Lake sediments are normally characterised by relatively rapid and continuous sedimentation over thousands of years. The physical properties of the sediments and the remains of plants and animals (including their stable isotopic composition) preserved within them render them excellent archives of the past environment and hence of past climatic change. In this section we first consider how climate affects lake ecosystems today, then discuss the sensitivity of the lake-sedimentary record as a reflection of past environment, outline how quantitative reconstructions of the past environment can be made using calibration functions, and discuss the need for multi-proxy approaches.

Lakes and climate

Lakes are complex ecosystems and have intricate relationships with climate today. Lakes include not only lake water but also the lake catchment. Both play an interactive role with climate. Temperature influences the catchment through the stability of the landscape (if ice, snow, or frost are present) and through terrestrial soil development and plant and animal communities. These influence the inwash into the lake of minerogenic and organic material and of base cations, nutrients, and dissolved organic carbon. Study of the minerogenic part of the sediment can reveal the proportion and type of inorganic material and hence provide evidence for its likely origin - glacial, periglacial, flooding, avalanche, etc. The organic component includes terrestrial fossils (e.g. pollen, plant macrofossils, oribatid mites) that can yield information about the catchment and its biota, and hence climate.

Temperature affects a lake directly through water temperature, including summer stratification and the amount and duration of ice-cover. The types and abundances of lake organisms will depend on the temperature. Precipitation effects are linked with those of temperature. Vegetation type and soil stability of the catchment depend on the amount of rain and snow, and the length of winter snow-cover. Precipitation provides the main means by which material is transported to the lake via streams and slope-wash. Precipitation (or evaporation) may also affect a lake directly by changes in lake-level. Wind affects lakes by redistributing snow, by water disturbance affecting stratification, and by wave action that affects shoreline erosion.

Internal lake processes are controlled ultimately by climate (mainly temperature and wind), but they are also self-driven by successional processes. The balance between nutrient supply, productivity, water oxygenation, and summer stratification is important in producing organic lake sediment that accumulates in the relatively oxygen-deficient and stable environment of the deepest part of the lake. The fossils are incorporated into the sediment in a time sequence. A time scale is normally provided by radiocarbon dating followed by statistical age-depth modelling.

Sediments may sometimes be disturbed by factors such as avalanches, mass flow, and slumping causing massive sediment displacements. These can yield important environmental information, particularly of rare extreme events such as avalanches or floods.

Lake sediments as a record of past environments

Sediment composition reflects the stability of the catchment and the productivity of the lake. The minerogenic component derives mostly from erosion in the catchment, sometimes as a result of catastrophic events. In NORPEC we use minerogenic material produced by glaciers in the catchment as an indicator of changes in glacier activity. Combined with summer temperature estimates from biological proxies in lakes unaffected by glacial outwash, we can estimate winter precipitation through the Holocene. The palaeomagnetic record can be used as a time-scale, as a correlative tool to record changing sources of minerogenic material, and as an indicator of the integrity of the sedimentary record of an individual core. Stable isotopes can also be used to provide environmental and climatic records.

The organic component, including fossils, can be washed in from the catchment and/or produced within the lake. The most abundant terrestrial fossil type is pollen and spores which gives a climatic signal related to the regional vegetation that produced the assemblage. Important regional vegetational changes include revegetation after deglaciation and changes in tree-lines (link to Wenche: Dovre). In favourable circumstances, lakes also receive large plant remains (macrofossils) produced more locally in the catchment. They provide a more precise record of local vegetation. Animal remains such as oribatid mites also reach the lake, and these provide information on different aspects of the past environment such as local soil types and microhabitats within the catchment.

Fossils of aquatic organisms are usually plentiful. The most abundant fossils are diatoms, and their species composition and relative abundance reflect lake-water chemistry (e.g. pH, total P) and the length of the ice-free growing season. In addition, pollen, spores, and macrofossils of aquatic plants give a picture of the littoral habitat, where the occurrence and density of macrophytes is an important habitat factor. Insects, particularly chironomids, usually have an excellent fossil record. Chironomids appear to be particularly sensitive to lake-water temperature which, in turn is a function of climate, as well as to water depth and nutrient and oxygen status.

Quantitative environmental reconstructions

Limnic palaeoenvironmental studies reconstruct aspects of the past environment from sediment properties or fossil assemblages. The methodology of quantitative calibration or transfer functions has revolutionised palaeoenvironmental research. The primary aim is to express the value of an environmental variable (e.g. mean July temperature) as a function of biological data (e.g. pollen assemblages). This function is called a calibration or transfer function. Calibration functions form the basis of much quantitative palaeoclimatology and can be used to reconstruct, for example, mean July temperature (pollen, chironomids), mean January temperature (pollen), annual precipitation (pollen), atmospheric pCO2 (macrofossils), lake-water pH and dissolved organic carbon (DOC) (diatoms), and lake ice-cover (diatoms). They provide the means of quantifying 'proxy' data into palaeoenvironmental estimates that can be used in the validation of GCM inferences of past climate from climate model experiments.

Multi-proxy studies

Ideally as many types of evidence as possible should be combined in a multi-disciplinary study. Natural systems respond by a complex interaction between different components and forcing functions. In the case of lakes, these include direct lake-climate interactions and catchment-climate interactions and indirect lake-soil-vegetation-catchment interactions. Because of the complexity of the system, it is important to reconstruct climate or climate-related variables from as many proxy sources as possible (lake sediments, pollen, chironomids, diatoms, plant macrofossils, mites, stable isotopes) and to look for consistencies, differences, and anomalies in the records.

Over the last decade Quaternary science and palaeolimnology have experienced a revolution in quantitative approaches and techniques, many of which are directly applicable to quantitative environmental reconstruction. This project will use these techniques to develop quantitative inference models (calibration functions) to reconstruct past climate from different lake-sedimentary records. The diagram at the right shows July temperatures at Kråkenes, Norway, as inferred from pollen, plant macrofossils, chironomid midges, and cladoceran deposited in lake sediments. (source: H.H.Birks: PAGES Newsletter 2000-2:17-18. (http://www.pages.unibe.ch)