Abstract
X-ray diffraction analyses were made of the smaller than 2 J..Lm fraction from
about 1250 samples of the central Mediterranean Miocene to Recent and the
southeastern North-Atlantic Miocene in order to reconstruct climatic changes.
Relative quantities of the clay minerals chlorite, illite, pyrophyllite, smectite,
kaolinite and palygorskite and the accessory minerals quartz and goethite
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were
obtained with a new quantification method, combining peak-area and peakheight
measurements on the diffractograms. Random mixed-layers and sepiolite
were found but not quantified separately. Detailed calcareous nannofossil and
planktonic foraminiferal biostratigraphic data - and for the Late Miocene to
Pleistocene also geomagnetic data - allowed time-stratigraphic correlation of
thirteen clay mineral intervals and twenty six subintervals distinguished in the
most detailed records.
The mineral curves as well as a principal component analysis of the averages
per (sub)interval show the existence of regional differences in clay composition
within the central Mediterranean area and between the two study areas. These
differences were preserved in the course of time and are related to source
characteristics and sorting during transport. Smectite dominates the associations.
In most intervals the vertical record of strong smectite trends and fluctuations
is not a reflection of contemporaneous changes in soil formation induced by
climate. Lateral comparison between curves for Pliocene sections in Sicily,
Calabria and the Tyrrhenian suggests that the main mechanism is to be looked
for in changes in erosion and reworking from older sedimentary sequences,
possibly of Messinian Age. Changes were caused by variation in precipitation
and in relief formation. A more distant supply of clay particles as aeolian dust
derived from Paleogene sediment outcrops in North Africa is reconstructed for
the Late Miocene to Pliocene of the entire central Mediterranean. This signal is
suppressed by local riverine supply at times of tectonic uplift causing
diachronous sedimentary facies transitions. Also during more humid periods we
find an increased local supply, expressed by grey layers in sedimentary
rhythmites for which rhythmites a forcing by astronomic precession has been
proposed by earlier authors.
The influence of a deteriorating climate is envisaged for the Middle Miocene
and Pleistocene. Clay compositional changes in the Middle Miocene of the two
study areas also reflect rejuvenation of reliefs and possibly changing wind and
current patterns, however. The Pleistocene record of the Tyrrhenian shows
some influence of weathering of volcanic terrains in Italy. Mediterranean-wide
the Messinian clay associations indicate semi-aridity. There are indications for
marine-authigenesis in the middle Tortonian and in the Messinian of Sicily,
however. Therefore semi-aridity, possibly in combination with peneplanisation
and sea-level-rise, cannot be considered the only cause of the observed smectite
abundances
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