Chironomid-inferred late-glacial and early-Holocene mean July air temperatures for Krakenes Lake, western Norway

A chironomid data-set calibrated to July air temperatures, based on 44 lakes in western Norway, is used to reconstruct mean July air temperatures from late-glacial and early-Holocene fossil chironomid assemblages at Krakenes Lake. The calibration function is based on Weighted Averaging Partial Least Squares regression and has a root mean square error of prediction (RMSEP) of 1.13 degrees C, a r(2) of 0.69, and a maximum bias of 2.66 degrees C. All these statistics are based on leave-one-out cross-validation. A calibration function based on summer surface-water temperatures has a poorer performance (RMSEP = 2.22 degrees C, r(2) = 0.30, maximum bias = 5.29 degrees C). The reconstructed July air temperatures at Krakenes rise to 10.5 degrees C soon after deglaciation, are about 11.5 degrees C in the Allerod, decrease to 9.5-10 degrees C in the Younger Dryas, and rise rapidly within 15 yrs to 11.5 degrees C at the onset of the Holocene. There is a two-step rise to 13 degrees C or more in the early-Holocene. The likely over-estimation of Younger Dryas temperatures and under-estimation of early-Holocene temperatures probably result from the limited temperature range represented by the existing calibration set. The data set is currently being expanded to include lakes with warmer air temperatures (> 14 degrees C) and with colder air temperatures (< 8 degrees C).