Short- and long-term sedimentation on Montportail-Brouage intertidal mudflat, Marennes-Oleron Bay (France)

The intertidal mudflat of Montportail-Brouage is 4 km wide and er;tends ove r about 30% of intertidal area of the Bay of Marennes-Oleron. The sampling area was a cross-shore transect (east-west) in the mid-part of the mudflat, so as to overlap the different geomorphological features, from the shore t o the central channel. Particularly distinctive were zones of shore-oblique ridges and runnels, and small channels. The short-term erosion-sedimentati on processes related to these bedforms, and their effects on the long-term stability of these structures are poorly known. Thus, sediment budget is di fficult to assess in the long term, due to temporal variations. Monthly sed imentological surveys were performed from March 1997 to May 1998. Six stati ons were sampled by coring for determination of water content, dry density, grain size and carbonate content of the sandy fraction, measured on the to pmost 5 cm. Within the ridge and runnel zone, this sampling was done on bot h structures along with the determination of depth profiles of macrofauna s pecies. Radiographic profiles to a depth of 50 cm were made at each of the 6 stations for description of the sedimentary facies, and radioisotope prof iles (Be-7, Pb-210(exc)) were made at four stations. Wet bulk density appea red to be roughly constant (1.39-1.44 kg m(-3)) over the whole mudflat belo w 2 or 3 cm. For the levels 0-1 and 1-2 cm, the bulk density showed much mo re variation, in particular in the runnels. These variations were due to ep isodic deposits of fluid mud (wet bulk density < 1.25 kg m(-3)). The sand c ontent decreased from the lower part of the mudflat to the upper part, part icularly in the fluid mud. On the contrary, the carbonate content of the sa nd fraction increased in the upper part by accumulation of foraminifera ass ociated with shell lag deposits. Be-7 measurements revealed a short-term se dimentation of fluid mud (up to 21.04 cm yr(-1)), particularly in runnels f rom the upper mudflat, thus indicating a temporary deposit, easily resuspen ded by wave action.