Late Quaternary fluvial dynamics of the Jarama River in central Spain

摘要

The Jarama River and its catchment provide a valuable natural archive to study and reconstruct past environmental conditions in central Spain. This region is highly prone to changes in the hydrological cycle under predicted climate warming in particular with respect to aridity periods, rainfall variability and the occurrence of extreme flood events. This paper presents 15 exposures covering a time span of the last 44 ka, which were documented in the field and of which seven exposures were soil-chemical analyzed regarding soil texture, organic matter, as well as carbonate and iron content. Ages are based on radiocarbon dating on 32 samples. Latest Holocene sediments were found within the channel-belt, where permanent remobilization of sediments takes place due to the migration of the meandering river course. In more distal floodplain positions, sediment sequences show a complex fluvial architecture referring to periods of varying character and intensity of alluviation as well as periods of geomorphic stability indicated by soil formation. Furthermore, sedimentation patterns vary along different river sections. Aggradations of coarse gravels took place between ~40 and 18 ka cal BP over the entire valley floor. Alluviation of fine material was documented between 17 and 16, at ~7.5, between 5.1 and 3.3, at 2.8, between 2.1 and 1.5, at ~1.0, and around 0.4 ka cal BP until recent times. Astonishingly in late Holocene times, between 4.2 and 3.1 ka cal BP, aggradations of coarse gravels even in distal floodplain areas overlap with sedimentation of fine material in adjacent river sections pointing to a complex constellation of parameters involved. Periods of soil formation were detected around 43, at 31, between 16 and 12.6, after 7.5 until 5.1, between 2.8 and 2.1 and at times after 1.5 ka cal BP. Phases of geomorphic stagnation were found at 38 ka cal BP and for a duration of 200 years at 3.0 ka cal BP and refer to absent geomorphic or pedogenic processes. All these patterns are an expression of the interaction between climatic variations, tectonic impulses and human influences. A comparison with other terrestrial archives enabled us to reconstruct late Pleistocene and Holocene palaeoenvironmental conditions on a regional scale, and furthermore to link specific stages of floodplain development to prevalent influencing variables. Thus, with emphasis on the fluvial system response, a model of a cause and effect relationship is presented, that concentrates on rapid climate changes, long-lasting climatic deteriorations and the role of human interventions.