Modeling Dust and Dissolved Iron Deposition to the Southern Ocean

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Title: Modeling Dust and Dissolved Iron Deposition to the Southern Ocean
Author: Johnson, Matthew Stephen
Advisors: Yang Zhang, Committee Member
David DeMaster, Committee Member
Nicholas Meskhidze, Committee Chair
Abstract: Aeolian dust deposition has proven to be a critical source of iron to the high nitrate low chlorophyll oceanic regions around the globe. This research was conducted to quantify mineral dust and dissolved iron fluxes to the Southern Atlantic sector of the Southern Ocean, which is postulated to be the largest oceanic region where marine productivity is limited by the micronutrient iron. To quantify mineral dust and dissolved iron fluxes the 3D global chemistry transport model GEOS-Chem, implemented with a prognostic iron dissolution scheme (GEOS-Chem/DFeS), was applied to the Patagonia and South Atlantic Ocean domain between October 2006 and September 2007. Our model simulations are focused on topics such as the model performance of Patagonian dust mobilization, transport, and deposition to the South Atlantic Ocean, mineral iron dissolution within advecting Patagonian dust and deposition of soluble iron to the surface waters of South Atlantic Ocean. Sensitivity simulations were conducted to estimate the magnitude and rates of dissolved/soluble iron deposited associated with natural sources of SO2 (volcanic emissions and oxidation of Dimethyl Sulfide) along with different mineralogical compositions of Patagonian soil. Daily, monthly, seasonally, and yearly averaged model outputs of mineral dust emissions, transport, and deposition with associated iron dissolution and deposition would be compared to past literature, Patagonian dust reports, MODIS real-time imagery, and MODIS AOD values to better understand the performance of GEOS-Chem/DFeS. GEOS-Chem deposited an annual magnitude of ~18 Tg of mineral dust to the South Atlantic Ocean during our yearlong simulation. This proved to fall within the range of annual dust deposition estimated by past modeling and measurement studies focused on our researched domain. The model also demonstrated the ability to capture dust advection and deposition seasonality, while being in compliance with climatic conditions favorable for dust emission and transport, in-situ mineral dust measurements in Antarctic, and MODIS AOD variations. It has been proposed that the majority of dust can be deposited to the world’s oceans in just a few large episodic dust outbreaks annually. GEOS-Chem predicted that during low dust concentrations over 65% of mineral dust can be deposited to the South Atlantic in the top 5% of dust deposition days. The fact that GEOS-Chem was in agreement with 75% of large dust storms reported in Patagonia provided confidence that the model accurately simulates large dust outbreaks. During our yearlong model simulation and analysis on mineral dust treatment, we show that GEOS-Chem is capable of capturing dust source regions in Patagonia, transport pathways, and the seasonality of mineral dust deposition to the South Atlantic Ocean. To simulate iron dissolution during atmospheric transport an iron dissolution scheme was implemented to the base model GEOS-Chem. The model simulates dissolution/precipitation of each mineral, within dust aerosols, using calculations based on solution pH, temperature, and the specific surface area of the mineral. During my research the iron dissolution module was implemented into the newest version of GEOS-Chem (v8-01-01) and improved with the prescribed mineralogical composition of dust and specified Fe content of different clay minerals and iron oxides to represent realistic values of Patagonian topsoil. Also we implemented equations that would now allow for iron dissolution within smectite and illite (clay minerals). GEOS-Chem/DFeS calculated a yearly total of ~0.012 Tg of bioavaliable iron was deposited to the South Atlantic surface waters. It was also shown through evaluation of dissolved iron fractions that acid mobilization of Patagonian dust is limited due to the lack of anthropogenic acidic trace gases, which is most likely due to the pristine nature of our researched domain. However, with the initial iron solubility prescribed to be 0.45%, any amount of elevated dust deposition influenced oceanic primary productivity through production of chlorophyll and the related decrease in CO2 concentrations. Sensitivity studies were conducted to better understand the influence of clay laden-iron dissolution, initial iron solubility, doubling of natural SO2 sources and volcanic eruptions on the magnitude and spatial patterns of DFe deposition to the SAO. While clay mineral dissolution did prove to have a substantial impact on dissolved iron production, initial iron solubility proves to the dominant source of dissolved iron deposited to the South Atlantic Ocean. Overall GEOS-Chem/DFeS displayed the capability to simulate both total mineral dust and dissolved iron deposition to the South Atlantic Ocean. However, the extreme lack of in-situ measurements of both dust and dissolved iron fluxes did not allow for the analysis of the actual accuracy of the models performance. With the improved iron dissolution scheme and the ability to quantify chlorophyll production and CO2 sequestration due to mineral dust deposition, this model proves to have the ability to be applied to many future research experiments involving topics such as future climate change. In the future, increased knowledge of the relationship between atmospheric deposition of mineral dust and oceanic biological processes could be obtained through further modeling studies such as the one conducted during my master’s thesis research.
Date: 2010-04-27
Degree: MS
Discipline: Marine, Earth and Atmospheric Sciences

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