Pyrite accumulation in salt marshes in the Eastern Scheldt, southwest Netherlands

Abstract

Pore water composition, pyrite distribution and pyrite crystal morphology of sediments from salt marshes in the Eastern Scheldt, southwestern Netherlands, were examined from July 1984 to October 1986. Hydrology and marsh vegetation were the chief determinants of pyrite accumulation. In the bare sediments of pans in the low marsh, highly reducing conditions prevailed just below the surface. At these sites, practically all the incoming detrital pyrite (0.5–1% FeS2) was preserved. The in-situ formation of pyrites was negligible in these anoxic sediments. All incoming detrital pyrite was oxidized in the surface layers (0–10 cm) of the medium-high marsh overgrown withSpartina anglica. Pyrite was formed at a rate of 2.6–3.8 mol S-FeS2m−2yr−1 in a narrow range of depths (15–20cm), at the interface of the oxidizing and underlying reducing sediment. At this interface the concentration profiles of Fe2+ and dissolved ΣS intersected. The role of the rhizosphere is discussed in connection with pyrite formation. No further pyrite formation occurred deeper in the sediment. This resulted in the build up of high concentrations of dissolved ΣS and acid volatile sulfides (AVS). The decrease with depth in oxalate-extractable Fe indicated that most of the iron oxyhydroxides (70–80%) had been transformed to pyrite. Another 10–20% of oxalate-extractable Fe was present as AVS. The abundance of framboidal pyrite particles and the high concentrations of AVS and dissolved ΣS indicated that the formation of pyrite occurred via iron monosulfide intermediates There was a linear relationship between the organic carbon and the S-FeS2 content in theSpartina overgrown reducing sediment. The mean C/S ratio was 4.2.