The Climate System
EESC 2100 Spring 2007
Lectures - Monday and Wednesday, 11:00 AM - 12:15 PM
Lab - Tuesday, 4:10 PM -7 PM
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
- 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
I. Photosynthesis & respiration generalized reactions
- Carbon dioxide, water & solar energy yield (in algae & other "green" plants)
carbohydrate & oxygen (Fig
- 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
- 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
- Phosphorus-bearing mononucleotide (in RNA & DNA) (Fig
- ATP; main energy transport molecule in cells containing N & P (Fig
- Production of organic molecules includes large amounts of N & P as well
III. Depth profiles of concentrations in ocean influenced by organisms
- Temperature & salinity depth profiles result in large density gradients (Fig
- Dissolved oxygen depleted in deep water & especially near base of thermocline
- Nitrate, silicate & phosphate almost completely removed from surface
- Partial depletion of inorganic carbon from surface waters (Fig
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
- 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
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
- 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
- low concentrations of dissolved oxygen in N Pacific (Fig
X. Classification of element behavior in ocean with respect to biological
- Elements totally depleted in surface waters: bio-limiting (Fig
- Elements partially depleted in surface water: bio-intermediate
- Elements which show no vertical gradient due to biological processes:
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
- Assume steady state of amounts & concentrations in reservoirs
- Residence time of water in ocean relative to river influx: 40,000 years (Fig
- Chloride residence time in ocean relative to river influx: 120 million
XII. Carbon species in atmosphere, ocean, sediment system.
- Atmosphere carbon gases: CO2 and CH4.
- Dissolved carbon in ocean: many species (Fig
- Particulate carbon in ocean and sediments: CaCO3 and organic
- Forests 654GtC
- Soils 1567 GtC.
- Exchange of carbon between land plants and the Atmosphere
- 120 GtC/yr. participates in photosynthetic reactions. (gross primary
- 60 GtC/yr. contributed to plant material (net primary production
- Most NPP is respired by heterotrophs (heterotrophic respiration Rh)
- Net ecosystem production (NEP) = NPP-Rh = 10GtC/yr.
- 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
- The effect of increased atmospheric carbon dioxide on plant metabolism.
- Increased rates of photosynthesis
- Reduced water loss and increased growth.
- An average 33% increase of NPP for a doubling of atmospheric
carbon dioxide concentration in some plants.
- Reduced plant growth when atmospheric concentrations exceed 800
to 1000 ppm. Some ecosystems max out well below this level.
- Effect of anthropogenic nitrogen fertilization
- 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.
- Effect of other pollutants
- Ozone causes leaf injury and reduces plant growth.
- Nitrates and sulfates in rain cause acidification of soils and
reduce plant growth.
XIV. The ocean carbon reservoir (Fig
- Ocean carbon more than 50 times that in atmosphere (38000 vs. 730 GtC).
- Ocean processes that effect the reservoir
- Annual two-way exchange of carbon between the atmosphere and ocean
- Complete exchange of Carbon between ocean and atmosphere takes about
- Ocean carbon is in several forms: dissolved carbon dioxide (1%),
bicarbonate (91%) and carbonate (8%).
- Net Primary production (NPP) of the ocean is 45 GtC/yr.
- 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
- About 0.1 GtC/yr of export production (EP) is incorporated in sediments
on the sea floor.
- Anthropogenic effects
- 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.
- 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.
- 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.
- Warming of the surface ocean will reduce the rate of carbon dioxide
uptake because carbon dioxide is less soluble in warm water than in
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.
- Garrison, T. (1993) Oceanography, An Invitation to Marine Science.
- Broecker, W. S. (1974) Chemical Oceanography. Chapter 1, PP
- 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.