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. (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) + 138 O 2 Algal Protoplasm  bacteria 106 CO 2 + 16 HNO 3 + H 3 PO 4 + 122 H 2 O + trace elements The oxidation of the NH 3 in organic matter to NO 3 is referred to as nitrification

3. Apparent Oxygen Utilization (AOU) Apparent Oxygen Utilization or AOU. AOU is defined as: AOU = O 2 ' - O 2 where: O 2 ' = value of O 2 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. O 2 is the dissolved oxygen actually measured in the same water sample.

4. Nutrients versus AOU

5. Oxidative and Preformed Nutrients versus Depth 1 mol O 2 = 106/138 mol CO 2 + 16/138 mol HNO 3 + 1/138 mol H 3 PO 4 consumed = 0.77 CO 2 + 0.12 HNO 3 + 0.0072 H 3 PO 4 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. Waters will move mostly along surfaces of constant density. Surface density, isopycnal outcrops

10.  P = [PO 4 ] - [PO 4 ]  = R PO4/O2 x AOU  N = [NO 3 ] - [NO 3 ]  = R NO3/O2 x AOU on   = 27.0 to 27.2 Takahashi et al, 1985

11. Remineralization Ratios versus Depth Anderson and Sarmiento, 1994) average for 400m to 4000m P N C O 2 1 : 16±1 : 117±14 : 170±10

12. It is clear that more O 2 (~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 (CH 2 O). Real plankton have 65% protein, 19% lipid and 16% carbohydrate (from NMR studies) The higher O 2 demand suggests that sinking organic matter has more of a lipid-like nature. Instead of: CH 2 O + O 2 = CO 2 + H 2 O More like: CH 2 + 3/2 O 2 = CO 2 + H 2 O Real plankton biomass is more like C 106 H 177 O 37 N 17 S 0.4 instead of C 106 H 260 O 106 N 16 Complete oxidation requires 154 moles of O 2 instead of 138 Compare with Hedges Model

13. Time series of northern hemisphere atmospheric concentrations and tritium in North Atlantic surface waters. When will CFCs not be a good clock? Atmospheric Record of Thermocline Ventilation Tracers Conservative, non-radioactive tracers (CFC-11, CFC-12, CFC-13, SF6) CFC-11 Propellent Styrofoam CFC-12 Air conditioning Refrigerators CFC-113 solvent SF6 Transformers Nike/Mercedes down 10% down 2% Tritium 3 H: t 1/2 = 12.5 y 3 H  3 He +  as H 2 0 (or HTO)

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

15. Example: Deep horizontal flow: 14 C

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

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

18. Example:Oxygen Utilization Rate calculated from AOU versus age Example for one density surface s θ = 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  OUR = 5.7 mol O 2 m -2 yr -1 Integrated OUR from 100m to depth

21. Comparison with O 2 Flux approach

22. OUR  New Production Convert the integrated O 2 consumption to the POC flux required Use Takahashi et al (1985) stoichiometric ratio to convert C to O 2 Integrated OUR x conversion = Integrated C oxidized 5.7 mol O 2 m -2 y -1 x 106C/172O 2 = 3.51 mol C m -2 y -1 For comparison in the last lecture we calculated the annual new production of C from the O 2 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 = O 2 sat -O 2 ΔO 2 = ΔO 2 sat - ΔAOU (from Deutsch et al)

28. Surface fingerprints: ventilation thermocline Mixed layer Atm. thermocline Mixed layer Atm. Decrease ventilation Air-sea O 2 flux ΔAOU subtropics time outcrop An increase in AOU due to decreased ventilation will cause changes in air-sea fluxes of both O 2 and CO 2 coincident with the ventilation change…

29. Surface fingerprints: export thermocline Mixed layer Atm. thermocline Mixed layer Atm. Increase export flux Export flux Air-sea O 2 flux ΔAOU outcrop time subtropics Similar AOU anomalies may be caused by increased export flux, with very different signatures of O 2 /CO 2 gas exchange.

30. Tritium 3 H:  1/2 = 12.5 y 3 H  3 He +  as H 2 0 (or HTO) 3 H = A conservative, radioactive tracer In rain in Ireland

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

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

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

34. Surface O 2 Saturation

35. Oxygen Distribution