BIOGEOCHEMICAL CYCLING OF TRACE ELEMENTS

The role of the ocean in the global biogeochemical cycling of trace elements is the subject of this chapter. The processes that introduce and remove these elements from the ocean are summarized in Figure 11.2 and discussed in detail next. 

Santschi, P. H. (1988), Factors Controlling the Biogeochemical Cycles of Trace Elements in Fresh and Coastal Marine Waters as Revealed by Artificial Radioisotopes, Limnol. Oceanogr. 33, 848-866. 

As already stressed, these techniques involve many analytical steps such as extraction, derivatization, separation and detection, which should be performed in such a way that decay of the unstable species does not occur. However, the control of the quality of measurements is often hampered by the lack of suitable reference materials for speciation analyses. Research is hence directed towards the development of new (if possible simple) analytical methods, the production of reference materials, and the monitoring of chemical species for various purposes (environmental risk assessment, toxicity studies, biogeochemical cycles of trace elements, etc.). 

Martin J.-M. and Windom H. L. (1991) Present and future roles of ocean margins in regulating marine biogeochem-ical cycles of trace elements. In Ocean Margin Process in Global Change (eds. R. F. C. Mantoura, J.-M. Martin, and R. Wollast). Wiley-Interscience, New York, pp. 45-67. 

Amongst the ash elements the most abundant in biomass is calcium, which is accumulated in leaves, in trunk wood, and in twigs. Potassium is dominant in annual NPP. The masses of trace metals in biogeochemical cycling of this Oak Forest ecosystem are roughly in correspondence to their respective average values for the... 

Then there are wider questions about the possible limiting roles of various trace elements in key biogeochemical processes Do trace elements other than iron limit phytoplankton growth and primary production Is the composition of phytoplankton assemblages controlled by trace elements Are various processes in the cycle of nitrogen limited by trace elements (e.g., N2 fixation by iron N2O reduction by copper) What are the links between trace elements and the reduced sulfur cycle in surface seawater ... 

Particles represent important agents of transport in global ocean cycles of many trace elements, of carbon, and of other substances. Once introduced into the oceans, many trace elements are removed from seawater by scavenging (sorption, com-plexation, and other forms of surface reactions) to particles (Goldberg, 1954 Turekian, 1977). Scavenging and burial in marine sediments represents the principal loss process influencing the biogeochemical cycle of many trace elements in the ocean (Li, 1981). 

At about the time that Honeyman and Santschi published their colloidal pumping hypothesis, lab and field studies showed that thorium and other metals in seawater are, indeed, associated with colloids (Baskaran et al., 1992 Moran and Buesseler, 1992 Moran and Moore, 1989 Niven and Moore, 1988). Subsequently, many more studies have demonstrated that colloids are abundant in seawater and that they play an important role in the marine biogeochemical cycles of organic carbon and of many trace elements (Guo and Santschi, 1997). 

In may be of interest to compare the fluxes of elements in biogeochemical cycles of the Oak Forest ecosystem with airborne deposition input. The latter were (in kg/ha/year) for N, 17.7 for Ca, 14.7 for Mg, 1.8 for K, 4.2 for Na, 1.4 for P, 1.1 for Fe, 0.07 and for Zn, 0.14. The deposition input of these elements falls into a range of 20% (calcium) to 4.5% (potassium) relative to the respective biogeochemical fluxes (seeTable 17). The airborne Fe input accounts for a mere 2.5%. Simultaneously, for some trace metals, like zinc, the deposition input is commensurate with the fluxes of the biogeochemical cycle. 

---