Source Of DFe Research Articles

Dust deposition: iron source or sink? A case study

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 deposition 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.

Dissolved iron distribution in the tropical and sub tropical South Eastern Pacific

Abstract. Dissolved iron (DFe) distributions (<0.2 μm) were determined in the upper water column (0–400 m) of the south eastern tropical and subtropical Pacific, in October–November 2004. Data were collected along a transect extending from the Marquesas Islands to the Chilean coast with most of the stations located in the south Pacific gyre. The concentrations of DFe presented large variability with highest values observed at both extremities of the transect. In the Chilean upwelling, DFe concentrations ranged between 1.2–3.9 nM. These high values result from inputs from the continental margin and are likely maintained by anoxic conditions in the water corresponding to the Oxygen Minimum Zone (OMZ). In subsurface waters near the Marquesas, that were also associated with the extension of the OMZ, DFe concentrations varied between 0.15–0.41 nM. Vertical transport of this water by mesoscale activity eastward of the archipelago may explain the dissymmetric east-west distribution of chlorophyll-a evidenced by satellite images. Using the new tracer Fe*=DFe−rFe:P (PO43−) we show that DFe was in deficit compared to PO43− resulting from the remineralisation of organic matter. This suggests that the Marquesas islands and the surrounding plateau are not a significant source of DFe. In the gyre, DFe concentrations in the upper 350 m water column were around 0.1 nM and the ferricline was located well below the nitracline. These low concentrations reflect the low input of DFe from the atmosphere, from the ventilation of the upper thermocline with water containing low DFe, and from the low biological activity within this ultra oligotrophic gyre.

Deep dissolved iron profiles in the eastern North Atlantic in relation to water masses

Concentrations of dissolved iron (DFe, 0.2μm) were determined at two stations in the Biscay Abyssal Plain (North East Atlantic) in March 2002. DFe concentrations in the surface layer (0.23–0.34 nM) were typical of winter conditions in this area. At 1000 m, DFe concentrations increased to 0.62–0.86 nM. This feature is consistent with the production of DFe by remineralization of the biogenic material. However, at this depth, Mediterranean Outflow Water (MOW) could be an additional source of DFe. Below 2500 m, DFe concentrations were constant (0.75 ± 0.04 nM). An interesting feature of the profiles was the intermediate maximum of DFe (1.19–1.12 nM) around 2000 m, associated with the Labrador Sea Water (LSW). We suggest that the iron enrichment of LSW occurred when this water mass reached the continental margin, likely in the vicinity of the Goban plateau. Vertical distributions were highly dependent on water masses encountered.