This article is part of the Spring 2020 issue of the Earth Science Matters Newsletter.
The antiquity of the Sahara Desert: New evidence from the mineralogy and geochemistry of Pliocene paleosols on the Canary Islands, Spain
Of the Earth’s warm deserts, the Sahara in northern Africa is by far the largest. Stretching from Morocco in the west to Egypt in the east, the Sahara has been sparsely occupied by humans for thousands of years. Natural and human history have played out on its landscapes for centuries; adventures in its vast emptiness are the stuff of legends. Perhaps this contributes to the controversy surrounding the Sahara’s age. Many investigators have proposed that the great desert dates to the Pleistocene which is the time period of the past approximately 2.6 million years (Ma) characterized by alternating glacial and interglacial periods (commonly referred to as the "Ice Ages"). Some claim the desert formed within the past few thousand years while others suggest a longer history, with the Sahara forming in the Pliocene between 5.3 and 2.6 million years ago.
To uncover evidence of the Sahara's early history, USGS geologists teamed with scientists from the Universidad de Las Palmas de Gran Canaria to investigate buried, fossil soils (paleosols) on the Canary Islands off the coast of Morocco. Because of its extreme aridity, the Sahara is the world's largest source of long-range-transported (LRT) dust. Each year, Saharan dust is transported westward across the Atlantic (Figure 1), to the Canary Islands, and as far west as the Caribbean and South America. The paleosols studied on the Canary Islands formed at various times in the past 5 million years. Mineralogical and geochemical data show that paleosols on the archipelago’s Fuerteventura and Gran Canaria Islands are of Pliocene to mid-Pleistocene age and developed, in part, from inputs of African dust. These paleosols contain quartz and mica, minerals abundant in Africa, but very rare in the basaltic rocks dominating the Canary Islands. The research group also examined relative abundances of key trace elements with minimal mobility: scandium, chromium, hafnium, thorium, and tantalum, along with other rare earth elements. Instrumental neutron activation analyses in USGS laboratories show that the Canary Island paleosols are composed of local rock and African-derived dust.
The USGS-Spanish team’s findings, coupled with a recently published Spanish study, indicate that 9 paleosols record delivery of African dust to the Canary Islands between ~4.8-2.8 Ma, ~3.0-2.9 Ma, ~2.3-1.46 Ma, and ~0.4 Ma. This long-term paleosol record of African dust input agrees with deep-sea records off the western Africa coast that imply increased dust fluxes to the eastern Atlantic Ocean at ~4.6 Ma. Taken together, these results indicate that the Sahara Desert existed as an arid-region dust source, at least intermittently, for much of the Pliocene, continuing well into the Pleistocene and today.
Beyond telling us the age of the Sahara, these findings have important implications. LRT dust can substantially impact the planetary radiation balance, with both warming or cooling effects, depending on dust composition, altitude, and the surface over which LRT is transported. Dust inputs to the ocean provide much of the iron that is often a limiting factor at the bottom of marine ecosystem food chains. Like the Canary Islands, Saharan dust is a major soil component for many Caribbean islands, parts of the southeastern United States, and parts of northern South America. In fact, periodic inputs of nutrient rich dust could be one of the contributing factors resulting in the tremendous plant diversity found in the Amazon Rain Forest. This research illuminates the nearly 5 million year history of the Sahara, along with its important role on this planet.
The paper “The antiquity of the Sahara Desert: New evidence from the mineralogy and geochemistry of Pliocene paleosols on the Canary Islands, Spain” appeared in the journal Palaeogeography, Palaeoclimatology, Palaeoecology.