1Chapter 9: Respiration - Chemical Tracers James W. Murray (4/19/01) Univ. Washington Respiration corresponds to the RKR reaction run backwards. This means that the dissolved oxygen concentration is a tracer for respiration. In the euphotic zone there is usually excess oxygen above atmospheric saturation because there is net biological production. Production of O2 by primary production is greater than consumption of O2 by respiration. Below the euphotic zone there is only consumption by respiration so the oxygen goes down as respiration proceeds. In some parts of the open ocean with restricted circulation all of the dissolved oxygen is consumed by respiration. Examples of these locations are the oxygen minimum zones in the eastern tropical north and south Pacific and Arabian Sea. At these locations respiration proceeds using nitrate to combust the organic matter. In some enclosed basins like the Black Sea, Cariaco Trench, Saanich Inlet and Framvaren Fjord respiration also uses up all the nitrate and sulfate is reduced to hydrogen sulfide (H2S) 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. Apparent Oxygen Utilization We can calculate the extent of respiration by calculating a parameter called the 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. The distribution of AOU throughout the ocean at 4000m is shown in the attached Figure 9-1 (from Broecker and Peng). The lowest values (50 µmol kg-1) are in the North Atlantic. The highest values (>190 µmol kg-1) are in the oldest water in the North Pacific. North-south sections of AOU in the western Atlantic and western Pacific Oceans are shown in Figures 9-2a,b. Some key intermediate and deep density surfaces (σo = 27.0, 27.5 and σ4 = 45.2) are indicated.2Fig 9-1 Distribution of AOU at 4000m in the world's oceans. Lowest values are in the Atlantic and the largest (>190 µmol kg-1) are in the Pacific. (from Broecker and Peng, 1982). Fig 9-2 a,b Sections of AOU through the Atlantic and Pacific3Regenerated Nutrients Once you've calculated the AOU in a water sample you can calculate the CO2, HNO3 and H3PO4 released by respiration. 1 mol O2 consumed = 106/138 mol CO2 + 16/138 mol HNO3 + 1/138 mol H3PO4 = 0.77 CO2 + 0.12 HNO3 + 0.0072 H3PO4 Preformed Nutrients Preformed nutrients are those initially present in seawater at the time of downwelling. Hence, preformed nutrient = total nutrient - regenerated nutrient. An example from Park (1967) of the relationships between AOU:PO4:NO3:Si is shown in the Fig. 9-3. The location of these profiles is at 44°N; 127°W (off the coast of Oregon). The nutrients of oxidative origin (Pox) were calculated from AOU and subtracted from total nutrients to get the preformed nutrient concentrations. Preformed nutrients are characteristic of waters originating in different regions and hence can be used as water mass tracers (e.g. Broecker, 1974; Broecker et al, 1985). Broecker (1974) originally proposed that O2 and NO3 and O2 and PO4 data can be combined in such a way that the alteration by respiration is cancelled. From the original RKR respiration equation given above, roughly 1/9 mole of N is released as NO3 and 1/135 mole of P is released as PO4 for each mole of O2 consumed. Thus the sum of 9NO3 + O2 was defined as "NO" and 135PO4 + O2 as "PO" . Both NO and PO should be nearly conservative tracers. An example of NO3, O2 and "NO" versus salinity for a single station in the western basin of the South Atlantic is shown in Fig. 9-4. You can see that O2 and NO3 show curvature with salinity but "NO" varies linearly, or conservatively, between two end members that represent the core of the AAIW (at 1150m) and the top of the NADW (at 2270m). Revised Stoichiometric Ratios Isopycnal surfaces are surfaces of constant density. Potential temperature and salinity are perfectly correlated on isopycnal surfaces. In the ocean most mixing and transport occur on isopycnal surfaces. The winter surface outcrops of various isopycnal surfaces in the North Atlantic Ocean are shown in Fig. 9-5. The densest waters that outcrop at the surface have a density of σo = 27.6. The north-south distribution of density surfaces in the 0 - 1500m depth range of the western side of the Atlantic, Indian and Pacific Oceans are shown in Fig. 9-6. The density surfaces shoal and outcrop at high latitude, then sink to maximum depths at mid-latitudes. They tend to shoal again near the equator where there is upwelling. To a first approximation the distributions of tritium (3H), which is a tracer for water, which has an atmospheric source, follow these density surfaces. Takahashi et al (1985) first argued that the correct approach for determining stoichiometric regeneration ratios was to utilize data along isopycnal surfaces. The change in NO3 and PO4 concentrations along isopycnal surfaces away from the surface outcrop can be estimated from:4567∆P = [PO4] - [PO4]° = RPO4/O2 x AOU ∆N = [NO3] - [NO3]° = RNO3/O2 x AOU They found that the stoichiometric ratios (on σo = 27.0-27.20) varied for different locations in the Atlantic and Indian oceans (Table 9-1). They argued that the widely used RKR values of P:N:C:-O2 of 1:16:106:138 should be replaced by 1:16:122+22:172 for the Atlantic and Indian Ocean. This approach was improved upon by Anderson and Sarmiento (1994) who calculated the stoichiometric ratios on 20 neutral surfaces in the South Atlantic, Indian and Pacific Basins between 400 to 4000m. Neutral surfaces are almost the same as isopycnal surfaces but are considered to be a more precise estimate of the horizons upon which water masses preferentially move and mix (McDougall, 1987). The -O2/P, Corg/P and N/P utilization ratios are shown in Fig. 9-7a,b,c. The P:N:C:-O2 ratios of remineralization below 400m are estimated with uncertainties as