IMPACT OF ATMOSPHERIC DEPOSITION ON PARTICULATE IRON FLUX AND DISTRIBUTION IN NORTHWESTERN MEDITERRANEAN WATERS

Iron distribution can be significantly influenced by the interactions between atmospheric inputs and internal recycling within the water column. This question was investigated in the northwestern Mediterranean Sea as part of the French DYFAMED program. Sediment traps were moored at 80, 200 and 1000 m depth at 42-degrees-44'N, 8-degrees-31'E during 1986-1987 to collect settling particles in which iron was analysed. During the same sampling period, concurrent atmospheric deposition fluxes of iron were measured at Capo Cavallo, Corsica, 20 nautical miles from our marine sampling site. Some short but intense atmospheric inputs (1.5-2.4 mg m-2 day-1 during 5-20 consecutive days) of mineral matter from Africa are shown to be sufficient to explain the yearly particulate iron flux leaving surface waters at 200 m depth. Zooplanktonic grazing activity, particularly 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 associated with alumino-silicates (refractory particulate iron) and most of iron associated with biological material (biogenic iron). There was about 3300 ppm of iron in the organic matter collected by the sediment traps at 200 m during summer and fall. At 1000 m depth this concentration was greater by a factor of 2. During the same period, the calculated ratio of refractory particulate iron (Fe(REFR)) to particulate aluminium for the large particles at 80 and 200 m depth ((Fe(REFR)/A1)TRAP = 0.43), was lower than the Fe/Al ratio usually measured in the Saharan aerosol (0.5 < Fe/Al < 0.7). This observation is underscored by the amplitude of the dissolved/particulate exchanges and suggests that most of the iron associated with sinking organic matter is provided by atmospheric input.