Grain-size of varved clays from the north-eastern Baltic Ice Lake: Insight to the sedimentary environment

- Sediment grain size is a proxy of the water depth in the glaciolacustrine lakes.
- It is a proxy of the sediment settling time - length of the winter - as well.
- The grain size indicates that the Baltic Ice Lake was permanently stratified.

Besides providing high resolution chronological information, varved sediments also are excellent environmental archives. We examined the grain size distribution of varved glaciolacustrine sediments as a proxy for estimation of the water depth and the duration of winter - a period with diminished sediment input from the melting glacier and restricted water circulation due to ice cover. The particle size at the top of the winter layer is assumed to reflect the time available for a particle to settle from the top of water column during the winter before water mixing and new sediment input in spring. Glacial varves from Pärnu Bay in SW Estonia, where a local varve chronology of 584 years was established previously is examined as a case study. X-ray absorption granulometer was used to determine the grain size distribution within 10 varves with 2-14 samples collected from each varve. The coarsest particle size found on top of the winter layer is calculated for each varve from measured grain size distributions using a novel methodology and compared to the reconstructed water depth of the Baltic Ice Lake to constrain the likely duration of the winter. A high variability of the constrained winter length with an average close to the duration of a calendar year was found. It is concluded that the coarsest particles on top of the winter layer have settled from intermediate depths due to water stratification, or less likely the actual water level was lower than reconstructed. We conclude that the methodology can be used to constrain environmental parameters in glacial lakes where varved sediments are formed.

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