Principal investigator: Knud Dideriksen
Conducted at the Banfield group at University of California, Berkeley and at NanoGeoScience, University of Copenhagen
Before life began, 3.8 billion years ago, Earth’s Archean atmosphere was devoid of oxygen; carbon dioxide levels were high and many elements, such as iron, existed in reduced form. The sediments from those early times offer a record of life’s beginning and the appearance of free oxygen. Banded iron formations (BIF) consist of millimeter- to centimeter-thick layers that extend over kilometers. Experts still debate whether the iron minerals formed from Fe(II) that was oxidised abiotically by light, or by the activity of organisms, and the precise role of oxygen is unknown. As an alternative hypothesis, the banded iron formations may also originate form precipitation of Fe(II) bearing silicates or carbonates.
Together with colleagues, I studied the formation and transformation of iron carbonates and oxides as well as the stable metal isotope fraction that results from redox reactions involving Fe(II). At relatively high supersaturation, iron carbonate nucleates as an amorphous solid, possibly through liquid-liquid separation like proposed for amorphous calcium carbonate. With time the amorphous iron carbonate crystalises as siderite. At the onset of oxidative weathering of the continents, chromate is expected to have been released to the Fe(II) bearing oceans. Together with Lasse Døssing, we determined the Cr isotope fractionation that occurs when chromate becomes reduced by Fe(II) at conditions resembling those expected for ancient oceans. Interestingly, the data suggest that a mixed valent iron oxide, green rust, formed as a transient phase as the Fe(II) was oxidised.
Oxidation of Fe(II) is highly likely to have initially produced the metastable Fe(III) oxide ferrihydrite. In the final part of the project, we studied the dynamics of the interaction between Fe(II) and ferrihydrite. Our results showed that extensive equilibration of Fe isotopes in solution and solid occurred prior to ferrihydrite transformation. During this period, the ferrihydrite evolved to have longer range atomic ordering. When Fe(II) is bonded to ferrihydrite, studies show that it can transfer an electron to the solid. Furthermore, experiments and modelling for Fe(III) oxides in general indicate that the electron can hop through the solid as a small polaron. We hypothesise that our results reflect spatially two separated half reactions of i) oxidative ferrihydrite growth and b) reductive ferrihydrite dissolution occuring on the particle and linked through polaron hopping of electrons.