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The elements to be discussed below are all biologically important and play a vital role in the metabolism and cellular synthesis by all organisms. The products of biosynthesis are deposited in the sediments and can be recognized in shales after having undergone diagenesis.

Each of these elements has a distinct distribution pattern in the ocean and their isotopes are incorporated into organic matter according to controls that are largely directed by kinetic processes, involving the action of enzymes as transporting agents and cell walls as diffusion boundries. The end products of biological and diagenetic processes results in the formation of kerogen, which concentrates in the shale, or hydrocarbons which may escape and be transported out of the shale. Both early diagenetic and later maturation processes each impose changes in the isotope composition of the original starting organic matter.

The average ratio of in terrestrial water is 22.6. Sulfur is an important element in the marine environment because it undergoes both organic and inorganic reactions which lead to formation of sedimentary materials.

It is now well established that biological systems are capable of selectively metabolizing the stable isotopes of sulfur (Thode 1951; Jones and Starkey, 1957; Harrison and Thode 1958; Kaplan and Rittenberg, 1964) . The process can be divided into assimilatory or dissimilatory, and reductive or oxidative functions. The maximum fractionation for various metabolic processes measured by Kaplan and Rittenberg (1962) can be seen in Table 2-l. The data in this table can be dompared with the pathways

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