Abstract
Research concerning the global distribution of aeolian dust sources has principally focussed on salt/clay pan and desiccated lacustrine emission areas. In southern Africa such sources are identified as Etosha Pan in northern Namibia and Makgadikgadi Pans in northern Botswana. Dust emitting from ephemeral river valleys, however, has been largely overlooked. Rivers are known nutrient transport pathways and the flooding regimes of ephemeral river valleys frequently replenish stores of fine sediment which, on drying, can become susceptible to aeolian erosion. Such airborne sediment may be nutrient rich and thus be significant for the fertilisation of marine waters once deposited. This study investigates the dust source sediments from three ephemeral river valleys in Namibia in terms of their particle size distribution and their concentrations of bioavailable N, P and Fe. We compare the nutrient content of these sediments from the ephemeral river valleys to those collected from Etosha and Makgadikgadi Pans and consider their relative ocean fertilising potential. Our results show that the ephemeral river valleys contain fine grained sediment similar in physical character to Etosha and Makgadikgadi Pans yet they have up to 43 times greater concentrations of bioavailable iron and enriched N and P macronutrients that are each important for ocean fertilisation. The known dust-emitting river valleys of Namibia may therefore be contributing a greater fertilisation role in the adjacent marine system than previously considered, and not-yet investigated. Given this finding a re-assessment of the potential role of ephemeral river valleys in providing nutrient-rich sediment into the aeolian and marine systems in other dryland areas is necessary.
Highlights
Land-ocean and land-atmosphere-ocean sediment transport provide pathways of nutrients essential for ocean productivity (Aufdenkampe et al, 2011; Kroeze et al, 2012; Bouwman et al, 2013)
Nutrient input is especially important in High Nutrient Low Chlorophyll (HNLC) regions, which includes the Southern Atlantic (Meskhidze et al, 2007), where surface water phytoplankton is limited by the absence of one or more nutrients required for photosynthetic growth (Martin et al, 1994; Quéguiner, 2013)
Global remote sensing and modelling assessments of dust have focused on the major single sources of emission in southern Africa, with the active dust emission sites of the ephemeral river valleys being largely overlooked, and excluded completely from any on-the-ground investigation. This paper addresses these issues for Namibian ephemeral river valleys by examining the sedimentary characteristics of valley sediments, the fine fraction and the bioavailable iron and macronutrient content, which are components considered important for ocean fertilisation
Summary
Land-ocean and land-atmosphere-ocean sediment transport provide pathways of nutrients essential for ocean productivity (Aufdenkampe et al, 2011; Kroeze et al, 2012; Bouwman et al, 2013). Marine phytoplankton provide half of the planet's primary production (Chavez et al, 2011; Worden et al, 2015) and the biological consumption of sediment-associated nutrients plays an important role in photosynthetic marine uptake of CO2 (Jickells et al, 2014; Hauck and Völker, 2015). The complexities of atmospheric processing of aeolian sediment-bound nutrients and their deposition on the ocean surface remains poorly understood (Mahowald et al, 2009; Baker and Croot, 2010; Buck et al, 2010), with correlation between dust storms and phytoplankton blooms being made (Calil et al, 2011; Tan et al, 2017) and challenged (Boyd et al, 2009; Shaw et al, 2010). The deposition of mineral aerosols containing greater amounts of essential Fe, N and P would be considered to have a higher fertilisation potential
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