Astronomical forcing in continental sediments. An integrated stratigraphic study of Miocene deposits from the Calatayud and Teruel basins, NE Spain

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 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.