Fluvial response to climate change and human activities, Burgundy, France.

摘要

Field mapping, stratigraphic and sedimentologic investigations, and a series of radiocarbon and optically-stimulated luminescence dates (OSL) define a record of Late Pleistocene and Holocene landscape change along 100 km of the Loire and Arroux Rivers in southern Burgundy, France. Seven informally designated allostratigraphic units have been differentiated (T8-T2), each representative of time periods when rivers were adjusted to prevailing hydrological regimes.;Glacial/periglacial episodes resulted in braided stream systems and aggradation of cross-bedded channel facies with intra-stratal cryogenic structures ca OIS 6, 80--40 ka, and 22--11 ka. Meandering rivers developed during interglacial episodes (maritime climates), resulting in sediment removal from valley axes and/or the deposition of single-story, bimodal channel/overbank facies, without intra-stratal cryogenic structures. The transition from braided to meandering streams is recorded as a shift in facies from erosionally bounded, trough cross-stratified sand and gravel to diffuse gravel sheets, between 11 ka and 7.3 ka. Rapid meandering and lateral reworking of older deposits between 6.4 ka and 3 ka resulted in thin sandy overbank deposition and organic sedimentation in abandoned channels, during a moist maritime climate. Channel width-depth ratios decreased and vertical accretion of silt and fine sand dominated the floodplain setting ca 2--1.2 ka, resulting from a shift to an increasingly Mediterranean style climate. An undulating scour and fill topography and a wide, straightened channel developed during the Little Ice Age under the influence of meridional circulation and frequent, large floods. Following the Little Ice Age, an increasingly zonal circulation resulted in dry conditions punctuated by large, infrequent floods and the deposition of coarse facies in overbank floodplain settings. Regional similarities in the timing of depositional episodes across France, and strong relationships between discharge regimes and atmospheric circulation over the last 115 years, support the hypothesis that climatic change is the dominant extrinsic control on the development of these fluvial successions.