Dust deposition directly affects the concentration of dissolved organic carbon in the ocean

Dust deposition to the ocean plays an indirect role in the carbon cycle due to stimulating the primary production by fertilisation. Additionally, it transports carbon to the ocean floor by acting as ballast for marine aggregates. Despite these recognized impacts, the direct influence of dust-seawater interactions on the carbon cycle remains poorly understood. Here, we study the effects of mineral dust on the dissolved organic carbon (DOC) concentration in seawater by performing sorption experiments through time series and stable carbon isotope analysis. We added two different amounts of dust to a solution of artificial seawater and ¹³C-labelled dissolved organic matter from Spirulina extract, creating a low and high dust-seawater ratio system. After 72 h, we observe a decrease in DOC for both systems, indicating the adsorption of DOC from the Spirulina extract onto dust particles. Analysis of the stable carbon isotope ratios of total organic carbon on the dust samples, before and after the sorption experiments, confirms these findings. Furthermore, our study shows that the net uptake of DOC on dust depends on the relative importance of adsorption, release and degradation of organic carbon. DOC release can become the dominant process based on the dust-seawater ratio and the initial organic carbon present on the dust, demonstrating that dust can act as both a sink and a source of organic carbon in the near-surface waters.