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Aridification of the Sahara desert caused by Tethys Sea shrinkage during the Late Miocene
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Aridification of the Sahara desert caused by Tethys Sea shrinkage during the Late Miocene
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Journal Article
Aridification of the Sahara desert caused by Tethys Sea shrinkage during the Late Miocene
2014
Overview
The drying of the Tethys Sea—the progenitor of the modern Mediterranean, Black and Caspian seas—weakened the northern extension of the African monsoon and led to the creation of the Sahara desert about 7 million years ago.
Sahara desert much older than we thought
Most evidence suggests that the modern Sahara desert first arose between two and three million years ago, coinciding with the initiation of major glaciations in the Northern Hemisphere. This study puts Saharan origins much earlier. Zhongshi Zhang
et al
. show that the shrinkage of the Tethys Sea — the progenitor of the modern Mediterranean, Black and Caspian seas — weakened the northern extension of the African monsoon and led to the creation of the Sahara desert about seven million years ago. Such a dramatic revision could lead to new investigations of the Sahara in fields as diverse as geology, evolutionary biology and climatology.
It is widely believed that the Sahara desert is no more than ∼2–3 million years (Myr) old
1
, with geological evidence showing a remarkable aridification of north Africa at the onset of the Quaternary ice ages
2
,
3
,
4
. Before that time, north African aridity was mainly controlled by the African summer monsoon (ASM)
5
,
6
,
7
,
8
, which oscillated with Earth’s orbital precession cycles. Afterwards, the Northern Hemisphere glaciation added an ice volume forcing on the ASM, which additionally oscillated with glacial–interglacial cycles
2
. These findings led to the idea that the Sahara desert came into existence when the Northern Hemisphere glaciated ∼2–3 Myr ago. The later discovery, however, of aeolian dune deposits ∼7 Myr old
9
suggested a much older age, although this interpretation is hotly challenged
1
and there is no clear mechanism for aridification around this time. Here we use climate model simulations to identify the Tortonian stage (∼7–11 Myr ago) of the Late Miocene epoch as the pivotal period for triggering north African aridity and creating the Sahara desert. Through a set of experiments with the Norwegian Earth System Model
10
and the Community Atmosphere Model
11
, we demonstrate that the African summer monsoon was drastically weakened by the Tethys Sea shrinkage during the Tortonian, allowing arid, desert conditions to expand across north Africa. Not only did the Tethys shrinkage alter the mean climate of the region, it also enhanced the sensitivity of the African monsoon to orbital forcing, which subsequently became the major driver of Sahara extent fluctuations. These important climatic changes probably caused the shifts in Asian and African flora and fauna observed during the same period
4
,
12
,
13
,
14
, with possible links to the emergence of early hominins in north Africa
15
,
16
.
