This project investigates the behavior of the natural isotopes of iron (Fe) and molybdenum (Mo) in soils. Differences in the mass of individual isotopes of Fe and Mo confer small variations in their behavior. Laboratory experiments suggest the direction and magnitude of these variations in isotope behavior in soils are tied to specific biogeochemical processes associated with the presence of soil organic matter, the composition of Fe minerals in the soil, and the soil oxygen status. However, field measurements of these isotopes are extremely limited in soils and this knowledge gap is a key barrier for developing them as diagnostic indicators of soil biogeochemical processes in ways similar to how carbon, oxygen, and nitrogen isotopes are commonly used. For instance, Fe and Mo isotopes hold great promise for determining past responses of soils to changes in climate, vegetation type, and water content. Our field measurements will be made across well-constrained gradients in soil age and climate that naturally exist across the Hawaiian Islands (Hawaiian Climate-Age Matrix). The Hawaiian Islands are an ideal natural laboratory for soil research because of their extreme isolation and a similar basaltic soil parent material across all the islands. In this research project we will: (1) establish the range of Fe and Mo isotopic variation across the Hawaiian Climate-Age Matrix; (2) characterize the composition of Fe minerals using advanced molecular-scale techniques; and (3) establish the relative influence of organic matter, Fe minerals and soil redox status (water and oxygen content) on Fe and Mo isotopic variation. In addition to developing a new isotopic probe of key soil processes, our work will provide detailed knowledge of the Fe solid-phase composition of the Hawaiian Climate-Age Matrix sites, which will benefit ongoing ecological and earth science research at those sites. It will also provide constraints on the baseline isotopic composition of Fe and Mo efflux from terrestrial weathering to rivers and oceanic sediments, which is critical for the use of these isotopes to interpret the past redox status of ancient marine sediments. Lastly, this work will train two graduate students and two undergraduate students over the three-year grant period as well as provide summer research experiences for three students each year from disadvantaged high schools.
In our part of this project, a specialized analysis tool, named Mossbauer spectroscopy, was used to identify iron oxide minerals in soils of different ages collected in Hawaii. This tools identifies the charge or valence (also known as the oxidation state or redox status) of the iron ion in the minerals. It also gives information about the location of other iron ions nearby and the type of electrostatic field that surrounds the iron ions. These measurements allow us to identify, at least in most cases, the specific type of iron mineral that is present. That location from which the soil samples were collected is ideal for studying the effects of climate and age on soils because of the very high variability in altitude and soil origin. Our results revealed specific differences in the types of iron minerals that were present, which allowed the other investigators in the porject to draw important conclusions about the age and history of the soils studied. They also compared the results from the iron study with a separate study they did of molybdenum (Mo). Redox manipulation experiments revealed an increase in labile Mo generated by short-term reducing conditions, with the mass of Mo mobilized increasing with increasing mean annual precipitation into the soil, despite a large drop in the abundance of potentially reducible Fe-(oxyhydr)oxide minerals onto which the Mo was hypothesized to be adsorbed. Our measurement of iron reduction in the field using passive sampling techniques is consistent with our hypothesis that soil Fe reduction potential increases as rainfall abundance increases. The outcome of this project is that we learned how to refine the operation of our instruments for studying iron. These instruments are extremely sensitive to vibrations (even those unnoticed by normal ambient conditions). This refinement led to results that could be interpreted at the highest possible level.
