Combining Physical and Chemical Information

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 estuaries to the oceans, and sedimentation and ultimate burial in the sediments. The physical/chemical form in which each metal is transported in the aquatic phases of the cycle will depend on the specific metal and its interactions with other dissolved and suspended constituents in accord with the principles discussed above. However, it is important to keep in mind that the physical processes of fluid flow and sediment transport must be combined with the chemical reactions to gain insight into the functioning of the complete system. This point has been well-stated by Turekian (1977) in a discussion of metal concentrations in the ocean  

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 sub- 

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 charactristics that are distinct from oceans or lakes. In estuaries, water renewal times are rapid (10 to 10 years compared to 1-10 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 

The efficient removal of iron in estuaries allows very little of the initial river-bome iron to escape the estuary to the coastal waters. Nearly all the iron entering the estuary is transported in particulate form to sediments on the estuary floor where, due to the high accumulation of organic matter, reducing 

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