Particulate manganese and iron in recent sediments of the Gulf of Lions continental margin (north-western Mediterranean Sea): deposition and diagenetic process

This study aims at apprehending the major forcing factors which govern the spatial and vertical Mn and Fe distributions in ten cores from the Gulf of Lions. Speciation by chemical leaching experiments helps understanding their chemical behaviour during the early diagenetic processes in a marine environment under strong continental influence. The distribution and chemical behaviour of Mn and Fe differ drastically. The strong dependence of Mn distribution on chemical conditions is expressed by a marked solid phase Mn enrichment in the oxic layers of sediment deposits. This enrichment increases with decreasing sedimentation rate (increasing water depth). It is illustrated by an enhanced portion of total Mn in the reducible phase and the precipitation of todorokite on calcareous supports in the upper part of the deepest cores. As a result, 65% of the margin total particulate Mn is thought to be stored in slope deposits in a relatively strongly held association. Conversely, the low chemical mobility of iron in the sedimentary column is indicated by uniform depth profiles and an exclusive location in the residual chemical phase in all the studied sites. The Fe distribution is largely controlled by the dispersion of particulate material on the margin from the Rhone River main source. A budget of Mn and Fe accumulation based on the average amount of total Mn and Fe content of sediments deposited during the last 100 yr, has been made taking into account diagenetic readjustments. The total Mn and Fe amount stored on the margin during the last century is estimated to be 1.5 x 10(6) and 54 x 10(6) t, respectively. These values are seven times higher than the total secular external inputs calculated on the basis of present day riverine and atmospheric fluxes. This large discrepancy is mainly explained by the sharp decrease of the Rhone River discharge after dam buildings between 1950 and 1958. A residual discrepancy dating before 1950, is likely to be due to seafloor erosion and gravity induced mechanisms. (C) 2001 Elsevier Science B.V. All rights reserved.