Sedimentary phosphorites comprise a major phosphorus (P) ore, yet their formation remains poorly understood. Extant polyphosphate-metabolizing bacterial communities are known to act as bacterial phosphate-pumps, leading to episodically high dissolved phosphate concentrations in pore waters of organic-rich sediment. These conditions can promote the precipitation of amorphous precursor phases that are quickly converted to apatite—usually in carbonate fluorapatite form [Ca10(PO4,CO3)6F2-3]. To assess the mechanisms underpinning the nucleation and growth of sedimentary apatite, we sampled P-rich sediments from the Namibian shelf, a modern environment where phosphogenesis presently occurs. The P-rich fraction of the topmost centimetres of sediment mainly consists of pellets about 50–400 μm in size, which in turn are comprised of micron-sized apatite particles that are often arranged into radial structures with diameters ranging from 2 to 4 μm, and morphologies that range from rod-shapes to dumbbells to spheres that resemble laboratory-grown fluorapatite–gelatin nanocomposites known from double-diffusion experiments in organic matrices. The nucleation and growth of authigenic apatite on the Namibian shelf is likely analogous to these laboratory-produced precipitates, where organic macromolecules play a central role in apatite nucleation and growth. The high density of apatite nucleation sites within the pellets (>109 particles per cm3) suggests precipitation at high pore water phosphate concentrations that have been reported from the Namibian shelf and may be attributed to microbial phosphate pumping. The intimate association of organic material with the apatite could suggest a possible role of biological substrata, such as exopolymeric substances (EPS), in the nucleation of apatite precursors. Importantly, we do not observe any evidence that the apatite particles are actual phosphatized microbes, contradicting some earlier studies. Nevertheless, these results further evidence the potential importance of microbially derived (extracellular) organic matter as a template for phosphatic mineral nucleation in both recent and ancient phosphorites.
Bibliographical noteFunding Information:
Estonian Research Council, Grant/Award Number: PUT696; Ministry of Education and Research of Estonia, Archimedes Foundation, Grant/Award Number: The Kristjan Jaak Scholarship program; Natural Sciences and Engineering Research Council of Canada, Grant/Award Number: Discovery grant RGPIN-165831; National Science Foundation, Grant/Award Number: EAR-1057119
The authors would like to thank the organizers and participants of the Regional Graduate Network in Oceanography Discovery Camp 2015 that is funded by the Agouron Institute and the Scientific Committee for Oceanographic Research (SCOR); the crew of R/V Mirabilis and the University of Namibia for access to coring sites and help with sample gathering; Nathan Gerein for SEM work in the University of Alberta; Liisa Lang for help with TEM analyses; and Leslie J. Robbins for helpful comments. Three anonymous reviewers are thanked for constructive comments and insights that have greatly improved the manuscript. This study was supported by the Ministry of Education and Research of Estonia mobility grant within Archimedes Foundation’s The Kristjan Jaak Scholarship program “Doctoral Study Abroad” to KM; Estonian Research Council under grant PUT696 to KK; Natural Sciences and Engineering Research Council of Canada Discovery grant RGPIN-165831 to KOK, and by a grant from the U.S. National Science Foundation EAR-1057119 to JVB.