Time-series analysis on selected geological sections - A search for cyclicity

Student thesis: Master's Thesis

Abstract

The climate is highly dependent on the amount of solar 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 d18O 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., 2010b). 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. Using spectral analysis, persistent cycles in the MS signal are distinguished. 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.
Date of Award2010
Original languageEnglish
SupervisorPhilippe Claeys (Promotor)

Keywords

  • time-series analysis
  • climate change
  • solar variability
  • Milankovic cycles
  • orbital forcing

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