Is Decaying Marine Plankton the Source of Selenium?

In general, the trace elements that have high contents in petroleum source rocks have closely interrelated abundances. The studies of Piper (1994) and Piper and Isaacs (1995a; 1995b) attribute this to their original association in marine plankton, the principal source of the organic material and the trace elements. In their model, the chemical degradation of organic material on the sea bottom of ancient oceans left trace elements behind more or less in proportion to their original abundance in the plankton. Marine-plankton-derived trace elements are most concentrated in very slowly deposited fine-grained sedimentary rocks, such as transgressive shales which formed condensed zones with high concentrations of organic carbon. In cases of extreme degradation, the organic carbon itself was completely removed, leaving only the resistant residue of the marine plankton–trace-element-enriched phosphate deposits (Piper and Medrano, 1994). As a result, these trace elements may be expected to have predictable correlations in organic carbon-rich marine shales (Figure 6), except where trace elements have been altered by processes acting on the rocks after their deposition.

Figure 6. Selenium is correlated with organic carbon in the Monterey Formation. (click for larger image) 9k

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