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An 81-cm sediment core from Chaiwopu Lake in arid northwest China was analyzed for <sup>137</sup>Cs activity and concentrations of major and trace elements. We used these data to discriminate between the influence of climate change and human activities on the geochemical change of the lake sediments over the past century. Elements Al, K, Ba, Ti, V, Fe, Ni, Mn, Li, and Be were mainly from detrital. Ca, Sr, and Mg concentrations were controlled by chemical weathering processes. Na came mainly from salt precipitation caused by a decline in water level. Enrichment factors for Pb and P in recent deposits are large, indicating they were influenced by human activies. Geochemical conditions during the past century can be divided into three stages: i) From ca. 1900 to the1950s element concentrations varied widely and frequently. In general, concentrations of typical mobile elements Ca, Sr, and Mg stay relatively high whereas values for other elements remained relatively low. This was interpreted to reflect variable climate under conditions of weak surface erosion intensity. ii) From the 1950s to the early 2000s, element concentrations display less variability. The Al, K, Ba, Ti, P, Cr, V, Fe, Ni, Mn, Co, Cu, Li, Zn, Be, Pb, and Na contents were generally higher, whereas contents of Ca, Sr, and Mg were on average lower. This indicates that the regional environment was conducive to surface erosion. Enrichment of trace metals and major elements in the sediment reflects enhanced human activities. iii) In the last decade, Pb and P exhibited a great increase, possibly associated with the input from fossil fuel combustion, sewage discharge and non-point-source pollution in the watershed. The lake volume decreased substantially because of groundwater extraction for municipal water, which resulted in a marked increase in salinity and enhanced Na precipitation

A 100-year record of climate change and human activities inferred from the geochemical composition of sediments in Chaiwopu Lake, arid northwest China