Oxygen in the Ocean http://eps.mcgill.ca/~courses/c542/ 1/28
2/28 Oxygen in the Oceans 1000 O2 (µm) O 2 (µm) 0 50 100 150 200 250 300 0 OMZ oxygen minimum zone photosynthesis respiration DEPTH (m) 2000 3000 North Pacific North Atlantic respiration and mixing 4000 5000
Dissolved oxygen determinations 1. Equilibrating or stripping with an inert gas and measure by G.C., M.S. or I.R. To gas chromatograph, Mass spectrometer or I.R. spectrometer Sample introduction N 2 septum Signal O 2 CO 2 frit He Elution time 3/28
Dissolved oxygen determinations 2. Diffusion of oxygen through a Teflon membrane and polarographic determination. Oxygen Seabird SBE-43 O 2 probe Dissolved oxygen is reduced to hydroxide at the cathode, while the silver anode is oxidised: O 2 + 2H 2 O + 4ē 4OH - 4Ag o + 4Cl - 4AgCl + 4ē and the resulting current is proportional to the concentration of oxygen. 4/28
5/28 Dissolved oxygen determinations 3. Quenching of a fluorescent dye trapped within a sensing foil (optode).
6/28 4. Direct measurement in solution (O 2 by Winkler Method). Mn 2+ Dissolved oxygen determinations + 2 OH - Mn(OH) 2 (Winkler reagent) Mn(OH) 2 + 0.5 O 2 MnO(OH) 2 2 Mn(OH) 2 + 0.5 O 2 2 Mn(OH) 3 MnO(OH) 2 + 4H + + 3I - Mn 2+ + I 3 - + 3H 2 O 2Mn(OH) 3 + 6H + + 3I - 2Mn 2+ + I 3 - + 6H 2 O I 3 - + 2S 2 O 3 2-3 I- + S 4 O 6 2- The thiosulfate titrant is standardized daily with KIO 3 : IO 3- + 6H + + 8I - 3I 3 - + 3H 2 O I 3 - + 2S 2 O 3 2-3 I- + S 4 O 6 2-
7/28 Dissolved oxygen determinations NS 2 O 3 2- = NIO3 - VIO 3 - /(VS2 O 3 2- - Vblank) Vblank = volume of titrant needed to titrate the reagent blank (KI + NaOH, H 2 SO 4 ) [O 2 ] = (VS 2 O 3 2- - Vblank) NS 2 O 3 2- - Vreagent [O 2 ]reagent (Vbottle Vreagent) [O 2 ]reagent = oxygen concentration in the combined reagents
Dissolved oxygen determinations 8/28
Dissolved oxygen determinations Reagent blank Probe calibration Dissolved oxygen (microm) 66.0 65.0 64.0 63.0 62.0 y = 0.3886x + 59.941 R² = 0.9859 6 8 10 12 14 Titrant volume (ml) Probe DO (microm) 400.0 350.0 300.0 250.0 200.0 150.0 100.0 y = 0.927x - 0.9407 R² = 0.9994 50.0 n = 47 0.0 0.0 100.0 200.0 300.0 400.0 Measured/Winkler DO (microm) 9/28
Oxygen solubility = f (temperature & salinity) ln C = -135.29996 + 1.572288 x 10 5 /T 6.637149 x 10 7 /T 2 + 1.243678 x 10 10 /T 3 8.621061 x 10 11 /T 4 S p (0.020573 12.142/T + 2363.1/T 2 ) From: Benson and Krause (1984) Limnol. Oceanogr. 29: 620-632 10/28
Oxygen in the Oceans 1000 O2 (µm) O 2 (µm) 0 50 100 150 200 250 300 0 OMZ oxygen minimum zone photosynthesis respiration DEPTH (m) 2000 3000 North Pacific North Atlantic respiration and mixing 4000 5000 11/28
Oxygen in surface waters 12/28
Oxygen supersaturation in the surface ocean TEMPERATURE ( o C) 20 21 22 23 24 25 26 0 20 40 O 2 (µm) 210 220 230 240 O 2 SATURATION 100 105 110 115 0 20 40 DEPTH (m) 60 80 100 60 80 100 120 140 120 140 13/28
Oxygen in surface waters Oxygen saturation (%) = 100 [G]/[G ] = 100 [G]/(P G /K G ) 14/28
Bubble injection 1. N 2 time N 2 O 2 Due to Greater Solubility of O 2 2. Diffusion Coefficients are Approximately Double in Bubbles 3. Bubbles are Pushed to Depths of 50 m 4. Air Injection- the total dissolution of the Air in a Bubble due to Hydrostatic Pressure Gas N 2 O 2 Ar CO 2 Ne He Kr δ Air % +7.7 +3.8 +3.5 +0.1 +11.6 +13.8 +1.8 From the total dissolution of a bubble (1cm 3 ) of air at STP (15 o C and S =35) δ Air (%) = (([G] meas /[G] equil )-1) 100 15/28
Gas supersaturation in the Ocean 16/28
Contribution of productivity/photosynthesis to oxygen supersaturation in the surface ocean Inert gases correct biological oxygen production for physical processes Ar has very similar physical properties to O 2 O 2 :Ar ratios are a qualitative measure of biological oxygen production Need to account for physical processes using inert gases 17/28
Oxygen distribution in the oceans 18/28
Effect of upwelling on surface ocean oxygen concentrations 10 [O 2 ] MEAS - [O 2 ] CALC (µmol kg -1 ) 5 0-5 -10-15 Surface Waters 5 o S 0 5 o N LATITUDE 19/28
Apparent Oxygen Utilization C 106 H 263 O 110 N 16 P + 138 O 2 106 CO 2 + 16 NO 3- + HPO 4 2- + 122 H 2 O + 18 H + + (trace elements) AOU = amount of dissolved oxygen used for respiration = [O 2 ] sat n [O 2 ] meas where [O 2 ] sat n = PO 2 (atm)/ko 2, with KO 2 being a function of T and S 20/28
Effect of upwelling on surfaceocean oxygen concentrations 10 [O 2 ] MEAS - [O 2 ] CALC (µmol kg -1 ) 5 0-5 -10-15 Surface Waters 5 o S 0 5 o N LATITUDE 21/28
Apparent Oxygen Utilization and water age 22/28
Dissolved oxygen and Apparent Oxygen Utilization 23/28
Apparent Oxygen Utilization AOU = measure of the amount of dissolved oxygen used for respiration = [O 2 ] sat n [O 2 ] meas C 106 H 263 O 110 N 16 P + 138 O 2 106 CO 2 + 16 NO 3- + HPO 4 2- + 122 H 2 O + 18 H + + (trace elements) [NO 3- ] o.o. = AOU * 16/138; [SRP] o.o. = AOU/138 Redfield called the nitrate and phosphate produced in this way as nutrients of oxidative origin, as opposed to the preformed nutrients present in the body of water before it left the surface. [NO 3- ] meas = [NO 3- ] preformed + [NO 3- ] o.o. = [NO 3- ] preformed + AOU*16/138 [SRP] meas = [SRP] preformed + [SRP] o.o. = [SRP] preformed + AOU/138 24/28
Preformed and oxidative nitrate [NO 3- ] meas = [NO 3- ] preformed + [NO 3- ] o.o. = [NO 3- ] preformed + AOU*16/138 25/28
Organic matter respiration C 106 H 263 O 110 N 16 P + 138 O 2 106 CO 2 + 16 NO 3- + HPO 4 2- + 122 H 2 O + 18 H + + (trace elements) Hence, when phytodetritus is oxidized, 1.3 moles of oxygen are required to remineralize an amount of organic material containing one mole of carbon. Whereas this ratio applies to the average organic matter formed in surface water, it does not necessarily apply to the organic matter falling to the deep sea. This material is partially decomposed, some is encapsulated in feacal pellets, and it does not necessarily have the same composition as the plant material synthesized in the euphotic zone. Furthermore, in some regions of the deep sea, where oxygen becomes severely depleted, organisms use nitrate as an oxidant instead of O 2. 26/28
From: Broecker and Peng (1982) Tracers in the Sea, Eldigio Press 27/28
Stoichiometry of organic matter respiration (4) ΔCaCO 3 = 0.5 (ΔAlk + ΔNO 3- ) = contribution of ΣCO 2 from CaCO 3 dissolution ΔOrg = ΔΣCO 2 - ΔCaCO 3 = change in ΣCO 2 from organic matter oxidation 28/28