The intertidal mudflat of Montportail-Brouage is 4 km wide and extends over about 30% of intertidal area of the Bay of Marennes-Oléron. 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 ( 7 Be, 210 Pb 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. 7 Be 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. 210 Pb exc measurements showed a long-term sedimentation rate which was 4 times higher in the lower part (0.97 cm yr −1) of the mudflat (near oyster reefs and mussel “bouchots”) than in the upper part (0.26 cm yr −1). In the middle mudflat, ridges and runnels exhibited a large difference of the long-term sedimentation rates, 0.70 cm yr −1 on ridges and only 0.07 cm yr −1 in runnels, that could be considered as a passageway for fluid mud transport. In the middle mudflat, where ridges and runnels occured, synopsis of observations indicated that a 30 yr old and 18 cm thick ridge layed on a more than 100 yr old basement mudflat. This bedform is maintened through short- and long-term sedimentations. In the runnels, always flooded by water even at low tide, the short-term sedimentation was 21.04 cm yr −1, but these sediments can be easily resuspended by tidal currents, inducing a weak long-term sedimentation (0.07 cm yr −1). In the ridges, the short-term sedimentation was 7.27 cm yr −1; however, due to potential impacts of drying processes, along biostabilization by epipelic diatoms during subaerial exposure at low tide, the short-term sedimentation contributed to a long-term sedimentation of 0.70 cm yr −1.
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May 19, 2006
H.-E Zschau + 5
Open Access