Time Series Analyses on selected geological sections: A search for cyclicity

energy intercepted by the Earth. However, until today, a
full understanding of how variations in intercepted solar
energy are translated into significant climate changes is
lacking. This thesis studies climate changes during two
very different geological periods: the Holocene and the
Devonian; and it examines the extent to which the Sun is
forcing them. Logically, for the two different geological
periods, another temporal resolution is used (years and
thousands of years respectively). This difference in
resolution makes it possible to study two very different
types of exogenous forcing of the climate: the influence of
variations in solar activity on the one hand, and the
influence of variations in the distribution of solar energy on
Earth, caused by changes in the Earth's orbit parameters,
on the other.
For the Holocene, a ?18O record of an U/Th dated
speleothem from Socotra (Yemen) is presented (De Geest
et al., 2006). This record serves as a proxy for variations in
the intensity of the inter-monsoon rainy seasons for the
period between 6000 BP and today. By comparing the ?18O
record with a reconstruction of solar activity (Steinhilber et
al., 2009), the obvious and statistical significant correlation
between both records suggests that variations in solar
activity play a crucial role in determining the intensity of
the rainy seasons on Socotra. Moreover, spectral analysis
demonstrates that the 205-years "De Vries / Suess" sunspot
cycle is the dominant forcing cycle in solar activity for the
Indian Monsoon Dynamics.
For the Devonian, a magnetic susceptibility (MS)
record is presented, obtained from Uppermost-Eifelian to
Late-Frasnian limestones along the southern border of the
Dinant Synclinorium (Boulvain et al., 2010). Large-scale
variations (tens to hundreds of meters) in the MS signal are
ascribed to sealevel fluctuations, while faster variations
(several meters) are interpreted as changes in the flux of
magnetic minerals towards the marine system, determined
by precipitation intensity. Spectral analysis highlights
persistent cycles in the MS signal. By involving both
chrono- and biostratigraphic information, as theoretical
knowledge about the proportions between of sedimentation
rates in different depositional environments, the various
cycles can be interpreted as precession (~17-18 ka) and/or
obliquity (~33 ka) cycles. To correctly understand these
astronomically forced cycles in the MS signal from a
climatologic and oceanographic point of view, a hypothesis
is proposed that explains, in a simplified way, how
precession and obliquity can account for a southward shift
of the Intertropical Convergence Zone (ITCZ), and for the
consequent increase in precipitation over the
palaeolocation of the Dinant Synclinorium. For the "La
Couvinoise" section, the well-developed precessional
cycles, together with the availability of an isotopic age,
lead to the construction of a "Floating Point" time scale,
with a temporal resolution of ~10 000 years.


References:
BOULVAIN, F. et al., 2010. Geologica Belgica, 13, 113-
117.
DE GEEST, P. et al., 2006. Karst Waters Institute Special
Publication 10: Archives of Climate Change in Karst, 103-
104.
STEINHILBER, F. et al., 2009. Geophysical Research
Letters, 36.