In many regions of the world's ocean, primary productivity is not limited by the major nutrients (nitrogen, phosphorous and silica) but by the micronutrient iron (Fe). One major source of Fe is the atmospheric transport and deposition of aerosols to the open ocean. The aerosols come from natural sources, such as soils and dust and biomass burning, and from anthropogenic emissions related to industrial processes and energy generation. Our understanding of the sources is limited by our ability to identify the origin of the Fe. Mechanisms of tracing the sources of aerosols include the use of the elemental ratios as specific sources have specific elemental signals. Fe isotopic variation has recently been demonstrated to be a potentially important tracer of Fe sources.
This project, a collaboration between investigators at Arizona State University and Northern Arizona University, will explore the use of Fe isotopes as a tracer of natural and anthropogenic sources of aerosols to assess their importance as a source of Fe to the open ocean. Fe is known to limit primary production in many high nutrient, low chlorophyll areas, so it is important to understand the origin of the Fe that is delivered to the oceans and its availability to marine microorganisms. Additionally, aerosols from different sources have variable size and solubility in seawater and therefore this also impacts Fe bioavailability. Examination of the isotopes of Fe in aerosols could help address these questions as the investigators' prior research has demonstrated distinct variations in the isotopic composition of aerosol Fe that arise from natural and anthropogenic sources.
The study will measure the Fe isotopic compositions of aerosol particles collected on Bermuda over a period of one year. Bermuda was chosen as seasonal differences lead to different aerosol types being deposited - summer winds flow from the east and carry Saharan soil dust and other aerosols, while winter winds originate from over North America. The project will compare the Bermuda results with that of key anthropogenic and natural aerosol materials that could be a source of Fe to the Atlantic Ocean. In addition, elemental analyses of these aerosols will provide an independent confirmation of the Fe isotopes results. Analysis of size-segregated samples will provide additional information and will be coupled with solubility experiments designed to assess the soluble Fe fraction.
Broader Impacts: The scientific impact of this work relates to obtaining a better understanding of the factors that impact the sources and availability of Fe, an important limiting micronutrient, to the ocean, and to marine microorganisms. As a part of the proposed project, the investigators will develop interactive educational activities to teach the major concepts of ocean nutrient availability and limitation to non-science students, which will be part of a new course "Habitable Worlds". Additionally, the proposed project will support graduate student training, and benefit under-represented groups.
When we think about the Earth’s energy balance and climate change, it is not often that we consider algal production in the open ocean. In fact, uptake of carbon dioxide by algae and phytoplankton may help to reduce atmospheric levels of carbon dioxide. The growth rate of marine algae depends on the nutrients which are available for uptake. In roughly half of the world’s oceans, the element iron limits algae growth, and the primary source of iron to the Earth’s oceans is atmospheric. About 97 % of the iron which deposits onto the ocean is dust from the Earth’s deserts and arid regions. This dust gets resuspended and then transported. About 3 % of the iron depositing onto the ocean is contained in particles resulting from the combustion of gasoline, wood, and other combustion sources. This distinction is important because the iron present in the combustion sources is likely more available to aquatic organisms than the crustal sources. The primary goal of this project was to differentiate the types of iron (crustal vs. combustion) depositing on the ocean surface near Bermuda (see attached photo of the particle sampler). Bermuda is an ideal natural laboratory because, during the summer months, the air masses originate from Northern Africa, so the particles are primarily from the Sahara Desert. During the other months, the air masses originate from North America, which comprises a mixture of crustal and combustion sources. In this project, we learned that large particles have a crustal iron composition over the course of the entire year. In the small particles, we also observed crustal iron during the Saharan months. However, during the North American months, we observed iron which was not of crustal composition. The actual source of the non-crustal iron from North America remained elusive in this study. Because the non-crustal iron is contained in the small particles, it is most likely combustion-based, meaning that it is either automotive, industrial, or a result of forest fires. Still, we determined that there is a mixture of combustion and crustal iron in the air over the open ocean, and the source of the combustion iron requires further study. In this study, one graduate student was trained in trace metal atmospheric sampling techniques and trace metal laboratory and analysis techniques. In addition, this proposal led to one of the PI’s acquiring a position where they could teach and mentor undergraduate and graduate students, as well as start an independent research laboratory.
