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 over 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 to the central channel. Particularly distinctive were zones of shore-oblique ridges and runnels, and small channels. The short-term erosion-sedimentation processes related to these bedforms, and their effects on the long-term stability of these structures are poorly known. Thus, sediment budget is difficult to assess in the long term, due to temporal variations. Monthly sedimentological surveys were performed from March 1997 to May 1998. Six stations were sampled by coring for determination of water content, dry density, grain size and carbonate content of the sandy fraction, measured on the topmost 5 cm. Within the ridge and runnel zone, this sampling was done on both structures along with the determination of depth profiles of macrofauna species. Radiographic profiles to a depth of 50 cm were made at each of the 6 stations for description of the sedimentary facies, and radioisotope profiles (Be-7, Pb-210(exc)) were made at four stations. Wet bulk density appeared to be roughly constant (1.39-1.44 kg m(-3)) over the whole mudflat below 2 or 3 cm. For the levels 0-1 and 1-2 cm, the bulk density showed much more variation, in particular in the runnels. These variations were due to episodic deposits of fluid mud (wet bulk density < 1.25 kg m(-3)). The sand content decreased from the lower part of the mudflat to the upper part, particularly in the fluid mud. On the contrary, the carbonate content of the sand fraction increased in the upper part by accumulation of foraminifera associated with shell lag deposits. Be-7 measurements revealed a short-term sedimentation of fluid mud (up to 21.04 cm yr(-1)), particularly in runnels from the upper mudflat, thus indicating a temporary deposit, easily resuspended by wave action.