GEOMORPHOLOGICAL SIGNATURES OF VARYING CLIMATE

A LONGSTANDING question in geomorphology(1,2) is whether the topograph y of a particular landscape is in balance with current climate-driven processes, or contains relict signatures of past climates. For the gla ciated landscapes of the Northern Hemisphere, the latter obviously app lies but the situation is far from clear in regions where climate-driv en processes have changed only in intensity, rather than character. We have addressed this question using a mathematical model of landscape evolution(3,4) and find that both cases-contemporary balance and relic t features-are possible. For a sinusoidal climate fluctuation, we find that all climate states (wet and dry) leave geomorphological signatur es only when there is no active uplift. With active uplift and the ass ociated increase in erosion, the topography tracks the current climate and any relict features are likely to reflect only the wettest condit ions previously experienced by the landscape. In both cases, the tempo ral evolution of the landscape in response to cyclic climate forcing i s complex, and leads to the unexpected result that valley density is l argest during periods dominated by slow downslope movement of sediment , rather than during times of strong fluvial incision, as would be ant icipated from steady-state models(5-7).