Lecture 16 Oxygen distributions and ocean ventilation

1. Lecture 16 Oxygen distributions and ocean ventilation Thermocline Ventilation and Deep Water Formation Oxygen Utilization rates

2. Aerobic respiration Oxygen is consumed and nutrients are released. (CH2O)106(NH3)16(H3PO4) + 138 O2 Algal Protoplasm  bacteria 106 CO2 + 16 HNO3 + H3PO4 + 122 H2O + trace elements The oxidation of the NH3 in organic matter to NO3 is referred to as nitrification

3. Apparent Oxygen Utilization (AOU) Apparent Oxygen Utilization or AOU. AOU is defined as: AOU = O2' - O2 where: O2' = value of O2 the water would have if it was in equilibrium with the atmosphere at the temperature and salinity of the water. This is called saturation. This implies that all waters are in equilibrium with the atmosphere (100% saturated) when they sink to become the deep ocean water. O2 is the dissolved oxygen actually measured in the same water sample.

4. Nutrients versus AOU

5. Oxidative and Preformed Nutrients versus Depth 1 mol O2 = 106/138 mol CO2 + 16/138 mol HNO3 + 1/138 mol H3PO4 consumed = 0.77 CO2 + 0.12 HNO3 + 0.0072 H3PO4 But vertical profiles are not the best way to study this problem.

6. The Ocean Conveyor What is it conveying? (at the surface? and at depth?)

7. Meridional Overturning Sinking and Remineralization Remineralization keeps the biological pump pumping!

8. Winter Outcrops of Isopycnal Surfaces

9. Surface density, isopycnal outcrops Waters will move mostly along surfaces of constant density.

10. DP = [PO4] - [PO4] = RPO4/O2 x AOU DN = [NO3] - [NO3] = RNO3/O2 x AOU on s = 27.0 to 27.2 Takahashi et al, 1985

11. Remineralization Ratios versus Depth average for 400m to 4000m P N C O2 1 : 16±1 : 117±14 : 170±10 Anderson and Sarmiento, 1994)

12. Compare with Hedges Model It is clear that more O2 (~170 moles) is actually required to respire sinking organic matter than was originally calculated from the RKR equation (138 moles). The RKR type organic matter has an oxidation state as for carbohydrate (CH2O). Real plankton have 65% protein, 19% lipid and 16% carbohydrate (from NMR studies) The higher O2 demand suggests that sinking organic matter has more of a lipid-like nature. Instead of: CH2O + O2 = CO2 + H2O More like: CH2 + 3/2 O2 = CO2 + H2O Real plankton biomass is more like C106H177O37N17S0.4 instead of C106H260O106N16 Complete oxidation requires 154 moles of O2 instead of 138

13. Atmospheric Record of Thermocline Ventilation Tracers Conservative, non-radioactive tracers (CFC-11, CFC-12, CFC-13, SF6) down 2% CFC-11 Propellent Styrofoam CFC-12 Air conditioning Refrigerators CFC-113 solvent SF6 Transformers Nike/Mercedes down 10% Tritium3H: t1/2 = 12.5 y 3H  3He + b as H20 (or HTO) Time series of northern hemisphere atmospheric concentrations and tritium in North Atlantic surface waters. When will CFCs not be a good clock?

14. Changing atmospheric gases Comparison of atmospheric history of tritium and 14C

15. Example: Deep horizontal flow: 14C

16. Tritium Contours (as of GEOSECS) A good tracer for thermocline ventilation ¼ of surface

17. Example: Thermocline ventilation: Tritium Tritium/Helium Age (yr) Why does Tritium concentration slightly different from Tritium/Helium Age? see Jenkins (1998) JGR, 103, 15,817

18. Example:Oxygen Utilization Rate calculated from AOU versus age Example for one density surface sθ = 26.80 Jenkins (1982), Nature, 300, 246

19. Winter Outcrops of Isopycnal Surfaces

20. OUR versus Depth OUR decreases exponentially with depth below the euphotic zone (Z in m) according to: ln OUR = -(0.68+0.17) - (0.00295+0.00027) Z Integrated OUR from 100m to depth  OUR = 5.7 mol O2 m-2 yr-1

21. Comparison with O2 Flux approach

22. OUR  New Production Convert the integrated O2 consumption to the POC flux required Use Takahashi et al (1985) stoichiometric ratio to convert C to O2 Integrated OUR x conversion = Integrated C oxidized 5.7 mol O2 m-2 y-1 x 106C/172O2 = 3.51 mol C m-2 y-1 For comparison in the last lecture we calculated the annual new production of C from the O2 mass balance in the euphotic zone. From that approach the new production is (using 106C/172O2) = 3.1 mol C m-2 y-1 Two independent estimates – remarkably close agreement!

23. Composite cruise track

24. Apparent Oxygen Utilization How and why do we define the quantity called AOU?

25. Oxygen is a tracer of both physical and biological changes Apparent Oxygen Utilization AOU = O2sat -O2 ΔO2 = ΔO2sat - ΔAOU (from Deutsch et al)

28. Atm. Mixed layer thermocline ventilation outcrop Air-sea O2 flux subtropics ΔAOU time Surface fingerprints: ventilation Atm. Mixed layer Decrease ventilation thermocline An increase in AOU due to decreased ventilation will cause changes in air-sea fluxes of both O2 and CO2 coincident with the ventilation change…

29. outcrop Surface fingerprints: export Atm. Atm. Mixed layer Mixed layer Increase export flux thermocline thermocline Export flux Similar AOU anomalies may be caused by increased export flux, with very different signatures of O2/CO2 gas exchange. subtropics Air-sea O2 flux ΔAOU time

30. Tritium3H: t1/2 = 12.5 y 3H  3He + b as H20 (or HTO) 3H = A conservative, radioactive tracer In rain in Ireland

31. Example: Thermocline ventilation: CFCs Locate and define the outcrop of this isopycnal (constant density) surface

32. Tritium is a conservative tracer for water (as HTO) – thermocline penetration Eq Meridional Section in the Pacific

33. Deep Ocean Respiration AOU OUR = age Oxygen Utilization Rate:

34. Surface O2 Saturation

35. Oxygen Distribution