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 (delta O-18(L)) at Lochnagar follow a seasonal pattern similar to isotopes in local precipitation (delta O-18(P)), however changes in catchment residence time, snow accumulation, lake ice cover and lake stratification modify the temporal structure of delta O-18(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 delta O-18(L) and delta O-18(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 delta O-18 records in palaeoclimatology Are important, since it is rare that authigenic and biogenic minerals or organic compounds (from which delta O-18 and/or delta H-2 can be measured) are produced in a lake evenly throughout the year.