The Curious Case of Iron Oxide Nanoparticles in Nature: Determining confidence estimates in sediment records of environmental change

Abstract:

Modern topsoils, as well as many paleosols, display enhanced induced magnetizations compared to their parent materials. Such enhanced magnetism has been successfully correlated with local Mean Annual Rainfall (MAR), but a generally applicable deduction of MAR from magnetic properties remains difficult to establish. In fact, there are enough cases of observed departures from the above model to warrant a number of provisos that require knowledge of variability in parent material character, precipitation minus evaporation, and perhaps other local data. Another problem is that we lack the knowledge to construct a forward model that proceeds from fundamental parameters like mineralogy, particle size, non-stoichiometry and structural defects. Increasing knowledge about iron oxide, oxyhydroxide and sulfide nanoparticles, likely first stages of parent material alteration, should help in constructing competing forward models as defined above.

In this talk we try to learn from observed magnetic behavior of fully characterized synthetic ferrihydrite and lepidocrocite, their alteration to second phases such as goethite, magnetite, maghemite and hematite, as well as further alterations of some secondary phases to more stable ones. We find that the multiple pathways of alteration depend on structural order, impurities, non-stoichiometry and kinetic stability of end products. By ranking the effects of these parameters, we can try to modify current forward models of alteration based on thermodynamic stability of iron oxides in parent materials. The ultimate goal should be to determine degrees of confidence in our claims of past climatic or environmental conditions.