Formation of siderite‐Mg‐calcite‐iron sulphide concretions in intertidal marsh and sandflat sediments, north Norfolk, England

Abstract

Concretions cemented mainly by siderite, Mg‐calcite and iron monosulphide are common in late Holocene marsh and sandflat sediments on parts of the north Norfolk coast. Field experiments have shown that the concretions are actively forming in reduced sediments in which sulphate‐reducing bacteria are active. δ13C values ranging from −3 to −11·8% (mean −5·9%0) suggest that the carbonate in the concretions is derived partly from marine sources and partly from microbial degradation of organic matter. δ18O values ranged from −6·4% to + 0·8% (mean −1·0%) suggesting that carbonate precipitated in porewaters ranging from pure sea water to‐sea water diluted with meteoric water. Chemical analysis of porewaters showed no evidence of significant sulphate depletion at the depth of concretion formation. Some concretions have formed around fragments of wood or metal, but others contain no apparent nucleus. In field experiments siderite, FeS and Mg‐calcite were precipitated around several different nuclei within a period of six months. We suggest that siderite may form wherever the rate of iron reduction exceeds the rate of sulphate reduction, such that insufficient dissolved sulphide is available to precipitate all the available dissolved ferrous iron.

Plant Paleoecology in Deep Time1

“…Regardless, many plant parts retain original biochemical information (Jahren et al, 2004b(Jahren et al, , 2006 that can be used for paleoecological restoration. In addition, such debris may act as the nucleating agent Volume 95, Number 1 DiMichele & Gastaldofor soil nodules or concretions (e.g., Pye et al, 1990), if bacterial activity and geochemical conditions permit. By their nature, rooted-in-place, autochthonous assemblages allow correlation between paleosols and the plant community under which they developed.…”

Microbially-driven formation of Cenozoic siderite and calcite concretions from eastern Austria

“…The growth of concretions of meter size is assumed to take millions of years (Sellés-Martínez, 1996). In great contrast, Pye et al (1990) described modern siderite concretions up to 40 cm in diameter, which were formed in few decades, because the host sediment, a salt marsh at the north Norfolk coast (UK), has only been deposited since World War II.…”

“…The equilibrium model suggests that an entire volume of sediment is uniformly supersaturated in carbonate, and concretions form wherever appropriate nuclei are present (Coleman and Raiswell, 1995;Raiswell and Fisher, 2000). However, only few concretions with nuclei have been documented (Pye et al, 1990;Thomka and Lewis, 2013), whereas the majority of concretions lacks nuclei (e.g. Siegel et al, 1987;Kiriakoulakis et al, 2000;Lash and Blood, 2004;Loyd et al, 2012a).…”

“…The authigenic phases of concretions that cement the background sediment are composed of carbonate, phosphate, silica, sulfide, sulfate, and iron oxide minerals (Coleman et al, 1985;Mozley, 1996;Yli-Hemminki et al, 2014). Most common parageneses are carbonate concretions in clayey or sandy horizons, quartz or chert in limestones, and pyrite in black shales (Coleman et al, 1985;Sellés-Martínez, 1996), and the most common concretion-forming minerals are carbonates such as calcite, Mg-calcite, Fe-calcite, siderite, and dolomite (Siegel et al, 1987;Pye et al, 1990;Mozley, 1996). Concretion size varies from millimeters to meters, and estimated growth rates vary from tens to hundreds up to thousands of years for decimeter-sized concretions (Duck, 1995;Pratt, 2001;Thomka and Lewis, 2013).…”

“…They form during shallow burial of sediments, preferably within the first tens of meters when the surrounding sediment is still unconsolidated, and are, thus, products of early diagenesis (Blome and Albert, 1985;Pye et al, 1990;Mozley, 1996;Raiswell and Fisher, 2004). Sellés-Martí-nez (1996) provided an elaborate classification scheme of various concretionary bodies, distinguishing between cements, veins, nodules, and concretions.…”