A varve is an annual layer of sediment

Sediment

Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....

 or sedimentary rock

Sedimentary rock

Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....

.

The word 'varve' is derived from the Swedish

Swedish language

Swedish is a North Germanic language, spoken by approximately 10 million people, predominantly in Sweden and parts of Finland, especially along its coast and on the Åland islands. It is largely mutually intelligible with Norwegian and Danish...

 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

Sweden

Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....

 in 1862. Initially, varve was used to describe the separate components of annual layers in glacial lake

Glacial lake

A glacial lake is a lake with origins in a melted glacier. Near the end of the last glacial period, roughly 10,000 years ago, glaciers began to retreat. A retreating glacier often left behind large deposits of ice in hollows between drumlins or hills. As the ice age ended, these melted to create...

 sediment

Sediment

Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....

s, but at the 1910 Geological Congress, the Swedish geologist Gerard De Geer

Gerard De Geer

Baron Gerard Jacob De Geer was a Swedish geologist who made significant contributions to Quaternary geology, particularly geomorphology and geochronology. De Geer is best known for his discovery of varves.- Early life and family :...

 (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 rhythmite

Rhythmite

A rhythmite consists of layers of sediment or sedimentary rock which are laid down with an obvious periodicity and regularity. They may be created by annual processes such as seasonally varying deposits reflecting variations in the runoff cycle, by shorter term processes such as tides, or by longer...

s 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

Stratigraphy

Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....

. Varves form only in fresh

Fresh Water

Fresh Water is the debut album by Australian rock and blues singer Alison McCallum, released in 1972. Rare for an Australian artist at the time, it came in a gatefold sleeve...

 or brackish water, because the high levels of salt in normal sea water coagulates the clay into coarse grains. Indeed, clay flocculation

Flocculation

Flocculation, in the field of chemistry, is a process wherein colloids come out of suspension in the form of floc or flakes by the addition of a clarifying agent. The action differs from precipitation in that, prior to flocculation, colloids are merely suspended in a liquid and not actually...

 occurs at high ionic strength

Ionic strength

The ionic strength of a solution is a measure of the concentration of ions in that solution. Ionic compounds, when dissolved in water, dissociate into ions. The total electrolyte concentration in solution will affect important properties such as the dissociation or the solubility of different salts...

 due to the collapse of the clay electrical double layer (EDL) which decreases the electrostatic repulsion between negatively charged clay particles.

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

Edward Hitchcock

Edward Hitchcock was a noted American geologist and the third President of Amherst College .-Life:...

 suspected laminated sediment in North America could be seasonal, and in 1884 Warren Upham

Warren Upham

Warren Upham was a geologist, archaeologist, and librarian who is best known for his studies of glacial Lake Agassiz. Upham worked as a geologist in New Hampshire before moving in 1879 to Minnesota to study the resources and glacial geology of that state...

 postulated that light-dark laminated couplets represented a single years deposition. Despite these earlier 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

Dendrochronology

Dendrochronology or tree-ring dating is the scientific method of dating based on the analysis of patterns of tree-rings. Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year...

. This prompted him to suggest the coarse-fine couplets frequently found in the sediments of glacial lakes were annual layers.

The first varve chronology

Chronology

Chronology is the science of arranging events in their order of occurrence in time, such as the use of a timeline or sequence of events. It is also "the determination of the actual temporal sequence of past events".Chronology is part of periodization...

 was constructed by De Geer in Stockholm

Stockholm

Stockholm is the capital and the largest city of Sweden and constitutes the most populated urban area in Scandinavia. Stockholm is the most populous city in Sweden, with a population of 851,155 in the municipality , 1.37 million in the urban area , and around 2.1 million in the metropolitan area...

 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 into making incorrect correlations, which they called 'teleconnections', between continents, a process criticised by other varve pioneers like Ernst Antevs.

In 1924, the Geochronological Institute, a special laboratory dedicated to varve research 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

Long Island

Long Island is an island located in the southeast part of the U.S. state of New York, just east of Manhattan. Stretching northeast into the Atlantic Ocean, Long Island contains four counties, two of which are boroughs of New York City , and two of which are mainly suburban...

, U.S.A. to Lake Timiskaming

Lake Timiskaming

Lake Timiskaming is a large freshwater lake on the provincial border between Ontario and Quebec, Canada. The lake, which forms part of the Ottawa River, is in length and covers an area of almost . Its water level ranges between and above sea-level, with a mean annual average of . The lake is...

 and Hudson Bay

Hudson Bay

Hudson Bay , sometimes called Hudson's Bay, is a large body of saltwater in northeastern Canada. It drains a very large area, about , that includes parts of Ontario, Quebec, Saskatchewan, Alberta, most of Manitoba, southeastern Nunavut, as well as parts of North Dakota, South Dakota, Minnesota,...

, Canada

Canada

Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

, and created a North American varve chronology. Carl Caldenius visited Patagonia

Patagonia

Patagonia is a region located in Argentina and Chile, integrating the southernmost section of the Andes mountains to the southwest towards the Pacific ocean and from the east of the cordillera to the valleys it follows south through Colorado River towards Carmen de Patagones in the Atlantic Ocean...

 and Tierra del Fuego

Tierra del Fuego

Tierra del Fuego is an archipelago off the southernmost tip of the South American mainland, across the Strait of Magellan. The archipelago consists of a main island Isla Grande de Tierra del Fuego divided between Chile and Argentina with an area of , and a group of smaller islands including Cape...

, and Erik Norin visited central Asia

Asia

Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...

. By this stage, other geologists were investigating varve sequences, including Matti Sauramo who constructed a varve chronology of the last deglaciation in Finland

Finland

Finland , officially the Republic of Finland, is a Nordic country situated in the Fennoscandian region of Northern Europe. It is bordered by Sweden in the west, Norway in the north and Russia in the east, while Estonia lies to its south across the Gulf of Finland.Around 5.4 million people reside...

.

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.

In 2008, although varves were considered likely to give similar information to dendrochronology

Dendrochronology

Dendrochronology or tree-ring dating is the scientific method of dating based on the analysis of patterns of tree-rings. Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year...

, they were considered "too uncertain" for use on a long-term timescale.

Formation

Varves form in a variety of marine

Marine (ocean)

Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...

 and lacustrine

Lake

A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...

 depositional environments

Sedimentary depositional environment

In geology, sedimentary depositional environment describes the combination of physical, chemical and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will be formed after lithification, if the sediment is preserved in the rock...

 from season

Season

A season is a division of the year, marked by changes in weather, ecology, and hours of daylight.Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis relative to the plane of revolution...

al variation in clastic, biological, and chemical sedimentary processes.

The classic varve archetype

Archetype

An archetype is a universally understood symbol or term or pattern of behavior, a prototype upon which others are copied, patterned, or emulated...

 is a light / dark coloured couplet deposited in a glacial lake

Glacial lake

A glacial lake is a lake with origins in a melted glacier. Near the end of the last glacial period, roughly 10,000 years ago, glaciers began to retreat. A retreating glacier often left behind large deposits of ice in hollows between drumlins or hills. As the ice age ended, these melted to create...

. The light layer usually comprises a coarser laminaset of silt

Silt

Silt is granular material of a size somewhere between sand and clay whose mineral origin is quartz and feldspar. Silt may occur as a soil or as suspended sediment in a surface water body...

 and fine sand

Sand

Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...

 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

Clay

Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...

-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

Bioturbation

In oceanography, limnology, pedology, geology , and archaeology, bioturbation is the displacement and mixing of sediment particles and solutes by fauna or flora . The mediators of bioturbation are typically annelid worms , bivalves In oceanography, limnology, pedology, geology (especially...

. Consequently, varves commonly form under anoxic

Anoxic sea water

Anoxic waters are areas of sea water or fresh water that are depleted of dissolved oxygen. This condition is generally found in areas that have restricted water exchange....

 conditions.

A well-known marine example of varve sediments are those found in the Santa Barbara basin, off California

California

California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...

.