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When these one-cell algae die, they drift down, shrouding the lake floor with a thin, white layer.
Based on the results of some duplicated countings of selected subsamples and independent counting of different subsamples collected from the same horizon, we estimate that the counting error is less than 1.5%, corresponding to 150 yr for 10,000 varve years.
Such varves are deposited in proglacial lakes annually because of the seasonal changes in the ablation of the glacier and the amount of meltwater feeding the lake.
Since then the definition of a varve has been extended so that it can be used to describe any layer which is deposited annually, the varves in proglacial lakes being only one example.
At the bottom of Lake Suigetsu, thin layers of microscopic algae have been piling up for many years.
The alternating layers of dark and light count the years like tree rings.
From the laminated sediments we selected terrestrial-origin macrofossils such as leaves, branches and insects for AMS 14C measurements.