Abstract
The chemical forms of phosphorus (P) in sediments are routinely measured in studies of P in modern and ancient marine environments. However, samples for such analyses are often exposed to atmospheric oxygen during storage and handling. Recent work suggests that long-term exposure of pyrite-bearing sediments can lead to a decline in apatite P and an increase in ferric Fe-bound P. Here, we report on alterations in P speciation in reducing modern Baltic Sea sediments that we deliberately exposed to atmospheric oxygen for a period of either one week or one year. During oxidation of the sediment, extensive changes occurred in all measured P reservoirs. Exchangeable P all but disappeared during the first week of exposure, likely reflecting adsorption of porewater PO4 by Fe(III) (oxyhydr)oxides (i.e. ferric Fe-bound P formation). Detrital and organic P were also rapidly affected: decreases in both reservoirs were already observed after the first week of exposure to atmospheric oxygen. This was likely because of acidic dissolution of detrital apatite and oxidation of organic matter, respectively. These processes produced dissolved PO4 that was then scavenged by Fe(III) (oxyhydr)oxides. Interestingly, P in authigenic calcium phosphates (i.e. apatite: authigenic Ca-P) remained unaffected after the first week of exposure, which we attributed to the shielding effect of microfossils in which authigenic Ca-P occurs in Baltic Sea sediments. This effect was transient; a marked decrease in the authigenic Ca-P pool was observed in the sediments after one year of exposure to oxygen. In summary, we show that handling and storage of wet sediments under oxic conditions can lead to rapid and extensive alteration of the original sediment P speciation.
Highlights
There exists a suite of geochemical characterizations that are used to reconstruct the conditions under which marine sediments have been deposited, both in modern and ancient marine systems
Most labile P phases are converted into apatite (Ca-P), which is the principal long-term sediment P reservoir [3,4]
Our results show that exposure of reducing sediment samples to atmospheric oxygen can lead to rapid and comprehensive alteration of sediment P speciation
Summary
There exists a suite of geochemical characterizations that are used to reconstruct the conditions under which marine sediments have been deposited, both in modern and ancient marine systems. Of particular interest is the redox state at the sediment-water interface during sediment deposition, as this greatly affects the mobility and cycling of essential nutrients and metals. Phosphorus (P) speciation can be a valuable tool, because the chemical distribution of P in marine sediments is strongly dependent on the redox state of bottom water and sediment. The ferric Fe oxide-bound P pool forms through scavenging of dissolved phosphate (PO4) by Fe(III) (oxyhydr)oxides and can be an important P reservoir in oxic surface sediments. Ferric Fe-bound P is generally insignificant or absent in reducing sediments where Fe(III) (oxyhydr)oxides cannot form or persist. P speciation holds valuable information on redox conditions during sediment deposition and early burial. Most labile P phases are converted into apatite (Ca-P), which is the principal long-term sediment P reservoir [3,4]
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