbonate ion) CO 3 (carbonate ion) Species interconvert readily Perturbations to one part of CO 2 system leads to redistribution of species Reactions not always intuitive! What s exactly is going on chemically with the CO 2?? Ocean Acidification: Starting with the Science National Research Council 3

4 Equations for CO 2 Speciation Equilibrium of gaseous and aqueous CO 2 : CO 2(g) CO 2(aq) Reactions in water: CO 2(aq) + H 2 O H 2 CO 3 HCO 3- + H + HCO 3- CO 3 + H + Hint: when seawater ph is between 7.5 and 8.5: CO 2(aq) + H 2 O + CO 3 2HCO 3 - CO 2 Speciation vs. ph CO 2(aq) Seawater values shown --- freshwater curves are shifted left CO 2(g) CO 2(aq) CO 2(aq) + H 2 O HCO H + HCO 3 - CO 3 + H + aob.oxfordjournals.org 4

5 Dissolved Inorganic Carbon (DIC, Total CO 2, ΣCO 2 ) DIC [CO 2(aq) ] + [H 2 CO 3 ]+ [HCO 3- ]+ [CO 3 ] At seawater ph, >99% of CO 2 species are HCO 3- and CO 3, so we can simplify: DIC [HCO 3- ]+ [CO 3 ] Partial Pressure of CO 2 (P (P CO2 ) In the atmosphere: P CO2 partial pressure of CO 2 in the atm, out of a total pressure of 1 atm (at sea level) Measurement: Determine fraction of atm that is CO 2 P CO2 = P total fraction CO2 5

6 In water: P CO2 atmospheric partial pressure of CO 2 when in equilibrium with the water (Henry s Law: P CO2 = K [CO 2 ] water Measurement: Equilibrate water with a small gas headspace Determine fraction of headspace that is CO 2 P CO2 =P total fraction CO2 P CO2 provides a common unit for comparing CO 2 concs in water and the atmosphere, which allows a quick evaluation of gas fluxes CO 2 (air) CO 2 (water) P CO2(air) > P CO2(water) Flux of CO 2 from air to water P CO2(air) = P CO2(water) No net flux of CO 2 P CO2(air) < P CO2(water) Flux of CO 2 from water to air 6

7 CO 2 System Calculations There are four CO 2 properties that can be measured: DIC, P CO2, ph, and carbonate alkalinity Any two of these properties can be used to determine the composition of the CO 2 system in water (i.e., concs of CO 2(aq), HCO 3 -, and CO 3 ) Traditionally, ph and alkalinity were measured Now: also use ΣCO 2 and P CO2 to better constrain calculations, improving accuracy CaCO 3 Precipitation/Dissolution A tricky subject when discussing CO 2 (or, more properly, P CO2 ) Ca 2+ + CO 3 2- CaCO 3 Does this reduce the CO 2 (P CO2 ) level of the seawater? No! Lost CO 2-3 will be replaced: HCO 3- CO H + But this H + release causes: HCO H + CO 2(aq) + H 2 O CO 2(g) CO 2(aq) CO 2(aq) + H 2 O HCO 3- + H + HCO 3- CO 3 + H + Thus, CaCO 3 precipitation causes a decrease in Total CO 2, but an increase in P CO2(aq) Hint: when ph is between 7.5 and 8.5: CO 2(aq) + CO 3 + H 2 O 2HCO 3-7

8 Mauna Loa Atmospheric CO 2 Record ppmv ppmv ppmv upload.wikimedia.org Ocean CO 2 Increase Tracks Atmosphere Ocean Acidification Pre- industrial (1700s) ph 8

9 In More Detail... Ocean Acidification: Starting with the Science National Research Council Ocean ph Byrne et al. 2010, Geophys. Res. Lett. 37 9

10 Change in seawater ph between 1991 and 2006 Change in seawater ph attributed to natural interdecadal/interannual DIC variability between 1991 and 2006 (due mostly to slowdown in downwelling, with more respiration at given depths) Change in seawater ph attributed to the uptake of anthropogenic carbon between 1991 and 2006 Global Oceanic CO 2 Flux High values at equator (esp. in the Pacific) and along west coasts are from upwelling and subsequent gas evasion to atmosphere Low values where there is high bioproductivity Sabine et al Low values where cooling of ocean increases solubility of gas and causes gas infusion to surface ocean 10

11 Ocean Water-column Anthropogenic Carbon (C( ant ) Sabine & Tanhua Ann. Rev. Mar. Sci. What s exactly is going on chemically with the CaCO 3?? Ocean Acidification: Starting with the Science National Research Council 11

12 Saturation State -- Carbonate Dissolution The saturation state of SW with respect to a calcium carbonate mineral (calcite, aragonite, magnesian calcite) determines whether the phase will dissolve or not E.g., the saturation state of SW with respect to calcite (CaCO 3 ) is: 2+ 2 [Ca ] [CO3 ] Seawater Ωcalcite 2+ 2 [Ca ][CO ] Calcite saturated seawater Depth at which Ω = 1 is called the saturation horizon 3 Oversaturated Ω > 1 Precipitation of mineral Undersaturated Ω < 1 Dissolution of mineral Saturation State Varies with Depth 12

13 Ω > 1 Ω = 1 Ω < 1 Animation of the saturation state of surface water with respect to aragonite. Ω > 1 Ω = 1 Ω < 1 The magenta line, which first becomes visible in 2025 (in the Weddell Sea), separates saturated waters (orange colors) from undersaturated waters (blue colors). By the end of this century, undersaturation spreads throughout the entire Southern Ocean (all ocean south of 60 S) and into a portion of the subarctic Pacific. (Orr et al Nature) 13

14 Another Model... Ω aragonite Feely et al. 2009, Oceanography 22:36-47 Calcification response patterns for 18 species of calcifying organisms subjected for 60 days to CO 2 -induced reductions in CaCO 3 saturation state of seawater. Net rates of calcification(+)/ dissolution( ) are expressed as a percentage of the organisms initial weight. Ries et al Geology 14

15 Possible Effects of Ocean Acidification Ocean Acidification: Starting with the Science National Research Council 15