Abstract
During the last decades, there is an increasing concern about global climate change as a
consequence of anthropogenic influences. Recently, a report presented by the working group of
the Intergovernmental Panel on Climate Change (IPCC) concluded that there is convincing
evidence that most of the warming observed over the last 50 years is
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the result of human
activities. The working group also predicts that climate will continue to change throughout the
21st century, due to human influence, such as the emission of greenhouse gasses. Clearly, it is
essential to understand the natural variability of global and regional climate change to
discriminate and quantifY man-induced changes. Long records ofpast changes in climate offer a
key role in identifYing the effects of anthropogenic influences and climate models used for
predicting global climate change in the future should also be able to reconstruct and 'predict'
past climate changes (Barron et al., 1995). The conclusions put forward by the IPCC are based
on measurements of temperature and on climate proxy data for the Northern Hemisphere over
the last 1,000 years inferred from e.g. tree rings, corals and ice cores. Of course, longer-term
natural climate variations also occur as demonstrated in studies of Pleistocene records. Studies of
Pleistocene climate records from ice cores, corals, varves and deep-sea sediments show that the
Earth's climate has varied on a millennial scale with periods of several thousand years (Pisias et
ai., 1973; Dansgaard et ai., 1984; Pestiaux et ai., 1988; Bond et ai., 1997). Millennial-scale
climate cycles are also present in the older part of the Pleistocene (Oppo et ai., 1998; Raymo et
ai., 1998; McManus et ai., 1999) and in the Pliocene (Steenbrink, 2001). The origin of these
cycles, however, is not well understood and may be attributed to internal forcing mechanisms of
the climate system (e.g. ice-sheet dynamics and atmosphere-ocean interactions) or to external
mechanisms (e.g. solar variability, long-term tidal variations and harmonics of primary orbital
frequencies).
Longer-term climate variability, on time scales of 10-103 kiloyears (kyr), is found in
Pleistocene as well as in Tertiary and older records and is manifested by changes in sediment
properties (e.g. lithology, colour or grain size variations), in fossil communities and in
geochemical and isotope composition of the sediments. Numerous studies have demonstrated
that cyclic changes observed in these records are related to orbitally induced variations in climate
(e.g. Shackleton and Opdyke, 1973; Hilgen, 1991a, b; Tiedemann et al., 1994; Shackleton et al.,
1995; Olsen et al., 1996; Van Vugt et al. 1998; Steenbrink et ai., 2000). Such climate proxy data
may be incorporated as constraints in climate models, which should result in more accurate
model predictions offuture (and past) climate changes on orbital time scales. The subject of this
thesis is to study such orbitally forced climate variations and to establish well dated climate proxy
records for part of the continental Neogene using a multidisciplinary and integrated stratigraphic
approach.
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