Abstract

In the debate over the origin of insular phosphorites, found in the top parts of more or less uplifted atolls, evidence is provided by both organic geochemistry (abundance and composition of humic compounds and lipidic biomarkers) and carbon-isotope geochemistry for the formation of phosphatic peloids, as well as phosphatized bioclasts, in environments containing mostly cyanobacterial and microbial organic matter. The composition of hydrocarbons and fatty acids suggests that this organic matter has been altered through bacterial processes. The abundance of humic compounds and the common presence of concentrically laminated structure of many peloids indicate that these particles formed during early diagenesis in a microbiologically active environment. Our data were obtained from (1) elemental analysis of bulk organic matter associated with atoll phosphorites as well as of their acid-soluble and humic compounds, (2) Rock-Eval pyrolysis of bulk organic matter and, (3) chromatographic analyses of hydrocarbons and fatty acids. The conclusion of this study is that, whatever the source of phosphorus (guano, supermarine reef organisms, marine waters, or possibly endo-upwelling), phosphatogenesis proceeded through growth of lagoonal cyanobacterial biomass, the phosphorus of which being partly recycled during subsequent diagenetic steps. If closed lagoons are indeed favorable for preservation of organic matter and, therefore, of phosphorus, a closed morphology is not a prerequisite for the course of such processes. Similar delta 13 C values in apatite CO 3 and in total dissolved organic carbon in recent microbial atoll deposits suggest that such deposits may be precursors to such peloidal insular phosphorites.

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