Nutrient elements in sea water

Photosynthesis requires a number of essential nutrient elements in addition to CO2 and H2O:

CO2 + H2O + nutrients ---> "Organic Matter" + O2

In the oceans, three nutrients are termed "critical" -- their availability (concentration) is much less than what is required by photosynthesizers. These three critical nutrients limit biological productivity in the oceans.
* Phosphorus, P: present as dissovled PO43-
* Nitrogen, N as dissolved NO3- (N2 cannot be used by algae or plants)
* Silicon, Si as dissolved H4SiO4 is required by diatoms! Si is not used in photosynthesis. But diatoms will not grow unless dissolved H4SiO4 is available.

Nutrient elements are non-conservative species in sea water. Their concentrations are controlled by life processes and thus vary with depth.
* Photosynthesis extracts nutrients efficiently from surface waters and keeps their concentrations very low.
* Respiration returns nutrients to sea water by decomposing organic matter.
* Vertical upwelling returns nutrients to surface waters where they can be used (again) by photosynthesizing algae.
[Ross Fig. 7-12; Text Figs. 10.4 and 10.5]

Organic substances in sea water.

These occur in relatively low amounts (a few ppm or less) as both
dissolved species (molecules and ions) and as
very fine particulates called colloids.
These substances represent the intermediate products of organic-matter decomposition. Although organic substances are always present, they don't tend to stock around very long. Their ultimate fate is:
* complete breakdown to inorganic components
* incorporation by organisms
* deposition and preservation in sediments

Controls on major dissolved constituents in sea water

Nutrient elements and organic substances are regulated and "cycled" by biological processes. But what about the major salt ions of sea water - the conservative species that comprise the salinity of sea water? Geologic and biologic evidence from ancient marine sedimentary rocks suggest that the salinity of sea water has remained constant for at least the last 1.5 billion years, and probably throughout ocean history.

We know that dissolved substances are entering the ocean all the time, mostly be river runoff. The probability that sea-water salinity has remained constant implies that there is a balance between input and removal of dissolved species. We call this situation a "steady state."

Inputs and outputs go on continuously.
Steady state occurs when input = output. Under these conditions, the mass or concentration of salt in the oceans remains approximately constant.

Input processes for major dissolved constituents [Text Fig. 5.14]

1. Volcanic eruptions. Volcanism releases gases containing Cl, S, and C to the atmosphere (and directly to the oceans). These gases dissolve in water to produce acid (and anions). Thus they represent the delivery of acid and anions to the land surface and to sea water.

2. Weathering of continental rocks. This is the most important process in terms of the total amount of dissolved constituents brought to the oceans.

Acidic waters + Rocks --> New minerals + Ions

(HCl, S-gases, H2CO3, organic acids)
(Na, K, Ca, Mg) - all cations
(Cl, HCO3, SO4) - all anions

3. Alteration of new oceanic crust at mid-ocean ridge spreading centers by the circulation of heated sea water. Hydrothermal alteration liberates some major elements to sea water - notably Ca and K. This process also delivers metals (Fe, Mn, Cu, Zn, etc.).

Outputs (removal) processes for major dissolved constituents

1. Biological processes. The secretion of calcium carbonate (CaCO3) and opaline silica (SiO2.nH2O) is the major removal process for HCO3, Ca, Si. A number of trace elements are also removed by adsorption on to very fine organic particles.

2. Evaporation from restricted basins. The precipitation of hydrogeneous "evaporite" minerals NaCl (halite) and CaSO4.2H2O (gypsum) removes Na, Cl, SO4

3. Formation of new minerals in ocean crust and within sediment. Reactions on and within the sea floor removes some elements by the formation of new minerals.
ions + basalt, sediment ---> new minerals

4. Adsorption of ions on very fine clay particles. The surfaces of clay particles are electrically charged. They can absorb ions from sea water.

5. Hydrothermal alteration of new oceanic crust. Hydrothermal alteration is not only a "source" for some elements, but also removes the major ions Mg and SO4 to form new minerals.

Input and output process are connected globally through plate tectonic processes:
* formation of new oceanic crust
* collision and subduction
* formation and uplift of new continental crust
* weathering and erosion of continental crust

Thus, there is a continuous, long-term cycle of matter through the lithosphere, atomsphere, and oceans (hydrosphere).

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