Environmental Engineering Reference
In-Depth Information
canyon formation and desiccation occurred in a dry basin which reached depths of
1.5 km and in which extensive evaporites were deposited. In a recent review of re-
search on this contentious issue, Ryan (2009) states that it was 'an extraordinary
event in which 5% of the dissolved salt of the oceans of the world was extracted
in a fraction of a million years to form a deposit more than 1 million km
3
in vol-
ume
The dolomite, gypsum, anhydrite and halite in the drill cores paint a sur-
prising picture of a Mediterranean desert lying more than 1 km below the Atlantic
Ocean with brine pools that shrank and expanded by the evaporative power of the
sun.' Much of the salt bed formation was influenced by orbital forcing, that is in-
solation changes related to the path taken by the Earth as it revolves around the
Sun and the way in which the Earth revolves on its axis, just as the advance and
retreat of the cold/glacial and warm/interglacial stages were influenced during the
Quaternary period (see below).
Some of these marginal Messinian evaporites deposited during the first step of the
crisis (Clauzon
et al.,
1996) were later uplifted by tectonic activity to form dry land
as in Messina, Sicily, northeast Libya, Italy and southern Spain. The time frame for
this process is contentious, with estimates varying between 1.5
...
10
6
years (Butler
et al.,
1999) and a relatively sudden event over 0.26
×
10
6
years (Clauzon
et al.,
1996; Krijgsman
et al.,
1999). Such conditions prevailed for more than 0.5
×
10
6
years, during which time there were several subphases of erosion. These numbered
between three and five (Gargani and Rigollet, 2007). The subsequent high sea-level
of the Atlantic Ocean allowed saltwater with a mixed flora and fauna to re-enter the
Mediterranean and may have been linked with stream piracy (Blanc, 2002) which
helped breach the divide. According to Favre
et al.
(2007), vegetation maps based
on pollen assemblages from 30 locations throughout the Mediterranean Basin indi-
cate that open habitats with steppe communities prevailed to the south, except some
savanna in the Nile region, while the north of the basin was characterized by forest
mosaics that varied in composition with relief.
Although this information reflects environmental change in the Mediterranean
Basin itself, the corollary is that the mountain regions within the basin were also
experiencing environmental change but there is much less palaeoenvironmental in-
formation from the mountain regions themselves. Willett
et al.
(2006) report that
the Messinian event was paralleled by increased erosion in the Alps, which they
attribute to increased rainfall. This was probably the case in the Mediterranean
mountains discussed in this topic, though not all. For example, Babault
et al.
(2006)
argue that the Ebro River basin of the Pyrenees became connected to the Mediter-
ranean Basin after rather than during the Messinian event, on the basis that there
are no incised 'canyons' typical of Messinian age landforms.
×
2.2.1 The Pliocene 5.3 to 2.6
×
10
6
years ago
Relative climatic stability that characterized the Tertiary period globally was com-
ing to an end as a cooling trend became established in a still warm and humid world.
