Award: OCE-2023011

The deposition of wind-borne aerosol particles, which originate from natural sources and human activities, is an important delivery mechanism for biologically essential and pollutant chemical trace elements (TEs) to remote areas of the ocean that are separated by great distances from other sources (such as rivers). As one example, the main source of iron to the open ocean is desert dust, and iron deposition is of great interest because it is an essential micronutrient and its availability limits phytoplankton growth in approximately 40 percent of the global ocean. Aerosols are absorbed by the ocean via dry deposition in which dry particles settle on the surface and through wet deposition in which particles are washed out of the atmosphere by rain. Quantifying trace element deposition to the open ocean has always been difficult because it is impractical to be on site for an extended period of time to collect samples. For this reason, scientists rely on models and isotopic tracers to estimate deposition rates. In this study we utilized the isotope beryllium-7 (Be-7) as a tracer of aerosol deposition. Be-7 is a naturally occurring radioactive isotope that is produced from the interaction of cosmic rays with oxygen and nitrogen atoms in the upper atmosphere and quickly attaches to aerosol particles. The deposition rate of Be-7 can be calculated from the amount, by area, of the isotope in the upper water column and its decay rate constant. The deposition rates of non-radioactive TEs, such as iron, lead, and aluminum, can then be estimated from the Be-7 flux and the ratio of the concentration of the TE to that of Be-7 on aerosol particles. The field work for this project was conducted in the Indian sector of the Southern Ocean aboard the French research vessel Marion Dufresne II. The cruise, which originated at La Runion Island on 11 January 2021 and concluded at Runion on 08 March, was part of the multidisciplinary, international GEOTRACES program. We collected 19 aerosol samples and seawater from 11 stations. Be-7 analysis was performed at Florida International University, and TEs were analyzed by our collaborator at Florida State University. The aerosol and seawater Be-7 data was used to calculate the aerosol bulk deposition velocity (Vb), which is the combined rate of removal from the atmosphere by wet and dry processes. We found a linear correlation of Vb with the rate of precipitation in samples collected north of 45 degrees south latitude, and this relationship is very similar to that observed in other regions including the Pacific, Atlantic, and Arctic Oceans. This suggests that wet deposition is more important than dry deposition and that precipitation rates based on satellite data can be used to remotely predict TE fluxes for much of the world ocean surface. Over the past ten years, our lab group has participated in similar expeditions to quantify atmospheric deposition in the Pacific, Indian, and Southern Oceans. When combined with results from this project, the full data set allows for a clearer understanding of the atmospheric inputs of natural and anthropogenic chemical elements across the vast Indo-Pacific basin. It also provides the direct observations necessary to test the accuracy of global atmospheric chemical transport models (GATCMs). Because aerosol deposition measurements over the open ocean are so sparse, these models have generally relied on parameters established from measurements over land, leaving the output ill-constrained when applied to ocean deposition. To this end, we developed a model specifically constrained by the Be-7 data. The results suggest that commonly used GATCMs may underestimate aerosol deposition by up to approximately 30 percent. Last Modified: 02/05/2025 Submitted by: MarkPStephens