| Rivers contribute a large fraction of trace metals to the oceans; but because riverine particles are trapped in estuaries, only dissolved metals ultimately reach oceanic waters. Reactions controlling the partitioning of metals between the dissolved and particulate fractions are investigated in the Gironde estuary (France). The estuarine behavior of dissolved metals (Cd, Ni, Zn, Cu, Pb, Mn) is reproduced quantitatively by simple steady state conservation model that accounts for the inorganic complexation of metals by seawater anions. The colloidal material (which appears to consist of a mixture of metal oxides and organic macromolecules) plays a minor role in the estuarine transport of most metals—with the exception of iron, whose behavior is controlled by the coagulation of iron oxides at low salinity.; Clays are the most abundant non-biogenic particles in the oceans and a new family of surface complexation models is developed to account for their adsorption properties. The new models take into account the existence of a permanent negative charge resulting from isomorphic substitution in the clay; ion exchange and surface complexation are thus described in a self consistent manner, and the effect of the permanent charge on surface complexation is quantified. Acid-base properties of montmorillonite and kaolinite, and in particular the increase in the Zero Proton Condition when ionic strength decreases, are correctly and quantitatively predicted. The models can also fit an extensive published data set on metal adsorption on montmorillonite.; The interactions of trace metals with marine organisms are important in determining the metal fluxes out of the surface oceans and the effect of trace metals on marine ecology and productivity. In the model marine diatom Thalassiosira weissflogii, the major use of Zn is in the enzyme carbonic anhydrase (CA). The major fraction of the cellular CA in T. weissflogii is found in the cytoplasm (and not in the chloroplasts), the remainder of the activity being external. The (unexpected) existence of a cytoplasmic CA may be linked to the functioning of the recently reported C4 photosynthetic pathway in diatoms. |
