169-1 The Evolving Relationship Between Phosphorites and the Microbial World: a Two-Billion-Year Saga of Environmental Change

Session: Phantastic Phosphorus and Its Historical Biogeochemistry

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Abstract:

Phosphate mineralization in marine sediments (phosphogenesis) results from microbial processes.  These processes include heterotrophic breakdown of organic molecules that releases PO₄, photoautotrophs that produce O₂ necessary to create redox gradients, and chemosynthetic bacteria that exploit these gradients, concentrate PO₄, and drive phosphate mineralization. The phosphate-bacterial connection began in the Paleoproterozoic and continues to the modern.  However, the location, size, and nature of sedimentary phosphate deposits have changed through time with geosphere and biosphere coevolution and progressive oxygenation of the oceans.  This has led to the nature and locus of phosphogenesis shifting, from small granular deposits in shallow Paleoproterozoic peritidal settings, to larger deposits found in shallow shelf settings in the Neoproterozoic, to massive phosphorite giants with diverse grain types in the Paleozoic and Mesozoic mid-shelf settings, and finally to the outer shelf in the Cenozoic.

The oldest granular phosphorite deposits are found in the Paleoproterozoic and occurred in peritidal environments with stromatolites that produced local oxygen oases. Wave action concentrated mm-scale laminae of phosphate peloids that include fossil bacteria. During the Marinoan Snowball Earth (ca. 640 Ma) in the Neoproterozoic, phosphogenesis occurred under sea ice in western Brazil.  Chemosynthetic bacterial communities thrived on the protected, siliciclastic-starved seafloor. Authigenic siderite and carbonate fluorapatite (CFA) laminae alternate with iron-oxide and glaciomarine sediments. Phosphogenesis was associated with organic matter burial and redox gradients.  The Permian Phosphoria phosphorite giant demonstrates the connection to extreme oceanographic conditions, which led to a wide diversity of environmental conditions and grain types.  The extreme conditions were required to create widespread anoxia and dysoxia on the seafloor. Phosphogenesis in outer shelf settings was associated with extreme organic matter burial, elevated trace metals, and a δ¹³C_(PO4–CO3) of -10 ‰, while nearshore environments were marked by low organic matter burial, low trace element concentrations, and a δ¹³C_(PO4–CO3) of -3 ‰.

Through time, phosphorite geochemistry and textures were controlled by environmental factors, but what has remained consistent is the association with organic matter and minerals that reflect redox gradients (including sulfide minerals), peloidal grains, and evidence of bacteria (body fossils, microbial sedimentary structures). Based on these associations and microtexture observations, it is clear that CFA results from bacterial passive mineralization, and the resulting phosphorite largely reflects the persistence of oceanographic conditions.

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