Arsenic in framboidal pyrite from recent sediments of a shallow water lagoon of the Baltic Sea

Abstract

Arsenic is a redox‐sensitive element of environmental relevance and often enriched in iron sulphides. Because sediments from the Achterwasser lagoon, a part of the estuarine system of the river Oder, south‐west Baltic Sea, show unexpectedly high pyrite concentrations of up to 7·5 wt% they were used to investigate the influence of authigenic pyrite on the mobility and burial of As in the coastal environment. Micro‐X‐ray‐fluorescence measurements of 106 micrometre‐sized pyrite framboids from the anoxic sediments show highly variable As concentrations ranging from 6 to 1142 μg g−1. Even within a 1 cm thick layer, the As concentration of different framboids varies greatly and no clear depth trend is visible throughout the 50 cm long sediment core. Pyrite can account for 9 to 55% (average 22%) of the total As budget of the sediments and the degree of trace metalloid pyritization for As ranges from 26 to 61%, indicating that authigenic pyrite formation is an important process in the geochemical cycling of As in coastal sediments. High‐resolution micro‐X‐ray fluorescence mapping of single pyrite grains shows that As is distributed inhomogeneously within larger framboids, suggesting changing pore water composition during pyrite growth. X‐ray absorption near edge structure spectra indicate that As is usually present as As(‐I) substituting S in the pyrite lattice. However, in samples close to the sediment/water interface a considerable part of As is in higher valence states (+III/+V). This can be explained by frequent re‐suspension of the surficial sediments to the oxic water column due to wave action and subsequent re‐deposition, leading to the adsorption of As oxyanions onto pyrite. Although reduced As(‐I) becomes more important in the deeper samples, reflecting decreasing redox potential and a longer time since deposition, the occurrence of oxidized As species (AsIII/AsV) in pyrite in the anoxic part of the sediment suggests formation under dysoxic conditions.

Comparison of metal enrichment in pyrite framboids from a metal-enriched and metal-poor estuary

“…To avoid problems associated with specificity of reagents in partial digestion chemical analyses we employ LA-ICPMS analyses to directly determine the composition of trace metals in pyrite. To achieve similar results, synchrotron μ-XRF has been used to analyze trace element content of pyrite (Berner et al 2006;Neumann et al 2013). While synchrotron μ-XRF is preferable due to its superior detection limits and spatial resolution, we show in this paper that LA-ICPMS is a useful technique when the framboids analyzed are large enough to obtain an adequate signal.…”

“…First, pyrite can be an important sink for metals and metalloids (such as As) in contaminated rivers and estuaries (Lowers et al 2007). Second, trace metal contents of shale can be used to interpret paleoenvironmental conditions (Algeo 2004;Anbar et al 2007;Berner et al 2013;Tribovillard et al 2006) and since diagenetic pyrite is one of the sinks for trace metals in sediments (Dellwig et al 2002;Huerta-Diaz and Morse 1992;Neumann et al 2013), the processes that led to enrichment of trace metals in pyrite are important. Third, recent ore deposit models suggest that diagenetic pyrite is the source for several significant gold deposits (Large et al 2007(Large et al , 2009; Thomas et al 2011).…”

Natural and anthropogenic variations in atmospheric mercury deposition during the Holocene near Quelccaya Ice Cap, Peru

“…Deposition of volcanic sulfate and Fe to Yanacocha from this eruption may have provided adequate reactants for framboid formation within the lake’s surface sediments and sequestration of Hg from the water column or volcanic ash. Further evidence for volcanism at ~3.2 ka comes from highly enriched As hvy , Ag hvy , and Tl hvy concentrations in Yanacocha sediments (Figure 5b), which, in addition to having an affinity for framboidal pyrite [ Schoonen , 2004; Neumann et al , 2013], are also emitted predominantly from volcanic sources [ Kellerhals et al , 2010]. If volcanism were responsible for the sharp increase in Hg concentration at ~3.3 ka, then the Hg is retained in the less dense fraction of sediment (< 2.8 g cm −3 ) or in nanometer-scale particles because of near-constant % Hg hvy during this time (Figure 5b).…”

Nickel and iron partitioning between clay minerals, Fe-oxides and Fe-sulfides in lagoon sediments from New Caledonia

“…mainly pyrite FeS 2 , but also mackinawite FeS and greigite Fe 3 S 4 which are usually included in the generic term of Acid Volatile Sulfides or AVS) Huerta-Diaz et al, 1992;Roberts and Turner 1993;Gagnon et al, 1995;Passier et al, 1996;Butler and Rickard 2000;Rickard and Morse, 2005;Neumann et al, 2005;Alvarez-Iglesias et al, 2012;Botsou et al, 2015). Among Fe-sulfides, pyrite and mackinawite have been extensively investigated for their strong capacity at trapping trace metals, either through analysis of natural sediments or through laboratory-controlled (bio)synthesis experiments (Huerta-Diaz et al, 1992;1998;Morse and Luther, 1999;Rickard and Luther, 2007;Burton et al, 2008;Berner et al, 2013;Neumann et al, 2013;Diaz-de-Alba et al, 2016;Le Pape et al, 2016;Ikogou et al, 2017;Wilkin and Beck, 2017). However, several studies on natural sediments have also shown that trace metals can be associated with silicate minerals and reducible Fe-and Mn-oxides (Lopez-Sanchez et al, 1996;Jones and Turki 1997;Morillo et al, 2004;Yuan et al, 2004;Cuong and Obbard 2006).…”