In lake sediments there are typically two principal proxy sources of environmental information that are suitable for study by means of stable isotope analysis: calcium carbonate and organic matter.

Calcium carbonate is commonly present as the mineral calcite or aragonite, formed either biogenically, as mollusc shells, ostracode carapaces or by charophytes, or by direct, apparently abiogenic precipitation from the water column (lake marl). The oxygen isotopic composition of these carbonates is determined by the isotopic composition of the lake water and the temperature of the water at the time of precipitation, both of which are a function of local climate. Carbon isotope ratios in carbonate minerals are little affected by temperature, but the isotopic composition of a lake's pool of dissolved inorganic carbon is strongly influenced by both biological (e.g rates of primary production and remineralisation) and inorganic (e.g. exchange between aqueous and atmospheric CO2) processes, all of which are climatically related. Thus the measured oxygen isotopic composition may provide information on features such as temperature variations, relative rates of inflow versus water loss by evaporation, or changes in the relative importance of the different air masses responsible for the regional water supply. A common problem with oxygen isotope studies of ancient carbonates is that we often operate with two unknowns - the temperature and isotopic composition of the water - but independent ways can often be found to minimise the resulting errors. In addition to rates of primary production, the carbon isotopic composition can provide insights into water column stability.

Sedimentary organic matter (OM) may be analysed for its hydrogen, carbon, nitrogen and oxygen isotopic ratios. For scientific and technical reasons, the NORPEC project is currently restricted to C- and N-isotope studies. Carbon isotopes in bulk lacustrine OM are determined by a number of factors, most notably the isotopic composition of the carbon source, carbon availability, and the photosynthetic pathway(s) utilised by the producing organisms. As with carbon isotopes in carbonates, isotopic studies of organic matter can give insights into rates of primary production and water column stability and, in addition, sources of organic matter. Nitrogen isotopes provide information on a similar range of environmentally related parameters and in some circumstances, rather precise information on the taxonomic affinities of the main primary producers. As factors such as nutrient supply, rates of primary production and water column stability are all related to local climate, isotopic studies of lacustrine OM provide proxy data that complement those obtained from isotopic analyses of lake carbonates. A common problem related to studies of lake sediments is that the bulk OM is typically variable mixtures of autochthonous and allochthonous (including terrestrial) material. In such circumstances, interpretation of the measured isotopic composition may not be straightforward. However, additional techniques such as smear-slide inspection, elemental and pyrolysis analysis can provide some measure of the relative proportions of various components.