Question

4. The vertical distribution of trace elements in the ocean provides clues about their reactivity and biogeochemistry. Although found in trace amounts in sea water, bioactive metals are of interest because they can influence productivity and the cycling of other substances. Discuss the five types of trace element distributions in the oceans. Provide examples of two trace elements that fit each type of distribution (U10 pts, G7 pts) a. b. Discuss conservative vs. non-conservative mixing between fresh water runoff and coastal waters. Assume H4SiO4 mixes conservatively while dissolved Al does not and that both species are found at higher concentrations in river water. Draw generic mixing diagrams for these elements across a salinity gradient (i.e, salinity 0 to 33) (U15 pts, G7 pts) Describe how dissolved Fe is lost in estuaries. In your description include the mechanism of removal, the cations most influential in the removal reactions, and a graph showing your expectation of the particulate Fe across the salinity gradient c.

Answer 1

As we know that the trace elements are those elements having concentrations within a range of 10 micro mol kg^-1 ( micro = 10 ^-6 )  to f mol kg^-1 ( f mol = fempto = 10^-15 ). Based on their chemical behavior in sea water and their distribution they are basically grouped into three categories. These are - Conservative, Nutrient and Scavenged.

a. The five types of distributions of trace elements in sea water can be discussed as follows.

( 1 ) CONSERVATIVE TYPE - Distributions. Conservative type trace elements are those which interact only weakly with particles and are in concentrations with constant ratio to salinity. They also have oceanic residence times greater than the mixing times of oceans ( greater than 10⁵ years ). Examples are - Molybdenum, Tungsten, and Rubidium e.t.c.

( 2 ) NUTRIENT TYPE- Distribution. AS their name suggests their distribution is controlled by biological cycling. Their distribution is controlled by their assimilation by plankton from surface waters and oxidation and remineralization of it in deeper waters. Because they are absorbed by phytoplanktons and biogenic particles in the surface waters and undergo decomposition and desolution in subsurface waters,  their concentrations are low in surface waters and increase in subsurface waters. Examples are - Zinc, Cadmium, Silver e.t.c.

( 3 ) SCAVENGED TYPE - Distribution. These elements are very reactive and do not stay in solutions therefore they have very short oceanic residence time of 100 - 1000 years.Their distribution reflects their source of origin. their concentration is maximum near their sources such as rivers, sediments and hydrothermal vents. Their concentration decrease with depth. Examples are - Aluminum, Cs¹³⁷, Zr⁹⁵ e.t.c.

( 4 ) HYBRID TYPE - Distributions. Distribution of some trace elements are dependent on both recycling and scavenging processes. They show Hybrid type distribution. Examples are iron and copper.

( 5 ) MIXED TYPE -Distribution. There are some elements which exist in more than one chemical form and show different distributions. Examples are Germanium, Arsenic, Selenium , e.t.c.

b. When afresh water runoff mixes with ocean water the different elements show either conservative or non conservative behavior.

Conservative mixing : It is attained when there is no loss or gain of dissolved components during mixing. The residence time of an element is much longer than the mixing time of the ocean which is about 1000 yrs. Conservative elements have a longer residence time than non conservative elements.

Non conservative mixing : It occurs when concentration of elements are significantly changed by biological or chemical reactions.

Boyle ( 1974 ) derived a mathematical relationship for the variation of flux of a dissolved element with salinity in an estuary. The result so obtained by putting the value of elements in the equation can be plotted on a graph. The equation is as follows.

dQ_c/ dQS = - Q_w ( S - S_r ) d²C/dS²

where

Q_w = Flux of the river water

Q_c = Flux of dissolved component carried by river water

S_r = Salinity of river

S = Salinity of sea or estuary.

( 1 ) As we have discussed earlier during conservative mixing there is no loss or gain of dissolved components the equation will be as follows.

dQ/dS = 0 = d²C/dS²

Therefore the plot of concentration of dissolved element against salinity will be a straight line. The conservative mixing of H₄SiO₄ against salinity can be plotted as follows.

CONSERVATIVE MIXING OF SALINITY

( 2 ) For non conservative mixing d²C/ds² will not be equal to 0. Therefore the plot of concentration of dissolved element against salinity will be a curve. Because an element could be either removed or added within the sea or estuary the curve will be above or below the straight line showing conservative behavior. The plot of non conservative mixing of Al against salinity can be plotted as follows.

NON CONSERVATIVE MIxING OF AL NaN CONSERV ATV 2 SALINITY