Quantifying landscape responses to human action through optical dating; examples from the Netherlands

Although it is widely recognized that humans alter the landscape both directly and indirectly, quantification of the induced landscape dynamics remains problematic. Suitable organic material for radiocarbon dating is often not available, either because the sediment of interests lacks organic material (e.g. drift sands), or because the age of the incorporated organic material does not reflect the timing of the landscape response (e.g. fimic soils). Over the past years, we have gained experience in quantifying anthropogenic induced landscape dynamics through the use of optically stimulated luminescence (OSL) dating methods. In this presentation we will present two examples based on multiple site studies: Firstly, we show how OSL dating provides accurate ages for the time of deposition and burial of Holocene reactivation phases of aeolian sand deposits (drift sands). We use the established chronologies to determine the rates of drift sand accumulation as well as periods of reduced aeolian activity. Such quantitative information is essential to make inferences about causes for drift sand activation (climate vs. human), and feedbacks involved. Secondly, we present OSL chronologies obtained for the build-up of fimic anthrosols through plaggic agriculture. In many places in NW-Europe, heath sods and manure were used to fertilize sandy soils, resulting in elevated fields suitable for agriculture. In these systems, resetting of the OSL signal takes place during ploughing of the fields, and OSL ages reflect the time that a soil horizon is buried below the plough depth. We find that OSL methods are particularly successful for determining the accumulation rate during later stages of plaggen agriculture; methodological challenges remain to accurately determine the timing of initiation of these agricultural systems. Combined modelling of landscape evolution and soil development, informed and constrained by dating results, can allow testing of hypotheses about this initiation.