Wagener T, Guieu C, Leblond N
Dust deposition, Iron, mediterranean, mesocosm, 52 m3, Elbo bay, France
Soil dust deposition is recognized as a majorsource of iron to the open ocean at global and regional scales.However, the processes that control the speciation and cycleof iron in the surface ocean after dust deposition are poorlydocumented mainly due to the logistical difficulties to investigatein-situ, natural dust events. The development ofclean mesocosms in the frame of the DUNE project (a DUstexperiment in a low Nutrient low chlorophyll Ecosystem)was a unique opportunity to investigate these processes atthe unexplored scale of one dust deposition event. Duringthe DUNE-1-P mesocosm seeding experiment, iron stocks(dissolved and particulate concentrations in the water column)and fluxes (export of particulate iron in sediment traps)were followed during 8 days after an artificial dust seedingmimicking a wet deposition of 10 gm?2. The addition ofdust at the surface of the mesocosms was immediately followedby a decrease of dissolved iron [dFe] concentrationin the 0–10m water column. This decrease was likely dueto dFe scavenging on settling dust particles and mineral organicaggregates. The scavenging ratio of dissolved iron ondust particles averaged 0.37±0.12 nmol mg?1. Batch dissolutionexperiments conducted in parallel to the mesocosm experimentshowed a increase (up to 600 %) in dust iron dissolutioncapacity in dust-fertilized waters compared to controlconditions. This study gives evidences of complex and unexpectedeffects of dust deposition on surface ocean biogeochemistry:(1) large dust deposition events may be a sink forsurface ocean dissolved iron and (2) successive dust depositionevents may induce different biogeochemical responsesin the surface ocean.