Southern African dust characteristics and potential impacts on the surrounding oceans

Date
2021-12
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: This study supports understanding the potential impact of dust aerosols from southern Africa have on the proximal ocean ecosystems. Dust can release essential nutrients and thus fertilize the ocean, which affects the food-web and the carbon dioxide concentrations in ocean waters, i.e. climate. Dust that fertilizes the phytoplankton communities in open oceans stimulates the drawdown of carbon dioxide from the atmosphere through the process of photosynthesis. Key here is that the nutrients must be released from the aerosol particles, as they can only be uptaken by biological organisms in soluble form. However, mineral dust emitted from arid environments differ from dust emitted through industrial activities; as a result the solubility and, by extent, potential impact on the ocean may also differ. In this study, I investigated mineral characteristics and solubility of dust from three distinct regions, one with a strong human footprint (Saldanha Bay), one with mostly natural mineral dust (Namib Desert) and one off shore over the Southern Ocean. The first study targeted Saldanha Bay, a town that hosts the largest port in South Africa, with exports of up to 60 million tons of iron and manganese ore annually and is home of a steel plant and a smelter. Satellite images and photos from the area have shown extensive dispersion of dust from the area. Solubility leaching experiments revealed that dust collected in this town is highly soluble (bioaccessible) for trace metals such as Fe (up to 28%), Cu (up to 33%), Pb (up to 45%) and Zn (up to 38%). Phytoplankton communities in open oceans are sometimes depleted in these trace metals, and thus, such high solubility of dust from Saldanha Bay can prove to be an important nutrient supplier to surrounding oceans. In addition, air mass trajectories revealed that this readily available dust most likely affects the southeast Atlantic and Indian Ocean. The major implication was that harbour towns can be essential sources of trace metals to proximal oceans. Secondly, I present results from the largest non-playa environment in the Namib Desert. Some of the prominent dust emitters located in the Namib Desert provide, feed and source of hundreds of tons of mineral dust to proximal surface ocean waters. The solubility of the mineral dust, in contrast to the mixed (anthropogenic and natural sourced) dust mentioned above, is much lower (e.g. up to 2 % for Fe, up to 7.8 % for Cu, up to 16% for Zn, up to 9% for Mn, 8% for Pb). These solubilities are in similar range to other semi-arid to arid major global dust sources such as the Sahara Desert in the Northern Hemisphere. Differences in mineralogy and particle size distribution at the sampling stations are the main drivers of the solubility. For example, the solubility of trace metals such as Fe, Zn and Pb is higher at the inland station than at the coastal station. The aggregated particles along the coastal stations are less soluble probably because of persistent foggy conditions and less sunlight. The inland stations had less foggy conditions and more fine-grained particles which are rich in FeO oxides. Air mass trajectory modelling indicated that this mineral dust, with its low solubility, typically travels towards the southeast Atlantic Ocean, but can also reach the nutrient poor areas in the Southern Ocean in some cases. This study highlighted the role of non-playa environments as important dust sources and that mineralogy coupled with particle size are closely related to trace metal solubility. In addition, to assessing the solubility and potential impact of dust with natural and anthropogenic imprint, we investigated the solubility of aerosols collected over the Southern Ocean south of South Africa on board on the research vessel SA Agulhas II. The solubility of the trace metals over the Southern Ocean ranges from 0.5 to 41% Fe, 9 to 48% Al, 0.1 to 88 % Mn, 25 to 72 % Zn and 3.3 to 39 % Co. This study found large variability in trace metal composition and fractional solubility in dust aerosols collected from three distinct environments in southern Africa and south of southern Africa. Our study highlighted the importance of southern African dust of both anthropogenic and natural sources as a nutrient supplier to surrounding coastal and open oceans.
AFRIKAANSE OPSOMMING: Hierdie studie ondersteun die begrip van die potensiële impak van stof-aërosole van Suider-Afrika op die proksimale oseaan-ekosisteme. Stof kan noodsaaklike voedingstowwe vrystel en sodoende die see bemes, wat die voedselweb en die kool stof dioksied konsentrasies in seewater, dit wil sê klimaat, beïnvloed. Stof wat die fitoplankton gemeenskappe in oop oseane bemes, stimuleer die onttrekking van koolstofdioksied uit die atmosfeer deur die proses van fotosintese. Sleutel hier is dat die voedingstowwe uit die aërosol deeltjies vrygestel moet word, aangesien dit slegs deur biologiese organismes in oplosbare vorm opgeneem kan word. Minerale stof wat uit droë omgewings vrygestel word verskil egter van stof wat deur industriële aktiwiteite vrygestel word; gevolglik kan die oplosbaarheid en, in mate, potensiële impak op die see ook verskil. In hierdie studie het ek mineraalkenmerke en oplosbaarheid van stof van drie afsonderlike streke ondersoek, een met 'n sterk menslike voetspoor (Saldanhabaai), een met meestal natuurlike minerale stof (Namib-woestyn) en een van die kus af oor die Suidelike Oseaan. Die eerste studie was gerig op Saldanhabaai, 'n dorp wat die grootste hawe in Suid-Afrika huisves, met uitvoere van tot 60 miljoen ton yster- en mangaanerts jaarliks en is die tuiste van 'n staalaanleg en 'n smelter. Satellietbeelde en foto's van die gebied het uitgebreide verspreiding van stof uit die gebied getoon. Oplosbaarheidslogingseksperimente het aan die lig gebring dat stof wat in hierdie dorp versamel word hoogs oplosbaar (biotoeganklik) is vir spoormetale soos Fe (tot 28%), Cu (tot 33%), Pb (tot 45%) en Zn (tot 38%) %). Fitoplankton gemeenskappe in oop oseane is soms uitgeput in - bane aan die lig gebring dat hierdie geredelik beskikbare stof heel waarskynlik die suidoos-Atlanties hierdie spoormetale, en so 'n hoë oplosbaarheid van stof van Saldanhabaai kan dus 'n belangrike voedingstofverskaffer aan omliggende oseane blyk te wees. Daarbenewens het lugmassa e en Indiese Oseaan affekteer. Die belangrikste implikasie was dat hawedorpe noodsaaklike bronne van spoormetale tot proksimale oseane kan wees. Tweedens bied ek resultate van die grootste nie playa-omgewing in die Namibwoestyn aan. Sommige van die prominente stofuitstralers wat in die Namib-woestyn geleë is, verskaf, voed en bron van honderde tonne minerale stof na die proksimale oppervlak-oseane water. Die oplosbaarheid van die mineraalstof, in teenstelling met die gemengde (antropogenese en natuurlike afkomstig) stof hierbo genoem, is baie laer (bv. tot 2 % vir Fe, tot 7.8 % vir Cu, tot 16 % vir Zn, tot en met 9% vir Mn, 8% vir Pb). Hierdie oplosbaarheid is in soortgelyke omvang as ander semi-droë tot dorre groot globale stofbronne soos die Sahara-woestyn in die Noordelike Halfrond. Verskille in mineralogie en deeltjiegrootteverspreiding by die monsternemingstasies is die hoofdrywers van die oplosbaarheid. Byvoorbeeld, die oplosbaarheid van spoormetale soos Fe, Zn en Pb is hoër by die binnelandse stasie as by die kusstasie. Die saamgevoegde deeltjies langs die kusstasies is minder oplosbaar, waarskynlik as gevolg van aanhoudende mistige toestande en minder sonlig. Die binnelandse stasies het minder mistige toestande gehad en meer fynkorrelige deeltjies wat ryk is aan FeO oksiede. Lugmassa- trajekmodellering het aangedui dat hierdie mineraalstof, met sy lae oplosbaarheid, tipies na die suidoostelike Atlantiese Oseaan beweeg, maar in sommige gevalle ook die voedingsarm gebiede in die Suidelike Oseaan kan bereik. Hierdie studie het die rol van nie-playa omgewings as belangrike stofbronne uitgelig en dat mineralogie tesame met deeltjiegrootte nou verwant is aan spoormetaaloplosbaarheid. Daarbenewens, om die oplosbaarheid en potensiële impak van stof met natuurlike en antropogenese afdruk te bepaal, het ons die oplosbaarheid van aërosols wat oor die Suidelike Oseaan suid van Suid- Afrika aan boord op die navorsings vaartuig SA Agulhas II versamel is, ondersoek. Die oplosbaarheid van die spoormetale oor die Suidelike Oseaan wissel van 0,5 tot 41 % Fe, 9 tot 48 % Al, 0,1 tot 88 % Mn, 25 tot 72 % Zn en 3,3 tot 39 % Co. Hierdie studie het groot variasie in spoormetaalsamestelling en fraksionele oplosbaarheid in stof aërosols gevind wat van drie verskillende omgewings in Suider-Afrika en suid van Suider-Afrika versamel is. Ons studie het die belangrikheid van Suider-Afrikaanse stof van beide antropogenese en natuurlike bronne uitgelig as 'n voedingstofverskaffer aan omliggende kus- en oop oseane.
Description
Thesis (PhD)--Stellenbosch University, 2021.
Keywords
Dust -- Environmental aspects -- Africa Southern -- Southern Ocean, Ocean-atmosphere interaction, Ocean -- Environmental aspects, Ocean -- Climatic factors, Bioavailability -- South Africa -- Southern Ocean, Solubility -- Effect of temperature on, UCTD
Citation