Varve

[
thumb|right|300px|Pleistocene age varves at Scarboro Bluffs, Toronto, Ontario, Canada. The thickest varves are more than half an inch thick. ]

A varve is an annual layer of sediment or sedimentary rock.

The word 'varve' is derived from the Swedish word "varv" whose meanings and connotations include 'revolution', 'in layers', and 'circle'. The term first appeared as "Hvarfig lera" (varved clay) on the first map produced by the Geological Survey of Sweden in 1862. Initially, varve was used to describe the separate components of annual layers in glacial lake sediments, but at the 1910 Geological Congress, the Swedish geologist Gerard De Geer (1858-1943) proposed a new formal definition where varve described the whole of any annual sedimentary layer. More recently introduced terms such as 'annually laminated' are synonymous with varve.

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.

History of varve research

Although the term varve was not introduced until the late nineteenth century, the concept of an annual rhythm of deposition is at least two centuries old. In the 1840s, Hitchcock suspected laminated sediment in North America could be seasonal, and in 1884 Warren Upham postulated that light-dark laminated couplets represented a single years deposition. Despite these early forays, the chief pioneer and populariser of varve research was Gerard De Geer. While working for the Geological Survey of Sweden, De Geer noticed a close visual similarity between the laminated sediments he was mapping, and tree-rings. This prompted him to suggest the coarse-fine couplets frequently found in the sediments of glacial lakes were annual layers.

The first varve chronology was constructed by De Geer in Stockholm in the late 19th century. Further work soon followed, and a network of sites along the east coast of Sweden was established. The varved sediments exposed in these sites had formed in glaciolacustrine and glacimarine conditions in the Baltic basin as the last ice sheet retreated northwards. By 1914, De Geer had discovered that it was possible to compare varve sequences across long distances by matching variations in varve thickness, and distinct marker laminae. However, this discovery led De Geer and many of his co-workers to making incorrect correlations, which they called 'teleconnections', between continents, a process criticised by other varve pioneers like Ernst Antevs.

In 1924 a special laboratory dedicated to varve research - the Geochronological Institute - was established. De Geer and his co-workers and students made trips to other countries and continents to investigate varved sediments. Ernst Antevs studied sites from Long Island, U.S.A. to Lake Timiskaming and Hudson Bay, Canada, and created a North American varve chronology. Carl Caldenius visited Patagonia and Tierra del Fuego, and Erik Norin visited central Asia. By this stage, other geologists were investigating varve sequences, including Matti Sauramo who constructed a varve chronology of the last deglaciation in Finland.

1940 saw the publication of a now classic scientific paper by De Geer, the "Geochronologia Suecica", in which he presented the Swedish Time Scale, a floating varve chronology for ice recession from Skåne to Indalsälven. Lidén made the first attempts to link this time scale with the present day. Since then, there have been revisions as new sites are discovered, and old ones reassessed. At present, the Swedish varve chronology is based on thousands of sites, and covers 13,200 varve years.

Formation

Varves form in a variety of marine and lacustrine depositional environments from seasonal variation in clastic, biological, and chemical sedimentary processes.

The classic varve archetype is a light / dark coloured couplet deposited in a glacial lake. The light layer usually comprises a coarser laminaset of silt and fine sand deposited under higher energy conditions when meltwater introduces sediment load into the lake water. During winter months, when meltwater and associated suspended sediment input is reduced, and often when the lake surface freezes, fine clay-size sediment is deposited forming a dark coloured laminaset.

In addition to seasonal variation of sedimentary processes and deposition, varve formation requires the absence of bioturbation. Consequently, varves commonly form under anoxic conditions.

A well-known marine example of varve sediments are those found in the Santa Barbara basin, off California. [cite journal
author = Thunell, R.C.
coauthors = Tappa, E., Anderson, D.M.
year = 1995
date = 1995-12-01
title = Sediment fluxes and varve formation in Santa Barbara Basin, offshore California
journal = Geology
volume = 23
issue = 12
pages = 1083–1086
doi = 10.1130/0091-7613(1995)023<1083:SFAVFI>2.3.CO;2
url = http://geology.geoscienceworld.org/cgi/content/abstract/23/12/1083
accessdate = 2007-04-27]

ee also

* Dendrochronology
* Dendroclimatology

References

* De Geer, G. (1940), "Geochronologia Sueccia Principles". Kungl. Svenska Vetenskapsakademiens Handlingar, Tredje Serien. Band 18 No.6.
* Lowe, J.J. and Walker, M.J.C. (1984), "Reconstructing Quaternary Environments". Longman Scientific and Technical.
* Sauramo, M. (1923), "Studies on the Quaternary varve sediments in southern Finland". Comm. Geol. Finlande Bulletin 60.
* Wohlfarth, B. (1996), The chronology of the Last Termination: A review of radiocarbon-dated, high-resolution terrestrial stratigraphies. "Quaternary Science Reviews" 15 pp. 267-284.