Molecular marker records of land use change

Evidence of a changing environment in the catchment area of a small lake (Gormire, Yorkshire, UK) over the past 3000 years is provided by the mean carbon number of n-alkanes and the ratio of lignin thermochemolysis products, together with pollen analysis and bulk organic geochemistry. Periods of deforestation, which commenced at 600 BC and AD 1200, display a significant decrease in organic carbon contents of the lake sediments, which probably reflects dilution by enhanced influx of clastic material. The mean carbon number of waxy (C27–C31) n-alkanes closely corresponds with the percentage of pollen derived from grass in the sediments. A higher-plant derived triterpenoid, tentatively identified from its mass spectrum as 28-carboxyursen-12-enol, appears exclusively in samples where tree and shrub-derived pollen is dominant. Thermochemolysis of lignin confirms that there was a significant change in the nature of material deposited in the lake sediments from woody to grass dominated. Changes in both the sedimentary C/N ratios and yields of lignin-derived phenols suggest that deforestation events led to enhanced aquatic productivity, initially through the development of reed-swamp vegetation.