Little Ice Age climatic fluctuations in the Namib Desert, Namibia, and adjacent areas: Evidence of exceptionally large floods from slack water deposits and desert soil sequences

Abstract

Knowledge of long-term rainfall variablity is essential for water management in Namibia. Data relevant to assess this variability are scarce because of the lack of long instrumental climate records and the limited potential of standard highresolution proxy records. In northern and eastern Africa the reconstruction of Holocene tropical lake-level changes has established alternating phases of desiccation and of high stands with lake-levels more than 100 m above the present level. This record of paleohydrological changes is impressive as compared to available data collected from modern instrumented observations. Such sudden and dramatic changes of the hydrologic regime within time scales that are relevant to human societies are not known from southwestern arid Africa (Namib Desert). Fluvial silts, accumulated in some Namib valleys, are interpreted as records of reduced precipitation in the catchments. Our investigations show that these fluvial silts are slack water deposits (SWDs) and reflect hydrologic - and climatic - conditions during the late Holocene that caused extreme flash floods in the valleys. Here we describe SWDs of some Namibian Desert valleys and present 14C dates of their ages. The youngest accumulation phase occurred during the Little Ice Age (LIA)(ca. AD 1300 to 1850). The biggest flash floods of the LIA, in most catchments, experienced water levels in the valleys that exceeded the most extreme floods of the last 100 - 150 years. In the northwestern Namib Desert, flash floods of the LIA were more frequent and more extreme than in the central Namib Desert. This may be caused by small shifts of the tropical-temperate-troughs in southern Africa and the south west Indian Ocean.