At the edge of the desert: The evolution of the Holocene environment in Lake Rkiz, Senegal River valley

Abstract

The analysis of faunal remains, pollen grains and other palynomorphs from a 4 m long core recovered at Lake Rkiz, Mauritania (16.911420 N, −15.172668 W, 0 m asl) documents the major phases of marine intrusion in the Senegal River system during the Holocene. Lake Rkiz was exposed to marine waters from 10 to 4 ka: a brackish lagoon developed at 10 ka, followed by a mangrove between 8.7 and 7 ka both in a calm environment. Rhizophora populations were never developed, however, either because they did not reach the core site at the northern extremity of the lake, or because they were restricted to the Senegal River bed. After 7 ka, the environment became increasingly unstable and the mangrove was replaced by a more clearly marine environment. During the last two millennia, a new, discrete phase of marine intrusion is recorded at Lake Rkiz. Contemporary to that already noted in the Ferlo River valley which feeds the Senegal River through Lake Guiers to the south, it also testifies to a positive oscillation of the sea level. The evolution of the regional vegetation of Lake Rkiz follows that of North tropical Africa with the expansion of a gallery forest of tropical humid affinity dominated by Celtis during the AHP. As elsewhere in the Sahara and Sahel, tropical plant types co-occurred with Saharan ones attesting to a mosaic-like landscape.