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How do varves in glacial lake sediments record seasonal changes?
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glacial landform
Eskers,
or narrow
ridges of
gravel and sand left by a retreating glacier,
wind through western
Nunavut, Canada, near the Thelon River.
Varves are annual sedimentary layers that record seasonal changes through the formation of distinct, paired sediment couplets in glacial and proglacial lakes. These deposits function as a natural calendar, reflecting the yearly climatic cycle.12
The seasonal process occurs as follows:
- Summer: As temperatures rise, ice melting and outwash transport sediment into the lake. Coarser materials, such as silt and fine-grade sand, settle relatively quickly to the lake bottom, creating a lighter-colored, thicker layer.12
- Winter: The lake surface typically freezes, halting new sediment input. In this calm environment, the finest particlesâclaysâslowly settle out of suspension, forming a thin, dark-colored layer.12
This annual cycle of coarse summer sediment and fine winter clay creates a "couplet" that serves as the hallmark of a varved deposit. Because the thickness of these layers varies according to the climatic conditions of the melt season, scientists can use varve chronology to establish precise timelines and correlate environmental data across different lake basins.3