Residence Times

outflow outflow

where dC/dt = rate of change in concentration. Inflow and outflow may include gain or loss through chemical reactions in addition to physical

TABLE 11-3

Residence Times for Some Elements in the Oceans2

Element Residence time (yr)

TABLE 11-3

Residence Times for Some Elements in the Oceans2

Element Residence time (yr)

Al

6.2 >

< 102

Ca

1.1 >

< 106

Cr

8.2 >

< 103

Fe

5.4 >

< 101

K

1.2 >

< 107

Mg

1.3 >

< 107

Mn

1.3 >

< 103

Na

8.3 >

< 107

Si

2>

c 104

Zn

5.1 >

< 102

aSee S. M. Libes, An Introduction to Marine Biogeochemistry, Wiley, New York, 1992, p. 638, for a more complete list.

aSee S. M. Libes, An Introduction to Marine Biogeochemistry, Wiley, New York, 1992, p. 638, for a more complete list.

processes. The residence time of a substance is defined as T = C/(dC/dt). It is the average time a substance remains in a particular reservoir such as a lake or an ocean before it is removed by some transport process such as flow of the water to another system, or incorporation into sediments. Some examples of residence time for the ocean are given in Table 11-3.

In freshwater lakes, we have a similar situation, but output involves outflow as well as the other processes. An inert soluble material should have T about equal to that of the water itself, since it flows in and out of a given region with the water. Shorter T values imply rapid formation of insoluble products (e.g., those of Al), while larger ones suggest a cycling process that may be inorganic or biological. For example, in upper layers one may have Fe3+ ^ Fe(OH)3. However, if much organic material is present in the sediments, giving low pE, the precipitated ferric compound may be reduced to soluble Fe2+, which is then reoxidized and recycled. Phosphorus and nitrogen are involved in biological cycles in both freshwater and oceans:

uptake by organisms ^ death and sedimentation ^ decomposition and solution

They often have long residence times.

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