Palaeogeography and dynamics of the deltaic wetland of Save River, Mozambique

Abstract

Many mangrove wetland systems in deltaic environments are negatively affected by massive sedimentation from river inflows. In this paper we use the example of the Save River delta to assess the palaeogeographic distribution of mangrove wetlands and to analyze their dynamics. To track past occurrences of mangrove wetlands in the study area we have integrated sedimentological data with siliceous microfossil analysis combined with AMS radiocarbon and OSL dating. The results show a fine-grained deposit with an approximate thickness of 2m, present at different sampling sites. In the upper deltaic plain, the deposit is interbedded between sand layers, while in the lower deltaic plain the deposit occupies the uppermost stratigraphic position. In most of the sampling sites the deposit shows a succession with brackish-marine diatoms at the bottom of the sequence while the upper part shows only scattered occurrences. Based on sedimentological and microfossil characteristics we have interpreted the layer to represent a mangrove wetland deposit. The development of the deposit in the study area is suggested to have been initiated around 3100cal. yr BP, induced by sea-level rise. Thereafter, the development followed the combined effect of a sea-level fall and delta progradation processes. In some areas, particularly in the proximal part of the delta, the mangrove deposit has developed progressively on top of the delta-front. From around 1300years ago (OSL) onwards, massive alluvial sedimentation impacted the mangrove ecosystem. However, the retreat of mangrove wetland coincided with a regional fall of sea level. At the edges of the alluvial deposit, the current mangrove ecosystem has reclaimed the habitat in some sectors where gully erosion has exposed the once extinct mangrove habitat.