New evidence of relative sea-level stability during the post-6000 Holocene on the Banc d'Arguin (Mauritania)

Morphological investigations on the northwest coastal plain of Mauritania (Banc d'Arguin) lead to a revised post-6000 yrs. BP sea-level curve. The region is characterized by extensive sand flats occasionally surmounted by low sand barriers. These barriers pick out a set of paleoshorelines attributed to the six episodes defined by Dia (2013). Morphological evidence of the extreme flatness of the coastal plain is provided by topographic transects based on total-station surveys. Index points distributed throughout the second half of the Holocene are defined in relation to the position of the barrier sole, which is adjusted to the upper limit of the berm uprush. The elevation of index points is related to the mean sea level (MSL) through an assessment of the various errors likely to affect the results. Over the period since ca. 7000 cal yr BP, the measured elevations remain within a few decimetres of the present-day sea level. This implies that, throughout this time interval, sea level in the Mauritanian region oscillated within the usual range of natural variations, whereas oscillations of larger magnitude prevailed elsewhere. Because the study area is tectonically stable, the glacio-eustatic adjustment due to hydro-isostatic effects is of minor importance in Mauritania and the relative sea level (RSL) has responded solely to the eustatic component. Therefore, we would expect to find some record of ocean syphoning through a gradual emergence of the land after the mid-Holocene marine transgression and the subsequent development of vertically stepped paleoshorelines. The absence of such evidence raises the question of whether the drop in RSL was compensated by continuing deglaciation during the post-6000 Holocene.