1Global Biogeochemical CyclesGEOS 489Guest LectureBiogeochemical Cycles in the OceansDr. Shari Yvon-Lewis(Dept. of Oceanography)Rm 412 O&M Building979-458-1816; email: [email protected] in SeawaterDissolved Inorganic Carbon (DIC, TCO2)CO2(gas) + H2O H2CO3H++ HCO3-2H++ CO32-Most DIC is present as bicarbonate (HCO3-), some is present as carbonate (CO3-2), and a small fraction is present as dissolved CO2gas (pCO2(w)) and carbonic acid (H2CO3). At typical surface water conditions, ~ 0.5% pCO2(w)+ 89.1% HCO3-+ 10.4% CO3-2The equilibrium constants Ki, for each of these reactions change slightly with changes in temperature, salinity, and alkalinity.K1= [H2CO3] / [H+][HCO3-]Phytoplankton only utilize pCO2(w)for photosynthesis, and only the pCO2(w)influences the air-sea exchange of CO2.3Because CO2gas combines chemically with water, the oceans have a great capacity to absorb CO2, much more so than for other gases such as Oxygen or Nitrogen.Atmospheric Gases by volume:Nitrogen (N2) - 78%Oxygen (O2) - 21%Argon (Ar) - 1%Carbon Dioxide (CO2) - 0.03%Dissolved Gases in the Oceans by volume:Nitrogen (48%)Oxygen (36%)Carbon Dioxide (15%)4At typical seawater pH values, a tiny fraction of the carbon is present as carbonic acid. The bicarbonate and carbonate ions act to buffer the pH of the oceans.5pCO2is the partial pressure of CO2gas in air or seawater.pCO2= [CO2] / H,where [CO2] is the concentration in moles/kg, andH is the solubility of CO2.The net air-sea flux of CO2is a function of the difference in partial pressure (pCO2(w)- pCO2(a)).6The pCO2and solubility of CO2gas in seawater are a function of temperature, TCO2, salinity, and alkalinity.•Temperature and Total CO2are strongest influences. −pCO2doubles with 16 degrees C temperature increase,−strong biological depletion of TCO2reduces pCO2by a similar magnitude•Solubility also influenced by alkalinity and salinity.−Reducing alkalinity increases pCO2, decreases solubility−Reducing salinity decreases pCO2, increases solubility7Alkalinity - is expressed charge equivalent terms, it is a measure of the number of H+ions required to neutralize the solution.Alk ~ [HCO3-] +2 [CO32-] + B(OH)4-Alk ~ [HCO3-] +2 [CO32-]Alkalinity can be measured by adding H+ to seawater and measuring the CO2gas emitted from,HCO3-+ H+-> H2O + CO2(gas)CO32- + 2H+-> H2O + CO2(gas)8Air-Sea Gas ExchangeThe flux across the gas-water interface occurs as molecular diffusion. This diffusion is always occurring even when the net flux is = 0.0 (i.e. flux in = flux out). The net flux will be from higher concentration (water or air) tolower concentration (water or air). Equilibration - is when the gas concentration in the atmosphere becomes equal to the concentration in the water, then the net air-sea flux will be = 0.0.9Air-Sea Gas ExchangeEquilibration Time - is the time it takes to reach equilibration given a transfer velocity and initial gas concentrations in the air and water.The timescale of equilibration for most gases (O2, N2) is a typically a few weeks.The timescale for CO2is much longer (~ 1 year) because CO2reacts chemically with the seawater.10(from Najjar, 1993)Flux = D / ∆Z (Cw - Csat),where D is the molecular diffusivity of the gas in seawater.11Flux of Gas Across the Air-Sea InterfaceFlux = dC/dt = D (dC/dz)Flux = (D/Hz) [PG(air) - PG(soln)]Flux = k [PG(air) - PG(soln)] = k ∆PGk is gas exchange coefficientz is laminar layer thicknessH is Henry’s Law constantD is diffusion coefficientChemical Oceanography, Millero12Air-Sea Gas ExchangeThe gas exchange coefficient, K, is often estimated as a function the Schmidt number and wind speed at a reference height (typically 10m).K = Sc-n* f(V)Schmidt number (Sc) =  / Dwhere  is the viscosity of the seawater,  is seawater density, and D is the molecular diffusivity of the gas. n ~ 0.67 at low wind speeds, ~ 0.5 at high wind speeds.Kgas= (Scgas/Scref)-n* f(V)Flux = K ([X(aq)] – [X(g)]) where [X(g)] = pX/H13Flux = KW([CO2(aq)] – [CO2(g)])http://www.pmel.noaa.gov/pubs/outstand/feel2331/feel2331.shtml14Air-Sea Gas ExchangeIncreasing wave height at the surface ocean increases the surface area over which air-sea exchange can occur.Breaking waves generate sea spray and bubble injection, both of which help to dramatically increase the air-sea flux of dissolved gases.Traditionally wind speeds are used in estimating the air-sea gas flux as a proxy for turbulence / surface roughness.Recently, satellite algorithms have been developed to measure surface roughness directly using radar backscatter to estimate surface roughness due to small waves.These remote sensing algorithms may improve our estimates of air-sea gas exchange in the near future.15Causes of Changes in pCO2(aq)• Removal– Photosynthesis– Precipitation of CaCO3– Solar Heating– Upwelling (air/sea exchange)• Addition– Oxidation of Organics– Dissolution of CaCO3– Input from Fossil Fuels (air/sea exchange)Chemical Oceanography, Millero16A tla n tic (2 5oW )L A T IT U D E-4 0 -2 0 0 20 40 60pCO2 (µµµµatm)28 030 032 034 036 038 040 0A irA irPacific OceanLatitude-80 -60 -40 -20 0 20 40pCO2 (µµµµatm)300320340360380400420440460480500Surface Ocean pCO2• ~1 yr to equilibrate CO2(aq)with CO2(air), so it is not unusual to observe large ∆pCO2. • Most differences caused by variability in the oceans due to biology and ocean circulation• Elevated pCO2(aq)in equatorial region related to upwelling• Cold polar regions act as sinksChemical Oceanography, Millero17(from Takahashi et al., 1999)Changes due to the Oxidation of Plant Material∆∆∆∆AOU (mM)0.00 0.04 0.08 0.12 0.16 0.20 0.24 0.28% Change-50050100150200250TCO2CApHpCO2(carbonate alkalinity)Chemical Oceanography, Millero19What controls carbon export?i.e. efficiency of biological pumpPrimary Production  Biological pump and the ocean C sink-an inverted pyramid<5 to >15% (decades)~1% (centuries)~0.1% (millennium)From Ken Buessler (http://cafethorium.whoi.edu/Fe/Why-Iron.html)20pCO2in SeawaterpCO2 (µµµµatm)200 400 600 800 1000 1200DEPTH (m)010002000300040005000North AtlanticNorth Pacific• Surface seasonality – blooms drawn down pCO2(aq)(the plant material must be exported to deep waters and oxidized there to be removed from the atmosphere for 100s of years.• 1000m maximum – Pacific has higher surface productivity and more sinking organics