Recall: Ice core evidence for changes in atmospheric CO2Atm. CO2 intro 2Digression: physical chemistry of CO2 in the ocean 1Digression: physical chemistry of CO2 in the ocean 2Variables, knowns:Gas solubilityThe Revelle FactorTypical values of carbonate speciesDigression: physical chemistry of CO2 in the ocean 3CO2 exchange between the ocean and atmosphereGlobal Carbon FluxesThe ocean CO2 “pumps” after Tyler Volk The solubility pump:The biological pumpThe solubility pumpA few concepts advanced to explain low glacial CO2Broecker (1982) sea level / shelf phosphorus interpretation of G/I pCO2 and oceanic δ13C evidenceBroecker’s 2-box interglacial ocean:Broecker’s 2-box glacial ocean:Broecker’s interglacial/glacial oceans:Effect of Broecker’s scheme on carbon isotopesComparison to data:Shackleton and Pisias (1984) planktonic-benthic δ13C recordToggweiler and Sarmiento (1984), Wenk and Siegenthaler et al. (1984), Knox and McElroy (1984) preformed phosphorus models for Toggweiler-Sarmiento (1984) 3-box model for interglacial / glacial CO2Construction of ocean chemical box models using matrices: the 3-box model as an exampleStep 1: Salinity 1Step 1: Salinity 2Step 2: phosphorusStep 3: alkalinityStep 4: carbon dioxideSwitch to 3-box model spreadsheetProblems with the preformed phosphorus hypothesisCoral-reef hypothesis (Berger and Keir, Opdyke)Boyle (1988) nutrient deepening hypothesisNutrient deepening illustrationProblem with the nutrient deepening hypothesis:Archer and Maier-Reimer “rain ratio / sedimentary dissolution hypothesisProblems with the “rain ratio” hypothesisTwo useful features of all the models that appeal to CaCO3 compensation:The nitrogen fixation / inventory hypothesis (McElroy, Broecker, others…)Problems with the nitrogen fixation / inventory hypothesisKeeling and Stephens sea-ice lid hypothesisSummary of the status of the glacial/interglacial CO2 problemReading (1)Reading (2)MIT OpenCourseWare http://ocw.mit.edu 12.740 Paleoceanography Spring 2008 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms.Atmospheric CO2and Ocean Chemistry12.740 Topic 11 Spring 2008Recall: Ice core evidence for changes in atmospheric CO2• Pre-anthropogenic pCO2was about 280 ppmV•Glacial pCO2was about 190 ppmV. Six Antarctic ice cores give same number.50100150200250300pCO2-500-450-400-350-300δD, ‰SMOW0 100000 200000 300000 400000Ice Age est. yrBPVostok δD, CO2 0-420 kyr (Petit et al., 1999)Atm. CO2 intro 2• In the early 1980’s, it seemed that the Dye 3 Greenland ice core showed relatively large, rapid (few hundred year) fluctuations in CO2during glacial stadial/interstadial fluctuations. These were not observed in the high resolution Byrd Antarctic ice core, and it is now thought that these apparent high CO2events were artifacts due to melt layers or interactions with dust.• So why did atmospheric CO2change with the glacial/interglacial cycles? Is the CO2change a chicken or an egg in the progression of climate change?• Many ideas have been proposed; almost an equal number have beendisposed of (or is it deposed?). Speaking informally, it is as if theories on glacial/interglacial carbon dioxide are radioactive with ~2 year half-lives.• Despite this situation, there is much to be learned about CO2in the ocean from those ideas, so a historical examination of them is still worthwhile.Digression: physical chemistry of CO2in the ocean 1A. Two useful conservative quantities (properties that mix linearly) are:ΣCO2= [CO2(aq)] + [HCO3-] + [CO3=]Alkalinity = [HCO3-] + 2[CO3=]+ [B(OH)4-] + [OH-] - [H+]+ (etc...)"Alkalinity" is a re-arranged charge balance equation which divides ions into those that have acid-base reactions and those that don't:e.g., in a system consisting of a solution of NaCl, MgSO4, NaCO3, and NaHCO3:[Na+] + 2[ M g++] -2[SO4=] = [HC O3-] + 2[C O3=] + [OH-]- [ H+]= AlkalinityAdding or removing CO2from a water sample does not change the alkalinity (convince yourself of this!)Digression: physical chemistry of CO2in the ocean 2D. Relevant equilibria ("apparent" thermodynamic constants). Because seawater is a “high ionic strength”solution (i.e., it is very salty), conventional thermodynamic constants (activity coefficients, etc.) are difficult to define because of strong ionic interactions. But because the composition of seawater (for major ions) is constant save for variations in total salt content, we can adopt “apparent constants” which incorporate the activity coefficients and ion interactions into the “apparent” constant (denoted by ‘):[C O2(aq)]i. CO2(gas) = CO2(aq) αs' =___________ ≈ 10-1.5pCO2[H+][H C O3-]ii. CO2(aq) + H2O = H++ H C O3-K1' =______________≈ 10-6.0[C O2(aq)][H+][C O3=]iii. HCO3-= H++ C O3=K2' = _____________≈ 10-9.1[HCO3-]iv. B(OH)3+ H2O = B( O H)4-+ H+[H+] [B(O H)4-] KB' = ______________________ ≈ 10-8.7[B(O H )3]These apparent constants are functions of temperature, salinity, and pressure.Variables, knowns:Variables:pCO2[CO2(aq)][HCO3-][CO3=][B(OH)3][B(OH)4-][H+]TS___________9 variablesKnowns:4 equationsTSΣB / S2 specified parameters(e.g. Alk, ΣCO2)_______________9 equations or specifiedGas solubilityGas solubilityCCl4CO2CFC-13N2ORnCFC-12100,00080,00060,00040,00020,000Solubility (mmol m-3 atm-1)00 5 10 15 20 25 30KrArCH4O2SF6N2NeHeSolubility (mmol m-3 atm-1)00 5 10 15 20 25 304,0003,0002,0001,000Plot of the solubility of various gases as a function of temperature. The solubility of all gases decreases with increasing temperature, but there exist large differences in the solubility of the different gases for a particular temperature. These differences can be understoodin terms of their molecular weight (ideal gas) or other factors, such as molecular interactions between the gas and the water (nonideal gas).Note the change in vertical scale between panels (a) and (b). Based on the empirical functions listed in table 3.2.2.Temperature (oC)Temperature (oC)(a) (b)Figure by MIT OpenCourseWare based on Sarmiento and Gruber (2006).The Revelle FactorTypical values of carbonate speciesFor example, for values of Alk and ΣCO2typical of pre-anthropogenic warm surface waters (Alk=2275, ΣCO2=1900; pH=8.3; pCO2=283) and cold deep waters (Alk=2375, ΣCO2=2260; pH=7.9):units: (µmol/kg)[CO2(aq)] [HCO3-][CO3=] Alk-ΣCO2[B(OH)3] [B(OH)4-]ws 8 1628 264 375 288 119cd 27 2150 85 115 349 58(ws = warm surface cd = cold deep)In deep waters one can