Luminescence and Carbon Dating of Fimic Horizons and Drift-sand Deposits in Cultivated Sandy Landscapes (SE Netherlands).
J. M. Van Mourik, Univ of Amsterdam, Institute for Biodiversity and Ecosystem Dynamics, Amsterdam, Netherlands
Aeolian sands, deposited during the Late Glacial of the Weichselian, cover an extensive part of the Netherlands. The geomorphology of this cover-sand landscape is dominated by ridges, plains and brooks. During the Holocene this landscape stabilized under vegetation and soils developed with cambic podzols under oak forest on the ridges, gleyic podzols under alder forest on the plains, and histosols in depressions and valleys of brooks. During the Late Holocene humans influenced this landscape. Shifting cultivation in the Bronze Age led to forest degradation, and during the Iron Age degradation increased as oak was used in the production of charcoal and parts of the deciduous forest were replaced by heath. The soils degraded from cambic to carbic podzols. After the introduction of plaggen agriculture farmers used heath sods to fertilize plots of arable land. Sods contain a small fraction of mineral grains and the application of sod-manure during a millennium or more resulted in fimic horizons (anthropogenic mineral A-horizons with thickness over 50 cm). On several sites on the heath, due to intensive sod digging, the protecting vegetation was destroyed and aeolian processes reactivated to become a drift-sand landscape. Hence, the development of fimic horizons and drift-sand erosion and deposition are closely related. Reliable dating of the introduction of plaggen agriculture is complicated and the calculation of sedimentation rates of fimic, and drift-sand deposits is difficult without reliable absolute dates. Traditional dating of fimic horizons and drift-sand deposits was based on pollen analyses and radiocarbon dating of soil organic matter. However, the types and distribution of organic matter differs between materials. In thin section, the organic matter in the fimic horizons and the A-horizons of paleosols is distributed in aggregates in voids between mineral grains with the internal fabric reflecting an excremental origin. This soil organic matter is a mixture of decomposed manure (cattle) and sods from different sites with different inherited ages. In contrast the organic matter in the B-horizons of paleosols is distributed in cutans on mineral grains and their internal fabric shows a mixture of plasmic organic matter and fine dark particles. Humic acids as well as humines can be extracted for radiocarbon dating and both are available in the fimic and buried A horizons but in the buried B horizon only the humic acids are suitable for dating. The study showed that the radiocarbon Ďages' of the humic acids are generally younger than the humines - the humines accumulate during active soil development in comparison with the faster decomposable humic acids. However, the difference in radiocarbon ages between these fractions may be related to the period of active soil processes and this information could be useful. In contrast, the OSL dating should provide correct ages to reconstruct the start and the sedimentation rate of the fimic horizon since it dates when burial of the quartz grains in the sod began as long as the grains were fully bleached when exposed at the surface such as during ploughing. In comparison to OSL ages the radiocarbon dates of samples of fimic horizons systematically overestimate the age of fimic material. Furthermore, radiocarbon dating cannot separate periods of sedimentation and soil development within a cycle though the time of initial burial can be estimated. There is also a problem of contamination of samples by younger carbon that is not always easy to detect. At this stage, OSL dates of quartz grains provide an improved stratigraphical frame work for the periods of drift-sand accumulation and thereby also provide a better estimate of the rate of pedogenesis.