Impact of atmospheric deposition on particulate iron flux and distribution in northwestern Mediterranean waters
Résumé
Iron distribution can be significantly influenced by the i\11eractions betwecn atmaspheric
inputs and internai recyding within the waler column. This ques,tion \Vas investigated in the
norchwestern Mediterranean Sea as part of the French DYF AMED pro gram. Sediment traps \Vere
moored al SO, 100 and 1000 m depth al 42°44'N, 8°31'E during 19S'fr1987 ta colleet senling
panic\es in which iron \'..-as analysed. During the same sampling peri ad, concurrent atmospheric
deposilion fluxes of iron were measured at Capo Cavallo, Corsica. 20 nautical miles from our
marine sampling sire. Sorne shon but intense atmospheric inputs (1.5-2.4 mg m~1 day-l during
5-20 consccutive days) of minerai matter from Africa are shown ta be suÏficient ta explain the
yearly particulate iron flux leaving surface waters at 200 m depth. ZooplanklOnie grazingactivity,
panicularly intense from April to June, is mostly responsible for the sedimentation of iron. Faecal
pellets control the total iron flux because they incorporate both iron associaled with alumino·
silicates (refractory particulate Iron) and most of iron associated with biological material (biogenic
iron). Thcre \Vas about 3300 ppm of iron in the organic matter collected by the sediment traps at 200
rn during summcr and fall. At 1000 m depth this concentration was greater by a factor of2. During
the same period. the calculated ratio of refraclory paniculate iron (FeREFR) ta particulate
aluminium for [he large particles at 80 and 200 m depth ((FeREFR/AlnRAP :=: 0.43), wasl.oweJrw.an
the Fe/AI rario usually measurcd in the Saharan aerosol (0.5 < Fe/Al < 0.7). This observation is
underscored by the amplitude of the dissolved/paniculate exchanges and suggests that most of the
iron associated with sin king organic matter is provided by atmospheric input.