Organo-geochemical and stable isotope indicators ofenvironmental change in a marl lake, Malham Tarn, North Yorkshire,U.K.

Abstract

Sediments of the marl lake Malham Tarn located in NW Englandpreserve an environmental record since 12 Ka. Eight Holocene pollen zones wereidentified, and the δ13C of total organic carbon (TOC) showsthree stratigraphic divisions. The basal clay unit and overlayingsand/clay/marl unit have δ13C of −24‰which decreases at the base of the principal marl unit to a mean value around−30‰, whilst the topmost black marl unit δ13Cincreases to −28‰ at the surface. Representative samples of theseunits were selected for analysis of n-alkanes andn-fatty acids and their δ13C.Samples of modern Chara and peat were analysed forcomparison. The clay unit has a minor contribution of redeposited matureorganic matter and autochthonous algae, the marl unit a high contribution ofChara, and the dark marl unit has a high contribution fromhigher plants. Compound-specific δ13C revealssystematic differences between alkanes and fatty acids of different chainlength. The major shift in δ13C in the short and medium chainfatty acids are probably due to the decreasing influence of carbonate rockflour as source of DIC. The major shift in δ13C in the longchain n-fatty acids andn-alkanes could reflect the lower atmosphericCO2 concentration at Last Glacial. The negative shift of short chainfatty acids in organic rich dark marls reflects introduction of detrital peatinto the lake. The δ13C results show a dramatic change fromdominance of autochthonous plus eroded sources up to Pollen Zone IV, then slowcolonisation of the hinterland by higher plants, followed by constantChara contributions throughout the deposition of the marl,and a further increase of higher plant material after the rise in water levelin 1791.