Authigenic apatite and octacalcium phosphate formation due to adsorption–precipitation switching across estuarine salinity gradients

Oxmann, Julian F. and Schwendenmann, L. (2015) Authigenic apatite and octacalcium phosphate formation due to adsorption–precipitation switching across estuarine salinity gradients Biogeosciences (BG), 12 (3). pp. 723-738. DOI 10.5194/bg-12-723-2015.

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Abstract

Mechanisms governing phosphorus (P) speciation in coastal sediments remain largely unknown due to the diversity of coastal environments and poor analytical specificity for P phases. We investigated P speciation across salinity gradients comprising diverse ecosystems in a P-enriched estuary. To determine P load effects on P speciation we compared the high P site with a low P site. Octacalcium phosphate (OCP), authigenic apatite (carbonate fluorapatite, CFAP) and detrital apatite (fluorapatite) were quantitated in addition to Al/Fe-bound P (Al/Fe-P) and Ca-bound P (Ca-P). Gradients in sediment pH strongly affected P fractions across ecosystems and independent of the site-specific total P status. We found a pronounced switch from adsorbed Al/Fe-P to mineral Ca-P with decreasing acidity from land to sea. This switch occurred at near-neutral sediment pH and has possibly been enhanced by redox-driven phosphate desorption from iron oxyhydroxides. The seaward decline in Al/Fe-P was counterbalanced by the precipitation of Ca-P. Correspondingly, two location-dependent accumulation mechanisms occurred at the high P site due to the switch, leading to elevated Al/Fe-P at pH < 6.6 (landward; adsorption) and elevated Ca-P at pH > 6.6 (seaward; precipitation). Enhanced Ca-P precipitation by increased P loads was also evident from disproportional accumulation of metastable Ca-P (Ca-Pmeta) at the high P site. Here, sediments contained on average 6-fold higher Ca-Pmeta levels compared with the low P site, although these sediments contained only 2-fold more total Ca-P than the low P sediments. Phosphorus species distributions indicated that these elevated Ca-Pmeta levels resulted from transformation of fertilizer-derived Al/Fe-P to OCP and CFAP in nearshore areas. Formation of CFAP as well as its precursor, OCP, results in P retention in coastal zones and can thus lead to substantial inorganic P accumulation in response to anthropogenic P input.

Document Type: Article
Additional Information: WOS:000349793100006
Keywords: Phosphorus speciation, fluxes and burial; Phosphorus redistribution; Successive or direct crystallization; Saturation states and formation kinetics; Detrital apatite; CONVEX; SEDEX; Fractionation; Morgan; Available phosphorus test; Sequential extraction method; Phosphorus fertilizer application; X-ray absorption near-edge structure spectroscopy; Human alterations to the phosphorus cycle; Inundation frequency; Bay; Tidal flat; Mangrove; Salt marsh; Pastoral agriculture
Research affiliation: OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-MG Marine Geosystems
Refereed: Yes
DOI etc.: 10.5194/bg-12-723-2015
ISSN: 1726-4170
Date Deposited: 12 Feb 2015 07:31
Last Modified: 13 Apr 2017 09:20
URI: http://eprints.uni-kiel.de/id/eprint/27368

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