Authigenesis of biomorphic apatite particles from Benguela upwelling zone sediments off Namibia: The role of organic matter in sedimentary apatite nucleation and growth

Kaarel Mänd, Kalle Kirsimäe, Aivo Lepland, Chris H. Crosby, Jake V. Bailey, Kurt O. Konhauser, Richard Wirth, Anja Schreiber, Kaarel Lumiste

Research output: Contribution to journalArticle

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Abstract

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.

Original languageEnglish (US)
Pages (from-to)640-658
Number of pages19
JournalGeobiology
Volume16
Issue number6
DOIs
StatePublished - Nov 2018

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authigenesis
apatite
Namibia
nucleation
upwelling
organic matter
sediments
sediment
phosphate
phosphorite
phosphates
porewater
pellets
phosphogenesis
double diffusion
fluorapatite
nanocomposites
particle
polyphosphates
bacterial communities

PubMed: MeSH publication types

  • Journal Article
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

Cite this

Authigenesis of biomorphic apatite particles from Benguela upwelling zone sediments off Namibia : The role of organic matter in sedimentary apatite nucleation and growth. / Mänd, Kaarel; Kirsimäe, Kalle; Lepland, Aivo; Crosby, Chris H.; Bailey, Jake V.; Konhauser, Kurt O.; Wirth, Richard; Schreiber, Anja; Lumiste, Kaarel.

In: Geobiology, Vol. 16, No. 6, 11.2018, p. 640-658.

Research output: Contribution to journalArticle

Mänd, Kaarel ; Kirsimäe, Kalle ; Lepland, Aivo ; Crosby, Chris H. ; Bailey, Jake V. ; Konhauser, Kurt O. ; Wirth, Richard ; Schreiber, Anja ; Lumiste, Kaarel. / Authigenesis of biomorphic apatite particles from Benguela upwelling zone sediments off Namibia : The role of organic matter in sedimentary apatite nucleation and growth. In: Geobiology. 2018 ; Vol. 16, No. 6. pp. 640-658.
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abstract = "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.",
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