Grant: Discovery Grants Program - Individual

Summary of Proposed Research Sediment deposited in oceans 2.5 to 3.5 billion years ago in many ways was very different from that accumulating today. Without multicellular organisms, especially those with hard shells, sedimentation relied on minerals that precipitated from the water itself, and, of course, were delivered to the sea by rivers and wind. The non-significant amounts of oxygen in the atmosphere-ocean system promoted metals such as iron and manganese going into solution and the lack of organisms that extract elements such as silicon from seawater led to their saturation. This situation was further exacerbated by higher heat flow from the young interior of Earth causing the development of heightened hydrothermal activity in the upper crust and extensive hot springs delivering dissolved elements to seawater leached from the volcanic rocks underlying ocean basins. This resulted in seas saturated with dissolved silica and abundant Fe2+ and Mn2+ in solution. Iron formation accumulated on the bottom where conditions were favourable for iron and/or silica oxides to precipitate. Considering our primary source of iron ore are these iron formations, little is known of the factors that promoted their genesis. Previous work by my group on layers of magnetite in 2.7 billion year old delta top sediment explored why iron oxides were deposited in this setting. It is likely that at times of low sediment delivery between flood events iron oxides formed and rained on the bottom. When the river avulsed and moved away 73 meters of iron oxides were deposited. As in deltas today, such as the Mississippi, the Archean river probably brought nutrients to the shoreline that promoted oxygen production by the photosynthesizing organisms. This would have produced the iron oxides that rained down on the bottom. We are working at present on numerous cored drill-holes through the offshore equivalent of this delta system. This work will continue into the next grant period and be added to by graduate students describing and sampling cored drill-holes through: 1) thin iron formations that were deposited along a braidplain dominated coastline, 2) iron formation deposited offshore from a stromatolite-rich shallow carbonate shelf, 3) deep ocean iron formation deposited in a submarine fan/ramp complex, and 4) volcanic associated deep ocean iron formation not associated with siliciclastics. The core from multiple drill-holes available to us at each of these sites will allow comparison of the sedimentation style and geochemistry of the iron oxides between the sites, and also laterally and vertically within each site. From this models can be developed to explain why chert layers are only present in deep water iron formation, why when present the chert layers regularly alternate with the iron oxide layers and a host of other questions. These types of studies have not been done previously and will lead to a better understanding of how this type of poorly understood rock formed.