COMPETITIVE REDUCTION OF FERRIHYDRITE AND NONTRONITE AND IMPLICATIONS FOR LACUSTRINE SEDIMENTS

Detrital Fe (III)-bearing minerals (e.g. ferrihydrite) are often converted to Fe (II)-phases (e.g. pyrite, siderite) in lake basins through reduction in organic-rich, oxygen-depleted sediment. Various phases may subsequently form depending on complex redox reactions in diagenetic pore waters, including greigite, goethite, hematite, magnetite and others. These processes are poorly understood, but are important contributors to the mineral record in lacustrine sediments, affecting our understanding of paleolimnology and paleomagnetic records.

To better understand the diagenetic history of Fe-bearing phases in lakes, we are conducting abiotic water-rock interaction experiments to isolate variables identified from natural systems. Fe-bearing phases such as ferrihydrite (fhy) and clay minerals such as Fe rich nontronite (nont.) were used in this experiment. We performed abiotic reduction experiments in carbonate brine to test the hypothesis that ferrihydrite reduction is kinetically more favorable than that of nontronite. Trials were run with varying mineral proportions and trial durations. Samples had 100% nont, 80/20 nont/fhy, 50/50 nont/fhy, 20/80 nont/fhy and 100% fhy. Trials one through three ran two, six and ten hours respectively. Samples were reduced using a sodium dithionite-citrate-bicarbonate solution (Stucki et al, 1984). We characterized the relative abundance of Fe-bearing phases pre- and post-reduction by detailed X-ray diffraction analysis.

The sample containing 100% nontronite exhibited small changes in its octahedral layer after each trial. In trial one through three, 060 reflection values shifted from 1.5012 Å, 1.5015 Å and 1.5019 Å. This suggests partial reduction of a component of the Fe (III) in its octahedral layer. There was no siderite production from the reduction of nontronite, suggesting that Fe (II) remained in the smectite structure rather than being released into solution. In contrast, every sample containing ferrihydrite showed loss of ferrihydrite with subsequent precipitation of siderite (Fe (II)-carbonate). These results indicate the importance of ferrihydrite in lacustrine iron biogeochemistry, with important implications for interpreting the sedimentary mineralogy and paleomagnetic records of lacustrine sediments.