The Holocene sedimentary and pyroclastic accumulations of two crater lakes (Mbalang,Tizong) of the volcanic plateau of Adamawa (Cameroon): Palaeoenvironmental reconstruction

Abstract

This paper presents the results from field observations, geochemical and mineralogical analyses, radiocarbon dating and magnetic susceptibility of the Holocene sediments of two crater lakes, Mbalang and Tizong, located within the woody savanna of the Adamawa Plateau (Cameroon). In the Mbalang core (M4) with an age of 7318 cal. BP at the base, the first 2000 years record indicates a relatively wet climate deduced from the recurrence of quartz-rich sandy microbeds. The next millennia are marked by a monotonous sedimentation interpreted as a relatively dry phase. The accumulation rate falls from 1.00–1.40 to 0.70 mm/yr in the last 100 years. The base of the Tizong core (T2) is dated to 4105 cal. BP. The sedimentation is frequently interrupted by interstratifications of pyroclastic beds from c. 3263 to 1391 cal. BP, well evidenced by their high magnetic susceptibility values. The high accumulation rate (1.58 mm/yr) integrates both allochthonous volcanic ashes and autochthonous intense proliferation of Chlorophyceaea. The Ngaoundere region, in the southern forest region, reveals a humid phase between c. 7943 and 5603 cal. BP. The later long-lasting dry tendency makes it difficult to particularly distinguish the c. 3210–1945 cal. BP dry periods recorded in the forest region of Cameroon. Nevertheless, signs from Mbalang pollens and Tizong waters bear witness to an important drying tendency during this period. In Lake Chad, after the successive highest lake levels recorded between c. 8500 and 6300 cal. BP, the level of this lake was declining for the last three millennia.