On various effects of dust on the concentration of dissolved organic carbon in the sea

After input from rivers, the aeolian input of particles is probably the main source of nutrients in the ocean. Dust also plays an important role in the transport of organic material into the deep sea. However, little is known about the adsorption capacity of mineral dust for dissolved organic material (DOM). In this study, we investigate the effects of dust-seawater interaction on the carbon cycle. We collected dust samples in the form of silty crusts (grainsize < 63 µm) from the ephemeral rivers Kuiseb and Omaruru in Namibia, Southern Africa. To characterize the source material, we analysed for total organic carbon (TOC), specific organic compounds, ¹³C/¹²C isotope ratio, elemental composition and specific surface area. Sorption experiments were performed through time series and isotope analysis. We added different amounts of dust to a mixture of artificial seawater with DOM and ¹³C labelled DOM, both extracted from Spirulina algae and each in naturally occurring concentration. Experiments with a moderate amount of dust showed a decline in dissolved organic carbon (DOC) over time. This finding was also confirmed by analyses of ¹³C/¹²C isotope ratio on dust samples, before and after suspension in seawater DOM solution. Both results indicate that dust adsorbed a significant portion organic carbon from the Spirulina DOM solution. However, the net uptake of DOM on mineral particle surfaces depends on the ratio between DOM concentration and the amount of dust used. If dust already contains organic components, which was the case in our experiment, at least some of these organic compounds will dissolve in contact with seawater if the ratio between the amount of dust or primary organic material contained and the DOM concentration in the initial test solution exceeds a certain value. In this case, the input of dust represents a source of DOM in the near surface seawater. Our study reveals that terrestrial, mineral dust may act as both a sink and a source for DOM in surface waters of the ocean. Subsequent studies will have to clarify the extent of the impact of these effects on the carbon cycle.