The researchers studying high-altitude glaciers failed to pick up radioactive signals from three ice cores collected from the 19,849 feet Naimona'nyi glacier on the southern margin of the Tibetan Plateau.

In some places, for some months each year, those rivers are severely depleted now, the researchers said.

In a paper published in Geophysical Research Letters, researchers from the Byrd Polar Research Center explain that levels of tritium, beta radioactivity emitters like strontium and cesium, and an isotope of chlorine are absent in all three cores.

The absence of radioactive signals in the top portion of these cores is a critical problem for determining the age of the ice in the cores, the researchers said.

The signals, remnants of the 1962-63 Soviet Arctic nuclear blasts and the 1952-58 nuclear tests in the South Pacific, provide well-dated benchmarks to calibrate the core time scales.