Observations on Metals in Natural Systems

All the factors mentioned in the previous sections play a role in the movement of metals through their overall biogeochemical cycle injection into the atmosphere, deposition onto land or water surfaces, transport via rivers and 

Why are the oceans so depleted in these trace metals Certainly it is not for the lack of availability from rock weathering or because of constraints imposed by the solubility of any unique compound of these elements. The reason must lie in the dynamics of the system of delivery of the metals to the oceans and their subsequent behavior in an ocean that cannot be simulated by simple in vitro experiments involving homogeneous reaction kinetics. 

In this section, an overview of the net transport of metals through rivers, estuaries, and the oceans provides an example of how these chemical and physical forces interact. 

Estuaries exhibit physical and chemical characteristics that are distinct from oceans or lakes. In estuaries, water renewal times are rapid (10 to 10 years compared to 1 to 10 years for lakes and 10 years for oceans), redox and salinity gradients are often transient, and diurnal variations in nutrient concentrations can be significant. The biological productivity of estuaries is high and this, coupled with accumulation of organic debris within estuary boundaries, often produces anoxic conditions at the sediment-water interface. Thus, in contrast to the relatively constant chemical composition of the 

F g- 15-15 Schematic representation of Fe and organic matter interactions in an estuary. POM = particulate organic matter DOM = dissolved organic matter PIM = particulate inorganic matter. 

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