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Abstract
Journal of Sedimentary Research, Section
A: Sedimentary Petrology and Processes
Vol. 67 (1997)No.
5. (September), Pages 891-897
Organic Matter in the Genesis of High-Island Atoll Peloidal Phosphorites:
The Lagoonal Link
Jean Trichet, Abdelouahad Fikri
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 d13C values in apatite
CO3 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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