Carbonate sediments are composed chiefly of biogenic fragments. The biogeochemistry of the shells or skeletons of organisms contributing to the sediments reflects some of the variables in the chemistry of the water in which they lived. An important factor determining the composition of the trace-element biogeochemistry of the shells appears to be the availability of certain trace elements in the waters in which the organisms lived. The trace-element composition of fresh water differs from that of sea water. Three elements particularly showing differences in concentration between fresh and marine water are barium, iron, and manganese. The average amount of barium in marine water is about 0.05 ppm.; in fresh water the barium content may exceed that of marine water, be less, or be approximately the same. The average iron content in marine water is about 0.008 ppm. in contrast to 1.0 ppm. for river water. The average manganese content of marine water is about 0.003 ppm. and of river water is near 0.03 ppm. Lagoonal waters contain more iron and manganese for uptake by organisms than do waters of an open-marine environment.

A study of marine, lagoonal, and fresh-water gastropods and pelecypods shows that the shells of non-marine and lagoonal mollusks contain a greater abundance of barium, iron, and manganese than do those of marine mollusks. The difference between marine and non-marine is independent of taxonomic rank or mineralogy of the shells. Modern molluscan shells as well as Recent carbonate sediments from Florida show, as a rule, a greater abundance of iron and manganese than do those from a marine environment. Scatter plots of one against the other of these three trace elements, despite some overlap, show groupings which correlate with environment.

Could trace elements become a prospective tool in environmental recognition? Perhaps they could under some special circumstances. The problem is that, during diagenesis, the mineralogy and chemistry of carbonate sediments, including the trace element assemblage, are drastically changed.

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