Seasonality and the isotope hydrology of Lochnagar,a Scottish mountain lake: implications for palaeoclimate research

The relationship between isotope ratios in precipitation and lake waters was monitored by bi-weekly measurements taken over a five and a half year period between May 2000 and September 2005 (a period of 1964 days) at Lochnagar, a remote mountain lake in eastern Scotland. Short-term changes in the oxygen isotope composition of lake water (δ¹⁸O_L) at Lochnagar follow a seasonal pattern similar to isotopes in local precipitation (δ¹⁸O_P), however changes in catchment residence time, snow accumulation, lake ice cover and lake stratification modify the temporal structure of δ¹⁸O_L over the seasonal cycle. Of particular importance is precipitation amount, which controls catchment and lake residence times, and determines the degree of phase lag and amplitude change between δ¹⁸O_L and δ¹⁸O_P. A simple mass balance model replicates these effects and demonstrates that the degree of phase lag and amplitude reduction is predictable given known input/volume ratio. The implications of these observations for the use of δ¹⁸O records in palaeoclimatology are important, since it is rare that authigenic and biogenic minerals or organic compounds (from which δ¹⁸O and/or δ²H can be measured) are produced in a lake evenly throughout the year.