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 col umn. 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. Du ring the same sampling period, concurrent atmospheric deposition fluxe s of iron were measured at Capo Cavallo, Corsica, 20 nautical miles fr om 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 particul ate iron flux leaving surface waters at 200 m depth. Zooplanktonic gra zing activity, particularly intense from April to June, is mostly resp onsible for the sedimentation of iron. Faecal pellets control the tota l iron flux because they incorporate both iron associated with alumino -silicates (refractory particulate iron) and most of iron associated w ith biological material (biogenic iron). There was about 3300 ppm of i ron in the organic matter collected by the sediment traps at 200 m dur ing summer and fall. At 1000 m depth this concentration was greater by a factor of 2. During the same period, the calculated ratio of refrac tory particulate iron (Fe(REFR)) to particulate aluminium for the larg e particles at 80 and 200 m depth ((Fe(REFR)/A1)TRAP = 0.43), was lowe r 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 ass ociated with sinking organic matter is provided by atmospheric input.