Die Auswirkungen des <i>CO₂</i>-Gehaltes der Archaischen Atmosphäre auf die Bildung von Siderit in Superior type Banded Iron Formations

Es wurde vorgeschlagen, Siderit (FeCO3) Archaischen Superior Type Banded Iron Formations zu verwenden, um eine Bestimmung des CO2-Partialdrucks in der Archaischen Atmosphäre durchzuführen. Dies impliziert, dass Siderit ein primäres Carbonatmineral ist, das direkt aus mit Eisen angereichertem, archaischem Meerwasser im Gleichgewicht mit dem atmosphärischem CO2 kristallisiert. Nach Experimenten, die in dieser Arbeit präsentiert werden, ist diese Hypothese nicht haltbar. Mit Wasser-Gas-Austausch-Experimenten unter kontrolliertem CO2-Partialdruck wurde getestet, ob Siderit als primäres Mineral in eisenhaltigem Meerwasser stabilisiert werden kann. Mit Eisen und Mangan angereicherte reduzierte Meerwasser-Proxys wurden mit reduzierten N2-CH4-CO2-H2-Gasphasen mit variablem CO2 reagiert. Die festen Phasen, die in eisenhaltigen Wasserzusammensetzungen stabilisiert werden können, sind amorphe Eisenhydroxycarbonate. Kristallines Siderit ist keine stabile Phase. Die Phasen, die aus mit mit Mangan angereichertem Wasser ausfallen, umfassen kristallines Rhodochrosit MnCO3 und möglicherweise amorphe mit Mangan angereicherte Phasen. Basierend auf diesen Ergebnissen muss davon abgeraten werden, Siderit in gebänderten Eisenformationen als CO2-Sensor für die archaische Atmosphäre zu verwenden. Stattdessen zeigen die Ergebnisse, die in dieser Arbeit vorgestellt werden, dass die realistischste Quelle für die Siderite in Banded Iron Formations eine Redoxreaktion zwischen organischem Material und primärem Hämatit ist. Die Fraktionierung von Eisen und Mangan, die in Banded Iron Formations beobachtet wird ist, basierend auf Experimenten, die in dieser Arbeit vorgestellt werden, eine kontinuierliche Oxidation des Archaischen Ozeans durch Sauerstoff, der von frühem, mikrobiellem Leben produziert wurde.Experimental data from siderite formation in Superior type BIFs on the CO2partial pressure in the Archaean atmosphere Siderite bands in mid to late Archaean Superior type Banded Iron Formations have been suggested as an indicator for the CO2-content of the Archaean atmosphere. Based on thermodynamical modelling it was calculated, that the these FeCO3 minerals suggest an CO2 partial pressure of around 35.000 ppmV in the atmosphere (Ohmoto et al. 2004). A significantly increased CO2-content in the Archaean is often advocated as a compensator for the so called “Faint young sun effect” . This effect describes the decreased solar luminosity in the earlier years of Earth’s history. Due to the lower radiation heat transmitted to earth by the sun, the surface temperatures should have been lower than today. Oxygen isotopes however show a rather increased surface temperature in the Archaean. To solve this paradox a massive “Greenhouse Effect” has been suggested. Massively increased CO2-contents would impose a warmer climate and thus compensate the lower solar luminosity. However aside from the theoretical constraints on the CO2-content of the Archaean atmosphere actual evidence from proxies remain rare. Therefore siderite would be an important indicator for CO2. Implicit in this proposition is the premise that these siderites in Archaen Banded Iron Formations are primary precipitates of an ocean water that has been in chemical equilibrium with the atmosphere at the time of their formation. This assumption has not been tested experimentally yet. In this thesis novel experiments are described, that test the possibility of siderite being precipitated from a Fe-enriched, reduced seawater proxy in equilibrium with a reduced, high CO2 atmosphere. To achieve this a seawater proxy was made from MilliQ®-water, NaCl and sodium-bicarbonate (NaHCO3) and enriched in Iron and Manganese. The proxy was flushed with N2-CH4-H2 atmospheres with varying CO2-content (0, 1000 and 5000 ppmV). It was tried to precipitate siderite via oversaturating the proxy in terms of iron-carbonate. It was found that under no tested set of conditions the proxy actually precipitated siderite. Instead an amorphous iron-carbonate-hydroxy phase was generated, that is described in the literature as “green rust”. However the addition of Manganese to the proxy induced the precipitation of Rhodochrosite (crystalline MnCO3). The latter is especially important, since siderite bands in Banded Iron Formations are nearly Mn-free. Because no obvious reasons can be found for the Archaean seawater to be Mn-free, whilst being enriched in hydrothermal Fe, the precipitation of Rhodochrosite points against this Method of generation for the siderite beds. Together with the absence of crystalline siderite, this results leads to the conclusion, that siderite beds in Archaean Banded Iron Formation should not be used as a CO2-proxy for the Archaean atmosphere

Siderite cannot be used as CO2 sensor for Archaean atmospheres

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