THE EFFECT OF HYPOTHETICAL PRECAMBRIAN OCEAN CHEMISTRY ON THE PHOTO-OXIDATION OF FE(II) AND SIGNIFICANCE FOR BANDED IRON FORMATIONS

Banded iron formations (BIFs) are spatially extensive chemical sedimentary deposits of alternating iron-rich and iron-poor layers most commonly found throughout the Precambrian (3.8? – 1.5 Gya), with sporadic occurrences observed in the late Neo-Proterozoic (~ 0.7 Gya). The iron minerals present are typically hematite (Fe2O3) and magnetite (Fe3O4) but iron-carbonate and iron-silicates can also be important constituents. It is generally accepted that the bulk of Fe(II) was supplied to the oceans by hydrothermal vents and would have remained stable under an anoxic Precambrian atmosphere. The many hypotheses for the mechanism of Fe-oxidation leading to the precipitation of BIF generally fall into two categories: (1) biogenic models, where oxygenic photosynthesis by cyanobacteria yields sufficient oxygen concentrations to oxidize Fe(II) locally, or anoxygenic photosynthetic bacteria that oxidize Fe(II) directly, and (2) abiogenic models, such as the photo-oxidation of Fe(II) by short-wave UV radiation.

In this work we explore the effect of varying ocean geochemistry on the leading abiogenic model by experimentally irradiating complex solutions of Fe(II) with UVC light at assumed Precambrian intensities (200 – 280 nm, ~ 0.03 mW/cm2). Experiments are conducted in 0.56 M NaCl in quartz Erlenmeyer flasks constantly bubbled with Ar to maintain anoxia and mix solutions. Control experiments are conducted in UV-opaque borosilicate reaction vessels under identical experimental conditions. Variables to be tested include Fe concentration (10 ppm and 100 ppm), pH (pH ~3, pH ~ 7), silica (absent and saturated) and pCO2 (absent and 0.03 atm as NaHCO3). Samples for pH, Fe(II) and Fe(III) are obtained every 24 h for 96 h. The photo-oxidation of Fe(II) is observed at pH ~ 3 and pH ~ 7 as evidenced by a decrease in Fe(II) concentration, an increase in Fe(III) and the visible presence of Fe-oxide precipitates. Concomitant to changes in Fe distribution, decreases in pH are observed, consistent with the hydration of Fe(III) and precipitation of Fe(OH)3. Initial experimental data show the effect of silica saturation is two-fold; both the photo-oxidation of Fe(II) as well as the precipitation of Fe minerals is inhibited. Data will also be presented showing the effect of variable atmospheric CO2.