Dust deposition: iron source or sink? A case study

Abstract

Abstract. A significant decrease of dissolved iron (DFe) concentration has been observed after dust addition into mesocosms during the DUst experiment in a low Nutrient low chlorophyll Ecosystem (DUNE), carried out in the summer of 2008. Due to low biological productivity at the experiment site, biological consumption of iron can not explain the magnitude of DFe decrease. To understand processes regulating the observed DFe variation, we simulated the experiment using a one-dimensional model of the Fe biogeochemical cycle, coupled with a simple ecosystem model. Different size classes of particles and particle aggregation are taken into account to describe the particle dynamics. DFe concentration is regulated in the model by dissolution from dust particles and adsorption onto particle surfaces, biological uptake, and photochemical mobilisation of particulate iron.The model reproduces the observed DFe decrease after dust addition well. This is essentially explained by particle adsorption and particle aggregation that produces a high export within the first 24 h. The estimated particle adsorption rates range between the measured adsorption rates of soluble iron and those of colloidal iron, indicating both processes controlling the DFe removal during the experiment. A dissolution timescale of 3 days is used in the model, instead of an instantaneous dissolution, underlining the importance of dissolution kinetics on the short-term impact of dust deposition on seawater DFe.Sensitivity studies reveal that initial DFe concentration before dust addition was crucial for the net impact of dust addition on DFe during the DUNE experiment. Based on the balance between abiotic sinks and sources of DFe, a critical DFe concentration has been defined, above which dust depoCorrespondence to: Y. Ye (ying.ye@awi.de) sition acts as a net sink of DFe, rather than a source. Taking into account the role of excess iron binding ligands and biotic processes, the critical DFe concentration might be applied to explain the short-term variability of DFe after natural dust deposition in various different ocean regions.

Ocean–Atmosphere Interactions of Particles

“…Sedimentation, including carbon export, can be quantified and this represents a major step forward of that methodology affected by the dust input to the surface seawater, as a dissolved iron (DFe) scavenging was observed rapidly after the seeding, withdrawing almost 1 nM DFe from the whole water column inside the three mesocosms amended with dust while the atmospheric particles are settling . Those results were satisfactorily simulated with a one-dimensional model of the Fe biogeochemical cycle, coupled with a simple ecosystem model (Ye et al 2011). When a second dust wet deposition was simulated, iron dissolution from the dust particles was then evidenced due to the excess Fe binding ligand concentrations produced by the enhanced biological activity (Wuttig et al 2013).…”

“…Complex iron chemistry in the ocean (Tagliabue et al 2009), adsorptive scavenging of dissolved iron and solubilisation of particulate iron (Ye et al 2011), varying iron content of dust particles as well as varying iron solubility in seawater ) have been investigated in individual modelling studies but an explicit representation of these processes is still lacking in most of the biogeochemical models that do include an explicit representation of the iron cycle. In addition, Krishnamurthy et al (2009) have shown that increasing iron deposition could not only lead to increasing marine productivity in HNLC regions, but also significantly enhance N-fixation in subtropical regions if iron limitation of N-fixation is included.…”

“…The mesoscoms experiment was simulated with a 1D model which takes into account dissolution and scavenging of iron on dust particles. The equilibrium between both processes allowed definition of a critical Fe concentration above which the balance between dissolution and scavenging is in favour of scavenging, resulting in a net sink of dissolved iron (Ye et al 2011). …”

PISCES-v2: an ocean biogeochemical model for carbon and ecosystem studies

“…(84). Model estimates (Ye et al, 2011) suggest a different affinity for these particles compared to biogenic particles, which justifies the split between biogenic and lithogenic materials in Eq. (50).…”

“…Thus, they may be transferred to the particulate pool, and settle to the ocean floor. Very few models have incorporated this potential important sink of dissolved iron (Ye et al, 2009(Ye et al, , 2011. In PISCES, we model this process following the approach chosen for DOM (see Sect.…”