Diatom oxygen isotopes in pro-glacial lake sediments from northern Sweden: a 5000 year record of atmospheric circulation

Abstract

We use a pro-glacial oxygen isotope record of diatom silica (δ 18O diatom) and a sedimentary proxy for glacier fluctuations to determine centennial–millennial scale climate change during the last 5000 years in northern Sweden. We show that the lake water isotopic composition predominantly reflects the isotopic composition of the precipitation. Superimposed on a general depletion trend of 3.5‰ over the past 5000 years we found that the isotopic composition of precipitation became depleted (>1‰ excursions) during four occasions centered at 4400, 3000, 2000, and after 1200 cal yr BP. Climate simultaneously sustained a positive glacier mass balance, that caused the catchment glacier to advance. A persistent change in the atmospheric circulation pattern could potentially have caused the registered changes in δ 18O diatom because different air masses hold characteristic δ 18O 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 δ 18O depleted air masses from the north/northeast during the past 5000 years. The δ 18O diatom depletion and glacier events all occur at times of relative ice-rafted-debris maxima in the North Atlantic, consistent with cold conditions and changes in surface wind directions. Our results confirm that changes towards a predominance of north/northeasterly winds occurred at these time intervals.