1 Carbon Cycle 9 Carbon cycle is critically important to climate because it regulates the amount of CO 2 and CH 4 in the atmosphere. Carbon, like water, continuously cycles between various reservoirs. Fig (Harvey, fig 2.6) illustrates the major carbon reservoirs as a series of interconnected boxes. Numbers in the boxes is estimated pre-industrial amount of carbon in gigatonnes (Gt, or billions of tonnes). Numbers on the arrows give the estimated pre-industrial flux (Gt per year) between the boxes (in the direction of the arrow).

2 9 Different studies agree on the following points: 1.The amount of carbon in the land biota is roughly comparable to the amount of carbon in the atmosphere. 2.The amount of carbon in the soil and in the detritus is about twice the amount in either the atmosphere or above-ground land biota. 3.The amount of carbon in the ocean mixed layer, which interacts directly with the atmosphere, is comparable to the amount of carbon in the atmosphere itself. 4.The overwhelming majority of the total carbon in the biosphere + atmosphere + ocean system is in the deep ocean. Carbon Cycle

3 Atmosphere Respiration (consumers) Photosynthesis (primary producers) Decomposition (soil & detritus) oxidized biospheric carbon (from Riverine input) Flux in = (120 – R) Land + (100 + aR) Ocean = 220 – (1 – a) R [Gt (C)/yr] Flux out = (120) Land + (100) Ocean = 220 [Gt (C)/yr] From these numbers, we conclude that there was a net CO 2 flux out [(1 – a)R] of the pre-industrial atmosphere.

4 When CO2 enters sea water, the following chemical reactions take place: CO2 dissolves in water Carbonic acid dissociates into ions Bicarbonate dissociates Net Reaction Carbon thus occurs as several species in seawater: CO 2 gas, H 2 CO 3, HCO 3 -, and CO 3 2- Atmosphere – Ocean Exchange Processes The Ocean Surface Layer – Fig 2.6 & 2.7

5 Carbon Cycle Things to note: The transfer of CO 2 between Atmosphere and Ocean driven by the difference in CO 2 partial pressures (pCO 2 ). The concentration of CO 2 as gas (pCO 2 ) in seawater is very small. –~ 90% of the inorganic carbon is in the form of bicarbonate HCO 3 - –~ 10% is in the form of carbonate CO 3 2- –Less than 1% is in the form of CO 2 The oceans hold so little CO 2 because it is converted into other forms of carbon.

6 Photosynthesis uses CO 2 for soft tissue construction Skeleton construction creates calcium carbonate Biological PumpSome of the soft tissue and skeletal material (CaCO3) ends up in the deep ocean through sinking dead micro-organisms (flux B ~ 10 Gt (C) per year). Biological processes in surface layer:

7 Harvey, Figs 2.7 & 2.8 Dissolved Inorganic Carbon (DIC) DIC Dissolved Inorganic Carbon; Collective name for carbon in the form of (dissolved) CO 2, HCO 3 - and CO 3 2- –Note that an older (but still used) name for DIC is Total CO 2 =  CO 2 Concentration Profiles (DIC and TPO 4 ) –Photosynthesis (and hence biological growth) depletes surface waters of nutrients (DIC, phosphate, nitrate,…). Maximum nutrient concentrations around depth of 1 km, max DIC at somewhat greater depth. –The profiles take the form they do mainly because Nutrients removed from solution in surface waters by photosynthesis, Nutrients are returned to solution in deep water as organic matter is decomposed, and Atmospheric CO 2 dissolves more readily in cold waters at high latitudes, which sink to the deep sea-floor on account of their low temperature and increased density.

8 Processes that balance downward transfer of DIC DIC transferred upward by –Turbulent Diffusion Because DIC increases with depth, turbulent diffusion tends to transfer DIC upwards. –Convective (Thermohaline overturning) Causes upward flow of DIC because the water that sinks tends to have a lower DIC concentration than the water that rises (which has absorbed DIC at the bottom transported there by the biological pump). However, as warm surface waters flow poleward, they cool and absorb more CO2 from the atmosphere. As a result, the DIC concentration in sinking water increases to that in the water that upwells at lower latitude. Consequently, net vertical transfer of DIC due to advective overturning is rather small.

9 Carbon Pumps Three factors account for the excess DIC in the deep ocean: (1)The water in the deep ocean is colder and thus can hold more CO2 at equilibrium with the atmosphere, (2) the deep ocean contains remineralized CO2 from organic particles that sink from the surface ocean, and (3) the deep ocean contains CO2 derived from the dissolution of CaCO3 in particles that sink from the surface ocean. Oceanographers refer to these factors as the three carbon pumps, 1.the solubility pump, 2.the biological pump (also known as organic or soft tissue pump), and 3.the carbonate pump

10 Why Atmospheric CO 2 Concentration Is So Low Can now see that there are two reasons why the atmospheric CO 2 concentration is low and why most of the carbon is in the oceans: 1.Because of the chemistry of DIC, and the fact that most of oceanic carbon does not occur as dissolved CO 2. 2.Because DIC concentration is Low in the surface mixed layer, but high in the deep ocean. The atmosphere is in direct contact only with the mixed layer, and is largely isolated from the deep ocean.

11 Summing up The biological pump maintains low DIC concentration in the surface layer. –This is possible even though DIC is in much greater concentrations in the deep ocean, and the mixing processes within mixed layer are intense, –because mixing between mixed layer and deeper water is much slower. Low DIC in surface water plus the chemistry of DIC results in low pCO2 of surface water. Low pCO2 of surface water results in low atmospheric pCO2, –since atmospheric pCO2 tends to adjust to the pCO2 of the water with which it is in contact.

12 Check out this paper Distribution of Anthropogenic CO2 in the Oceans