Nutrients and Ocean Chemistry

Take away ideas and understandings

Biologic processes cause most of the observed changes in ocean chemistry. Know and understand the typical depth profiles of biolimiting, biointermediate, and bioinert elements.
Understand why the ocean is salty. The definitions of Alkalinity and elemental residence times and why they are important.
Understand how the ocean carbonate system buffers (balances) ocean pH.
Understand atmosphere-ocean CO₂ gas exchange and the relative roles of the ocean and atmosphere C reservoirs.
Ocean chemistry strongly linked to land (chemical weathering), ocean biology, atmosphere (gas exchange), and ocean mixing (Fig A).

I. Photosynthesis & respiration generalized reactions

Carbon dioxide, water & solar energy yield (in algae & other "green" plants) carbohydrate & oxygen (Fig 1)
1 mole of CO₂ consumed as 1 mole of O₂ produced
Major influence on chemistry of surface ocean
Respiration as "inverse" process releases biochemical energy for organisms (Fig 2)
Some organic particles in surface ocean sink to deeper water before respiration

II. Representative organic molecules synthesized by all organisms

Nitrogen-bearing base from DNA & RNA (adenine) (Fig 3)
Phosphorus-bearing mononucleotide (in RNA & DNA) (Fig 4)
ATP; main energy transport molecule in cells containing N & P (Fig 5)
Production of organic molecules includes large amounts of N & P as well as C

III. Depth profiles of concentrations in ocean influenced by organisms

Temperature & salinity depth profiles result in large density gradients (Fig 6)
Dissolved oxygen depleted in deep water & especially near base of thermocline
Nitrate, silicate & phosphate almost completely removed from surface waters
Partial depletion of inorganic carbon from surface waters (Fig 7)

IV. Why the ocean is salty

Weathering of continental rocks by chemical and physical processes and dissolution of rocks to their elemental constituents.
These dissolved products are transported by rivers to oceans where they accumulate. Some accumulate very slowly, others more quickly.
The ocean has, on average, 35 ppt salinity (3.5%), 99.9% of which is composed of the elements shown in Figure 8.

VI. Covariation of nutrient concentrations in sea water

Nearly constant ratio of N to P in much of the ocean 16:1 molar ratio (Fig 15, Fig 16)
Surface waters with little N or P, while deep waters much higher
The N:P ratio (the Redfield ratio) is the same in plankton as it is in water, reflecting the linkage between life and chemistry in the ocean.

VII. Gas exchange of O₂ and CO₂

Dissolved gases of surface mixed layer are exchanged with atmospheric gases (Fig 17) more rapidly than uptake or production during photosynthesis
Thus "extra" O₂ generated by green plants moves into the overlying atmosphere fairly rapidly
Surface ocean remains saturated in O₂ due to gas exchange, even in areas with very low rates of photosynthesis
Mixed layer of ocean always has sufficient CO₂ for photosynthesis, even in areas of strong upwelling and very high rates of primary production.

VIII. Schematic cycles of nutrient elements in the ocean

Two box model (Fig 18) of surface mixed layer (photosynthesis zone) and
Deep water: reoxidation (respiration) of organic N & P to inorganic nutrients

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

Deep water formation in North Atlantic & Antarctic (Fig 19)
Circumpolar transport (southern ocean) links the major oceans
Northward flow in Indian & Pacific Oceans
"Young" water in N Atlantic & "old" water in N Pacific
high concentrations of nitrate in N Pacific (Fig 20)
low concentrations of dissolved oxygen in N Pacific (Fig 21)

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

Elements totally depleted in surface waters: bio-limiting (Fig 22)
Elements partially depleted in surface water: bio-intermediate
Elements which show no vertical gradient due to biological processes: bio-inert

XI. Residence time calculations in sea water

Essentials of Residence Time calculations: Calculate the residence time (Fig 23) of a Columbia undergraduate student.
Reservoirs & fluxes (Fig 24)
Assume steady state of amounts & concentrations in reservoirs
Residence time of water in ocean relative to river influx: 40,000 years (Fig 25)
Chloride residence time in ocean relative to river influx: 120 million yrs (Fig 26)

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

Atmosphere carbon gases: CO₂ and CH₄.
Dissolved carbon in ocean: many species (Fig 27).
Particulate carbon in ocean and sediments: CaCO₃ and organic C.

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

Forests 654GtC
Soils 1567 GtC.
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.
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)

Ocean carbon more than 50 times that in atmosphere (38000 vs. 730 GtC).
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.
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.

Resources:

Garrison, T. (1993) Oceanography, An Invitation to Marine Science. pp 130-137.
Broecker, W. S. (1974) Chemical Oceanography. Chapter 1, PP 3-29.
Broecker, W.S. and T.H. Peng (1982) Tracers in the Sea. Chapter 1, pp 1-41.
Hays, J., Water: the vital fluid, April 1996

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

The Climate System

EESC 2100 Spring 2007