This layered deposit of sediment is called a varve, and the researchers used glacial varves
collected in the 1980s and 1990s for (http://dx.doi.org/10.1038/s41467-017-01273-1) their study , corresponding to a 1,000-year time period between 13,000 and 12,000 years ago. A thicker layer in a varve is an indicator of more melting since a larger volume of water would be needed to carry more debris.
The general nature of the sediments, consisting of typical glacial varves
, aquatic organic matter, and infrequent gravity-type deposition indicates sedimentation processes that involved vertical particle settling, ideal for reconstructing lake nutrient histories.