Evidence for the role of tropical plumes in driving mid-Holocene north-west Sahara rainfall

Abstract

The West African Monsoon (WAM), Atlantic north-westerlies and Mediterranean cyclones are significant sources of rainfall in north-west Africa, supplying moisture to the fringes of the Sahara. Rainfall patterns and the extent of the desert vary through time with strong evidence of a wetter Sahara during the early- to mid-Holocene (widely referred to as the African Humid Period). North of 28°N there is a particular lack of palaeorainfall reconstructions, with higher spatial- and temporal-resolution required to constrain the mechanisms responsible for past sub-tropical climate change, and the impacts of environmental change on human developments. We provide palaeorainfall reconstructions from stalagmites from today's arid north-west Sahara, inland from the coast and south of the Atlas Mountains in Morocco (30–32°N). The records show increased rainfall between 8.7–4.3 kyr BP. The timing, and oxygen isotopes of the speleothem growth, in comparison with other records, strongly suggest that South-of-Atlas rainfall continued after the decline of the West African Monsoon in the mid-Holocene. We propose that additional rainfall was supplied by increased tropical-plume rainfall in the South-of-Atlas region. We suggest that an increased North-South inter-hemispheric temperature anomaly, shifting the ITCZ northwards, increased the supply of tropical-moisture to tropical-plumes. For the first time this study provides evidence supporting tropical-plumes as an additional source of past-rainfall, helping to reconcile palaeo-archives and modelling studies. Increased South-of-Atlas rainfall improved habitability and increased recharge to rivers flowing south into the Sahara, which likely facilitated connectivity through the Sahara, during a key period in the development of land use and animal production.