DIATOM OXYGEN ISOTOPES IN PRO-GLACIAL LAKE SEDIMENTS FROM NORTHERN SWEDEN: A 5000 YEAR RECORD OF ATMOSPHERIC CIRCULATION

We use a pro-glacial oxygen isotope record of diatom silica (d¹⁸O_diatom) and a sedimentary proxy for glacier fluctuations to determine centennial-millennial scale climate change during the last 5000 years in northern Sweden. Together with results from previous studies, we show that the lake water isotopic composition predominantly reflects the isotopic composition of summer precipitation. Superimposed on a general depletion trend of 3.5 ‰ we find that the isotopic composition became depleted (>1 ‰ excursions) during four occasions. Climate simultaneously sustained a positive glacier mass balance, causing the catchment glacier to advance around 4400, 3000, 2000 and after 1200 calendar years BP. The d¹⁸O_diatom depletion and glacier events all occur at times of relative ice rafted debris (IRD) maxima in the North Atlantic consistent with cold conditions. A persistent change in the atmospheric circulation pattern could potentially have caused the registered changes in d¹⁸O_diatom because different air masses hold characteristics d¹⁸O signatures of their precipitation. The glacier mass balance primarily responds to the influence of summer temperature on ablation. We suggest that the most likely cause for the recorded changes in both these proxies is a steadily increasing but fluctuating dominance of colder and d¹⁸O depleted air masses from the north/northeast during the past 5000 years.