Lacustrine sediments

One of the best archives of terrestrial palaeo-climate information is lake sediment. Annual and decadal climate shifts influence sediment production and deposition in lakes. During the winter, lakes, at least in mountainous regions, are normally frozen and little clastic sediment enters the lakes. Organic production is effectively reduced due to little sunlight and snow-covered lake ice. In spring and early summer, strengthened sunlight, ice-free lake conditions, and nutrients released from melting ice and snow, support diatom blooms. Pollen from adjacent plants is also deposited in lakes during the summer season. Terrigenous silts and clays, deposited in the lakes from surface runoff from rainfall and snow-melt throughout the summer, settle out of suspension and form another sediment layer. This seasonal alternation of depositional regimes results in the annual production of laminae/ varves.

Sediments accumulating in proglacial lakes contain information about glacier fluctuations in the form of variations in particle size, sediment thickness, and organic minerogenic content (e.g. Karlen, 1976,1981,1988; Leonard, 1986; Nesje et al., 1991; Karlen and Matthews, 1992; Matthews and Karlen, 1992). Such sediments commonly form continuous records reflecting climate and glacier fluctuations, because the relative amount of minerogenic silt and clay eroded by the glacier varies with glacier activity. Glacier fluctuations may therefore be compared directly with variations in pollen influx and the relative pollen content, which reflect local vegetation changes. In lakes with rhythmic sedimentation, precise dating may provide information about climatic/environmental changes and different response times between physical and biological systems.

Several factors, including the size, depth and bathymetry of the lake, the altitude of the lake, its distance from the glacier, the proportion of the catchment glacierized, whether coarse sediments are trapped in upstream lakes, and the form of the surrounding hill slopes and especially their exposure to avalanche activity, must be seriously evaluated in lacustrine sediment studies. Lacustrine sediment studies normally include: loss-on-ignition (LOI) measurements, grain-size distribution (sedigraph), X-radiography of sediment cores, visual counting of laminae/ varves, 210Pb and accelerator mass spectrometry (AMS) radiocarbon dating, palaeomagnetic measurements, pollen counts, diatoms, plant macrofossils and chironomids.

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