FLUVIAL SYSTEM RESPONSE TO LATE CENOZOIC CLIMATE CHANGE - THE BRITISH PERSPECTIVE

Investigations of the geometry, sediment composition and timing of river systems over the British land area reveals a number of characteristic patterns, that reflect climate forcing over the late Cenozoic. Precession-forced changes are reflected by large, low energy catchments draining low relief landscapes with thick soil cover and thick biomass, delivering only fine-grained suspended load from the catchment interior to the coastal region. Any coarse-grained material moved at this time is a function of local availability rather than regional energetics and the products of this activity were deposited at long timescales which recorded the effects of neotectonics. Obliquity-forced changes, enhanced by erosion-initiated uplift, operated in large, well organised catchments in which climate cycles changed from permafrost (possibly with glaciation in the mountains) to temperate, and transported coarse grained materials across relatively large areas to the adjacent seas at relatively rapid sedimentation rates. With the onset of eccentricity-forced climate cycles and the development of lowland glaciation the major river catchments were disrupted or even obliterated, sediment was stored in newly created sumps in much altered, or newly created river valleys, and major fluvial features such as the North Sea Delta were switched off. Within this forcing pattern, local factors have had a distinctive role on river activity in a way that did not apply during earlier parts of the Cenozoic, and rivers reworked their sediment load in a spatially discontinuous, and temporally erratic pattern. These factors include: i) extensive lowland glaciation which locally redistributed sediment and stimulated subsequent paraglacially-driven activity with enhanced erosion and sediment transfer; ii) glacio-isostatic rebound which locally enhanced river energy and river activity, iii) extreme changes in shoreline position which caused equally extreme changes in channel and valley configuration causing local, rather than regional sediment storage and iv) very rapid and short lived changes of climate such as at the end of the Last Glaciation, which have caused highly unstable, and geographically discontinuous landforms and sediment bodies, a pattern that is being replicated at the present day by Human activity.