The Effects of Regional Oceanographic Conditions on the Geochemistry of Ferromanganese Crusts from the West-Central Pacific Ocean

Abstract

Ferromanganese (FeMn) crusts are of interest and clear importance both as a unique paleoceanographic archive of seawater chemistry and as a potential resource for valuable and critical metals. Presently, there is a need to shift FeMn crust research from purely exploration-based studies to more targeted expeditions that can precisely locate FeMn crusts with characteristics of interest for scientific research and resource evaluation. Understanding the influence of general oceanographic conditions on FeMn crust formation is key to defining permissive regions. This thesis investigates variations in the geochemistry of FeMn crusts from the west-central Pacific that are associated with regional oceanographic conditions, including proximity to the oxygen minimum zone and high biological productivity maintained at the equator, as well as water depth controls and the effects of diagenetic phosphatization. Chemical analysis of over 60 FeMn crusts reveals that the oxygen minimum zones in the northern and southern tropics in the west Pacific exhibit strong controls on the composition of recent FeMn crust growth as well bulk FeMn crusts. Manganese contents are increased near the oxygen minimum zones, which is shown quantitatively for a broad region in this thesis for the first time. Iron is instead enriched in deeper waters where oxidation of iron is increased, which may be caused by higher carbonate ion concentrations and pH at depth and the availability of deep sources of iron. High biological productivity in the equatorial upwelling zone enriches iron, magnesium, and barium in FeMn crusts that are forming below this zone due to increased efficiency of the biological pump. Diagenetic phosphatization is commonly observed in crusts from depths above 2000 meters, but does not exceedingly dilute manganese oxides or elements of economic interest. Synchrotron analyses of phosphatized FeMn crust layers indicate that phosphatization does not change the speciation of lead as hypothesized based on previous sequential leaching studies. Overall, is it shown that the general oceanographic parameters studied here influence FeMn crust composition based on their geographic location and water depth, and these parameters can help create criteria for future crust exploration.