Sensitivity of deep lake temperature to past and future climatic changes: A modeling study for Lac d'Annecy, France, and Ammersee, Germany

[1] Oxygen isotope ratios (delta(18)O) of deep-lake ostracod valves can provide excellent estimates of the oxygen isotopic composition of past atmospheric precipitation (delta(18)O(P)). One of the major steps of such a delta(18)O(P) reconstruction is the calculation of the delta(18)O of the lake water (delta(18)O(L)) from the valve delta(18)O, which includes an appropriate estimate of the past water temperature during valve formation. In order to simulate changes in lake vertical temperature profile and specifically the bottom water temperature during ostracod valve formation, we have redeveloped a one-dimensional eddy diffusion thermal lake model including a lake ice model and refined it by including a seasonal light extinction model. This model was tested for two different sites, Lac d'Annecy and Ammersee. Sensitivity studies show that the ostracod calcification temperature is approaching constant 3.7degreesC for colder, but may significantly vary for warmer than present conditions. In Lac d'Annecy, such temperature effects can lead to a significant underestimation of positive delta(18)O(P) excursions. In Ammersee the underestimation is smaller because the hypolimnic temperatures are less sensitive to surface temperature changes. Our model can be used to better constrain such temperature effects and such further increase the quality of the delta(18)O(L) and delta(18)O(P) reconstruction.