Siderite cannot be used as CO2 sensor for Archaean atmospheres

Gäb, Fabian, Ballhaus, Chris, Siemens, Jan, Heuser, Alexander, Lissner, Moritz, Geisler, Thorsten and Garbe-Schönberg, Dieter (2017) Siderite cannot be used as CO2 sensor for Archaean atmospheres Geochimica et Cosmochimica Acta, 214 . pp. 209-225. DOI 10.1016/j.gca.2017.07.027.

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It was proposed to utilize siderite FeCO3 in mid to late Archaean Superior type banded as a proxy to constrain the CO2 partial pressure of Archaean atmospheres. Implicit in this proposition is that siderite was a primary carbonate mineral that crystallized directly from Fe2+ enriched Archaean seawater, in equilibrium with atmospheric CO2. To our knowledge that proposition has not been demonstrated to be valid. We test with water-gas exchange experiments under controlled CO2 partial pressures if siderite can be stabilized as a primary mineral in Fe2+ bearing seawater. Reduced seawater proxies enriched in Fe2+ and Mn2+ are equilibrated with reduced N2-CH4-CO2-H2 gas phases with variable CO2. The solid phases stabilized in Fe2+ enriched water compositions are amorphous ferrous iron hydroxy carbonates. Crystalline siderite FeCO3 is not found to be a stable phase. The phases precipitating from Mn2+ enriched water include crystalline rhodochrosite MnCO3 and possibly amorphous Mn-enriched phases. Based on these results we advise against using siderite in banded iron formations as a CO2 sensor for the Archaean atmosphere.

Document Type: Article
Keywords: Archaean atmosphere, Archaean seawater, Banded iron, formations Siderite
Research affiliation: OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-MG Marine Geosystems
Kiel University
Refereed: Yes
DOI etc.: 10.1016/j.gca.2017.07.027
ISSN: 0016-7037
Date Deposited: 06 Sep 2017 08:25
Last Modified: 19 Dec 2017 12:42

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