Varves are the best indicators of ancient climate change.

Varves has the similarity with the regular, annual rings of the trees.

by

Dr. Nitish Priyadarshi

The study of climates during the geological past, is one of the most topical areas of research in the geosciences at present. The threat of future climate change caused by higher levels of green house gases, which would drastically alter many aspects of our environment, has prompted much research to try to understand how our own complex climate system works. Only by understanding how climate has evolved over million of years can we identify important climate cycles with a frequency in excess of the short climate records we possess. These climate cycles have the potential to have a profound effect on our environment.

Understanding our climate history in the geological past is also important for climatologists trying to construct accurate numerical computer models of our present climate system to use for predicting future climate change. It is obviously not possible to check the accuracy of models that are predicting the future so climatologists must turn to the past to see if their models can accurately simulate ancient climates. It is therefore the role of the geoscientists to collect as many data as possible from the rock record.

There are many rocks/minerals whose formations are controlled by climate, such as bauxite, coal and varves.
This article aims to introduce the formation of varves and how they can be used to determine ancient climate conditions.
What is Varve?

A varve is any sedimentary bed or lamination deposit within the period of one year, or any pair of contrasting laminae representing seasonal sedimentation (as summer and winter) within the period of one year.

In its broadest sense the term is applied to the layer of sediment deposited in a single year. In practice, however, the use of the term is almost always confined to sediments deposited in glacial melt-water lakes. These consist of coarser layer, representing summer deposition, and a finer layer, representing winter deposition. Counting and correlation of these varves has led to the development of a detailed Pleistocene chronology for the Northern Hemisphere.

Varves form distinctive layers. A varve consists of two layers; a thick light colored layer of silt and fine sand which forms in the spring and summer and a thin dark colored layer of clay forming in the fall and winter. Since low temperatures are necessary in delaying the settling of clay particles, it is assumed that varve formation can occur only in glacial waters, particularly lakes on the margins of glaciers. The salt and electrolytes in seawater cause a homogenous mass, preventing the formation of varves.

The deposition of varved sediments is either caused by seasonal variations in the glacial melt-water discharge (glacial varves) or by seasonal variations in the river discharge (postglacial varves).

The study of varved clays goes back to about 1880, when De Geer, working in the Stockholm area in Sweden, “was struck by the marked cyclical banding of the varved clay. From the obvious similarity with the regular, annual rings of the trees. Geer got at once the impression that both ought to be annual deposits”.

According to Sauramo (1923), there are four characteristics of varve: the grain size, the colour, the plasticity, and the chemical composition. The dominating rhythemic arrangement of a varve is alteration between coarse and lighter summer units and finer and darker winter units. These granulometrically graded varves are termed diatactic varves. There are also non graded varves, with the seasonal rhythm expressed only in the colour ranges (darker winter units). These varves are known as symmict varves and are common in brackish water environment.
Of the many rhythmites found in the geological record, varves are one of the most important and illuminating to studies of past climate change. Varves are amongst the smallest-scale events recognised in stratigraphy. Varves form only in fresh or brackish water, because the high levels of salt in normal sea water coagulates the clay into coarse grains.

Varve analysis is the process of counting varves or annually laminated sediments to determine the rates of change in climate and various ecosystems. Varves form when glacial advances come in contact with bodies of water such as lakes. When this process occurs, layers of sediment form on the floor of the body of water. This technique provides an opportunity to acquire detailed chronological information about the composition, displacement, and climate of that region, at that time.
Reference:
Doyle, P. and Bennett, M. R. 1999. Unlocking the Stratigraphical Record, advances in Modern Stratigraphy. John Wiley & Sons, New York.

Fairbridge, R. W. and Bourgeois, J. 1978. The Encyclopedia of Sedimentology. Dowden, Hutchinson Ross, Inc. Pennsylvania.

Sauramo, M., 1923. Studies of the Quaternary varve sediments in southern Finland, bull. Comm.. Geol. Finlande, 60, 164p.

http://www.mnsu.edu/emuseum/archaeology/dating/dat_varve.html