Measurements of Radiocarbon Concentrations by Accelerator Mass Spectrometry in the Bottom Sediments from Lake Tilitso in Nepal, Himalayas

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Concentrations of radionuclides such as ¹⁴C, ¹³⁷Cs, ²¹⁰Pb, and ²¹⁴Pb, the contents of organic C and N, and ¹³C/¹²C ratios were measured for near surface sediments collected from Tilitso, a high altitude lake in Nepal, Himalayas. Living attached algae obtained from the streams feeding Lake Tilitso were also analyzed on their ¹⁴C abundances. The ¹⁴C concentration, Δ¹⁴C, was measured by direct detection of ¹⁴C atoms using a Tandetron accelerator mass spectrometer, on the acid-insoluble organic carbon that was extracted from each sediment or each algae sample. Activities of ¹³⁷Cs, ²¹⁰Pb, and ²¹⁴Pb in the sediments were measured with a coaxial-well-type high-purity-germanium detector.
The sedimentation rate was estimated to be 0.56±0.27 cm y^-1 by the ²¹⁰Pb method. The content of carbon as acid-insoluble organic compounds was from 0.5 to 0.7%, and such carbon was depleted in ¹⁴C, yielding Δ¹⁴C values between -855±5 and -905±4‰ (apparent ¹⁴C ages between 15,520±250 and 18,910±360 y BP). Values of Δ¹⁴C for attached algae samples were also low, ranging from -463±31 to -701±29‰ (apparent ages from 4,980±460 to 9,700±780 y BP). The unexpectedly low ¹⁴C concentrations of these sediment and attached algae samples can be reasonably explained by considering geological and climatic environments around Lake Tilitso.