Nutrients and Ocean Chemistry

Take away ideas and understandings

I. Photosynthesis & respiration generalized reactions

II. Representative organic molecules synthesized by all organisms

III. Depth profiles of concentrations in ocean influenced by organisms

IV. Why the ocean is salty

VI. Covariation of nutrient concentrations in sea water

VII. Gas exchange of O2 and CO2

VIII. Schematic cycles of nutrient elements in the ocean

IX. Distribution of nutrients and dissolved oxygen in the deep oceans of the world

X. Classification of element behavior in ocean with respect to biological influences

XI. Residence time calculations in sea water

XII. Carbon species in atmosphere, ocean, sediment system.

XIII. The Land Plant Carbon Reservoir (Fig 28, Fig. 3.1 from the IPCC 2001 third assesment)

  1. Forests 654GtC
  2. Soils 1567 GtC.
  3. Exchange of carbon between land plants and the Atmosphere
    1. 120 GtC/yr. participates in photosynthetic reactions. (gross primary production GPP).
    2. 60 GtC/yr. contributed to plant material (net primary production NPP).
    3. Most NPP is respired by heterotrophs (heterotrophic respiration Rh)
    4. Net ecosystem production (NEP) = NPP-Rh = 10GtC/yr.
    5. After losses to fires, tree harvesting, erosion and export of DIC (dissolved inorganic carbon) we have Net Biome Production NBP = to ®0.2 +/- 0.7 or ®1.4 +/- 0.7GtC/yr.
  4. Anthropogenic effects on land plants
    1. The effect of increased atmospheric carbon dioxide on plant metabolism. It causes:
      1. Increased rates of photosynthesis
      2. Reduced water loss and increased growth.
      3. An average 33% increase of NPP for a doubling of atmospheric carbon dioxide concentration in some plants.
      4. Reduced plant growth when atmospheric concentrations exceed 800 to 1000 ppm. Some ecosystems max out well below this level.
    2. Effect of anthropogenic nitrogen fertilization
      1. Causes increased NPP near nitrous oxide or ammonia sources and enhances the formation of modified organic matter in soils thus increasing the residence time of soil carbon.
    3. Effect of other pollutants
      1. Ozone causes leaf injury and reduces plant growth.
      2. Nitrates and sulfates in rain cause acidification of soils and reduce plant growth.

XIV. The ocean carbon reservoir (Fig 28)

  1. Ocean carbon more than 50 times that in atmosphere (38000 vs. 730 GtC).
  2. Ocean processes that effect the reservoir
    1. Annual two-way exchange of carbon between the atmosphere and ocean is 90GtC/yr.
    2. Complete exchange of Carbon between ocean and atmosphere takes about 400yrs.
    3. Ocean carbon is in several forms: dissolved carbon dioxide (1%), bicarbonate (91%) and carbonate (8%).
    4. Net Primary production (NPP) of the ocean is 45 GtC/yr.
    5. Most of NPP is consumed by heterotrophs in the near-surface waters, sinking particulate organic carbon (POC) and dissolved organic carbon (DOC) makes up export production (EP). 10 to 20 GtCyr. This sinking organic debris (the "biological pump") is oxidized by heterotrophs in the deep-ocean and becomes dissolved inorganic carbon (DIC). Because of this large DIC carbon reservoir in the deep-ocean the atmospheric carbon dioxide concentration is 200 ppm. lower than it would be without it.
    6. About 0.1 GtC/yr of export production (EP) is incorporated in sediments on the sea floor.
  3. Anthropogenic effects
    1. Addition of carbon dioxide to the ocean reduces the carbonate ion concentration and thereby reduces the solubility of carbon dioxide in seawater. This is a large effect for increasing the atmospheric carbon dioxide concentration by 100 ppm. (from 370 to 470 ppm) would decrease carbonate ion concentration by 40% more than would have been the case if carbon dioxide concentrations were raised from the pre-industrial 280 to 380 ppm. Thus the oceans ability to take up carbon dioxide is reduced as atmospheric concentrations rise.
    2. Although the deep ocean could dissolve 70 to 80% of the expected anthropogenic carbon dioxide emissions and the sediments could neutralize another 15% it takes some 400years for the deep ocean to exchange with the surface and thousands more for changes in sedimentary calcium carbonate to equilibrate with the atmosphere. Consequently atmospheric concentrations of carbon dioxide could become substantially elevated before the ocean is able to remove this added carbon dioxide.
    3. Because the deep-ocean has a fixed rate of mixing the higher the rate of emissions the lower the proportion of those emissions that will be taken up by the ocean.
    4. Warming of the surface ocean will reduce the rate of carbon dioxide uptake because carbon dioxide is less soluble in warm water than in cold water.

XV. Ocean and land biosphere as sinks for anthropogenic carbon dioxide.

Burning of fossils fuels consumes atmospheric oxygen and releases carbon dioxide. The total amount of fossil fuel burned during the last decade or so is well known from commercial transactions. Consequently the total amount added to the atmosphere is also well known (Fig. 29). [Linda fig. 29 is figure 3.4 from the 2001 IPCC report same source as for fig 28]The amount of oxygen consumed in the process of burning fossil fuels can also be calculated and is related in a direct and proportional way to the carbon dioxide produced. The line in figure 29 labeled –fossil fuel burning” gives the increase in CO2 concentration and decrease in O2 concentration that should occur if all the CO2 produced by fossil fuel burning went into the atmosphere and stayed there. It turns out that only about half of the fossil fuel carbon dioxide has ended up in the atmosphere. The rest has gone into the ocean and the biosphere. Since carbon uptake by plants releases oxygen then biosphere uptake will add oxygen to the atmosphere whereas ocean uptake does not. Figure 29 shows the record of atmospheric oxygen decrease and carbon dioxide increase (line with dates) and the estimated amount of CO2 that has gone into the biosphere and ocean.


Lecture text by Jim Simpson and Peter deMenocal. Updated Spring 2002.