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

Abstract

Carbonate concretions from two distinct settings have been studied for their petrography, carbon and oxygen stable isotope patterns, and lipid biomarker inventories. Siderite concretions are enclosed in a Paleocene-Eocene deep-marine succession with sandy to silty turbidites and marl layers from the Gosau Basin of Gams in northern Styria. Septarian calcite concretions of the southern Vienna Basin from the sandpit of Steinbrunn (Burgenland) are embedded in Upper Miocene brackish sediments, represented by calcareous sands, silts, and clays. Neither for the siderite, nor for the calcite concretions a petrographic, mineralogical, or stable isotope trend from the center to the margin of the concretions was observed, implying that the concretions grew pervasively. The δ 13 C values of the Gams siderite concretions (-11.1 to -7.5‰) point to microbial respiration of organic carbon and the δ 18 O values (-3.5 to +2.2‰) are in accordance with a marine depositional environment. The low δ 13 C values (-6.8 to -4.2‰) of the Steinbrunn calcite concretions most likely reflect a combination of bacterial organic matter oxidation and input of marine biodetrital carbonate. The corresponding δ18 O values (-8.8 to -7.9‰) agree with carbonate precipitation in a meteoric environment or fractionation in the course of bacterial sulfate reduction. Lipid biomarkers have been extracted before and after decalcification of the concretions in order to assess pristine signatures and to exclude secondary contamination. The siderite concretions did not yield indigenous biomarkers due to their high thermal maturity. The calcite concretions comprise abundant plant wax-derived long-chain n-alkanes, reflecting high terrestrial input. Bacterial-derived, terminally-branched fatty acids and hopanoids were found, but with overall low contents. The presence of framboidal pyrite, the moderately low δ 13 C values, and the biomarker inventory indicate that bacterial sulfate reduction contributed to the formation of the calcite concretions in a brackish environment. The low δ 13 C values of the siderite concretions, on the other hand, are best explained by bacterial iron reduction, since sulfate reduction and resultant hydrogen sulfide production would have inhibited siderite precipitation. This study documents a new example for an exception from the common pattern that siderite concretions preferentially precipitate in freshwater environments. The Gams siderite concretions formed within marine sediments, whereas the Steinbrunn calcite concretions formed in freshwater to brackish sediments. Karbonatkonkretionen aus verschiedenen Ablagerungsräumen wurden im

The microbially driven formation of siderite in salt marsh sediments

“…While there are studies looking at crystal morphology and isotopic composition of modern and ancient siderites (Abdul Hadi & Astin, ; Allison & Pye, ; Baumann et al, ; Moore, Ferrell, & Aharon, ; Raiswell & Fisher, ; Wittkop et al, ), and separate studies trying to nucleate siderite biotically or abiotically in the laboratory (Köhler, Konhauser, Papineau, Bekker, & Kappler, ; Mortimer, Galsworthy, Bittrell, Wilmot, & Newton, ; Mortimer & Coleman, ; Sanchez‐Roman et al, ), there are few studies that couple the analysis of modern siderite to laboratory experiments trying to grow it. In the East Anglian salt marshes (particularly along the north Norfolk coast, in the salt marshes named Warham, Stiffkey, and Blakeney), there are large siderite concretions (up to 20 cm in diameter) found actively growing in the sediment (Figure ).…”

“…A pH lower than 6 means carbonate precipitation will be significantly delayed, and if the pore fluid pH is greater than 7.2, precipitation of calcite or aragonite is favored (Roberts et al, 2013). The formation of siderite is often assumed to involve microbially mediated chemical reactions, such as bacterial iron reduction, that produce dissolved iron and generate alkalinity through the oxidation of organic carbon (Baumann, Birgel, Wargreich, & Peckmann, 2016;Van Lith et al, 2003;Roberts et al, 2013;Sel, Radha, Dideriksen, & Navrotsky, 2012). However, bacterial iron reduction alone tends to push the pH of the pore fluid higher than 7.2 (Soetaert, Hofmann, Middleberg, Meysman, & Greenwood, 2007;Zeng & Tice, 2014), which means that for siderite to form, more is needed than simply local conditions that favor bacterial iron reduction.…”

“…Despite some early studies exploring the mineralogy and solid-phase geochemistry of the siderite concretions in these salt marsh sediments, there has been less attention paid to the interplay between the microbes and aqueous geochemistry that leads to the precipitation of siderite (Allison & Pye, 1994;Coleman, Hedrick, Lovley, White, & Pye, 1993;Pye, 1984;Pye, Dickinson, Schiavon, Coleman, & Cox, 1990). Previous studies attribute the formation and growth of siderite concretions to high rates of "bacterial activity,' high organic matter content, tidal pumping, and high sedimentation rate (Baumann et al, 2016;Coleman, 1993;Pye, 1984;Pye et al, 1990). In this study, we examine the mineralogy and elemental distributions of siderite concretions collected from the East Anglian salt marsh sediment, particularly those of the Warham salt marshes, to better understand the geochemical conditions during concretion nucleation and subsequent growth.…”

Ichnofossils, Cracks or Crystals? A Test for Biogenicity of Stick-Like Structures from Vera Rubin Ridge, Mars

“…Bulbous structures (e.g., structure b in Figure 1) are embedded in the host rock of the stick-like structures. They are here interpreted as concretions, intended as confined bodies of clastic sediment lithified by authigenic minerals [57]. On Earth, formation of concretions is commonly mediated by biogenic processes such as microbial activity and the presence of organic matter-including mucus on burrow walls [58]-but fully abiotic processes can also explain the origin of some concretions [59].…”

A calcite crisis unravelling Early Miocene (Ottnangian) stratigraphy in the North Alpine–Carpathian Foreland Basin: a litho- and chemostratigraphic marker for the Rzehakia Lake System

“…Calcite was dissolved by a late diagenetic process, i.e. pore water chemistry was already independent from open water chemistry (Baumann et al 2016). 5.…”