Reconstruction of Lake Balkhash levels and precipitation/evaporation changes during the last 2000 years from fossil diatom assemblages

Abstract

Environmental changes in arid central Asia are prominently manifest through changes in the water balance. Lake Balkhash, the largest lake in Kazakhstan, is a shallow lake situated in a closed basin. We obtained a 600-cm-long core from the western part of the lake in 2007 and carried out analyses of fossil diatoms and ostracods, geochemistry, and grain size of the sediments. The age control of the core is based on radiocarbon ages of fossil ostracods and, in the uppermost part, the AD 1963 137Cs activity peak and the first appearance of 137Cs in AD 1952. We recognized two main diatom assemblages by detrended correspondence analysis, one characterized by the dominance of freshwater planktonic species, and the other dominated by brackish planktonic, brackish, and marine benthic, and freshwater benthic species. The first assemblage suggests higher lake levels, and the second suggests lower lake levels. The observed assemblage changes are consistent with lake-level observations recorded during the last 120 years. The diatom assemblage data from the core indicate that there were seven periods when the lake level was low during the last 2000 years; moreover, the pH of the lake changed together with the lake level. In addition, during periods when the lake level was low from AD 1260–2000, the abundance of fossil ostracods and the Ca and TOC contents were low in comparison with the periods before AD 1260. The causes of low water levels were essentially the same between Lake Balkhash and the Aral Sea during the studied period. One likely cause of the low water levels, especially those during the last 1200 years, is changes in solar activity. Furthermore, some environmental changes during the past 40 years can be explained by human activities.