varve
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A varve is an annual layer of
sediment Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...
or
sedimentary rock Sedimentary rocks are types of rock (geology), rock formed by the cementation (geology), cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or de ...
. The word 'varve' derives from the Swedish word ''varv'' whose meanings and connotations include 'revolution', 'in layers', and 'circle'. Of the many
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, b ...
s in the geological record, varves are one of the most important and illuminating in studies of past
climate change Present-day climate change includes both global warming—the ongoing increase in Global surface temperature, global average temperature—and its wider effects on Earth's climate system. Climate variability and change, Climate change in ...
. Varves are amongst the smallest-scale events recognised in
stratigraphy Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithost ...
. An annual layer can be highly visible because the particles washed into the layer in the spring when there is greater flow strength are much coarser than those deposited later in the year. This forms a pair of layers—one coarse and one fine—for each annual cycle. Varves form only in fresh or
brackish Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estuari ...
water, because the high levels of salt in normal sea water coagulate the clay into coarse grains. Since the saline waters leave coarse particles all year, it is nearly impossible to distinguish the individual layers in salt waters. Indeed, clay
flocculation In colloidal chemistry, flocculation is a process by which colloidal particles come out of Suspension (chemistry), suspension to sediment in the form of floc or flake, either spontaneously or due to the addition of a clarifying agent. The actio ...
occurs at high
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 a ...
due to the collapse of the clay
electrical double layer Electricity is the set of physical phenomena associated with the presence and motion of matter possessing an electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by Maxwel ...
(EDL), which decreases the electrostatic repulsion between negatively charged clay particles.


Etymology

The word 'varve' derives 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 ''not to be confused with the Geologiska föreningen ()'' Geological Survey of Sweden (, SGU) is a Government agencies in Sweden, Swedish government agency that was founded by Axel Erdmann in 1858, that supervises all mineral deposits activities ( ...
in 1862. Initially, "varve" referred to each of the separate components comprising a single annual layer in
glacial lake A glacial lake is a body of water with origins from glacier activity. They are formed when a glacier erodes the land and then melts, filling the depression created by the glacier. Formation Near the end of the last glacial period, roughly 10,0 ...
sediment Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...
s, but at the 1910 Geological Congress, the Swedish geologist Gerard De Geer (1858–1943) proposed a new formal definition, where varve means the whole of any annual sedimentary layer. More recently introduced terms such as 'annually laminated' are synonymous with varve.


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,
Edward Hitchcock Edward Hitchcock (May 24, 1793 – February 27, 1864) was an American geologist and the third President of Amherst College (1845–1854). Life Born to poor parents, he attended newly founded Deerfield Academy, where he was later principal, ...
suspected laminated sediment in North America could be seasonal, and in 1884
Warren Upham Warren Upham (March 8, 1850 – January 29, 1934) was an American geologist, archaeologist, and librarian who is best known for his studies of glacial Lake Agassiz. Biography Warren Upham was born in Amherst, New Hampshire and attended Dartmout ...
postulated that light-dark laminated couplets represented a single year's 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. 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 (from Latin , from Ancient Greek , , ; and , ''wikt:-logia, -logia'') is the science of arranging events in their order of occurrence in time. Consider, for example, the use of a timeline or sequence of events. It is also "the deter ...
was constructed by De Geer in
Stockholm Stockholm (; ) is the Capital city, capital and List of urban areas in Sweden by population, most populous city of Sweden, as well as the List of urban areas in the Nordic countries, largest urban area in the Nordic countries. Approximately ...
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 is a densely populated continental island in southeastern New York (state), New York state, extending into the Atlantic Ocean. It constitutes a significant share of the New York metropolitan area in both population and land are ...
, U.S.A. to
Lake Timiskaming Lake Timiskaming or Lake Temiskaming (, ) is a large freshwater lake on the Provinces and territories of Canada, provincial boundary between Ontario and Quebec, Canada. The lake, which forms part of the Ottawa River, is in length and covers ...
and
Hudson Bay Hudson Bay, sometimes called Hudson's Bay (usually historically), is a large body of Saline water, saltwater in northeastern Canada with a surface area of . It is located north of Ontario, west of Quebec, northeast of Manitoba, and southeast o ...
, Canada, and created a North American varve chronology. Carl Caldenius visited
Patagonia Patagonia () is a geographical region that includes parts of Argentina and Chile at the southern end of South America. The region includes the southern section of the Andes mountain chain with lakes, fjords, temperate rainforests, and glaciers ...
and
Tierra del Fuego Tierra del Fuego (, ; Spanish for "Land of Fire", rarely also Fireland in English) is an archipelago off the southernmost tip of the South America, South American mainland, across the Strait of Magellan. The archipelago consists of the main is ...
, and Erik Norin visited central
Asia Asia ( , ) is the largest continent in the world by both land area and population. It covers an area of more than 44 million square kilometres, about 30% of Earth's total land area and 8% of Earth's total surface area. The continent, which ...
. By this stage, other geologists were investigating varve sequences, including Matti Sauramo who constructed a varve chronology of the last deglaciation in
Finland Finland, officially the Republic of Finland, is a Nordic country in Northern Europe. It borders Sweden to the northwest, Norway to the north, and Russia to the east, with the Gulf of Bothnia to the west and the Gulf of Finland to the south, ...
. 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 Scania ( ), also known by its native name of Skåne (), is the southernmost of the historical provinces of Sweden, provinces () of Sweden. Located in the south tip of the geographical region of Götaland, the province is roughly conterminous w ...
to Indalsälven. Ragnar 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 (or tree-ring dating) is the scientific method of chronological dating, dating tree rings (also called growth rings) to the exact year they were formed in a tree. As well as dating them, this can give data for dendroclimatology, ...
, they were considered "too uncertain" for use on a long-term timescale. However, by 2012, “missing” varves in the Lake Suigetsu sequence were identified in the Lake Suigetsu 2006 Project by overlapping multiple cores and improved varve counting techniques, extending the timescale to 52,800 years.


Formation

Varves form in a variety of marine and lacustrine depositional environments from
season A season is a division of the year based on changes in weather, ecology, and the number of daylight hours in a given region. On Earth, seasons are the result of the axial parallelism of Earth's axial tilt, tilted orbit around the Sun. In temperat ...
al variation in
clastic Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus,Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3 chunks, and smaller grains of rock broken off other rocks by ...
, biological, and chemical sedimentary processes. The classic varve
archetype The concept of an archetype ( ) appears in areas relating to behavior, historical psychology, philosophy and literary analysis. An archetype can be any of the following: # a statement, pattern of behavior, prototype, "first" form, or a main mo ...
is a light / dark coloured couplet deposited in a lake basin. During summer months, light laminae (thin layers) composed of silica and calcium carbonate are deposited. This is the consequence of dead
phytoplankton Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater Aquatic ecosystem, ecosystems. The name comes from the Greek language, Greek words (), meaning 'plant', and (), mea ...
, and other micro-organisms that create their skeletons or shells out of silica or calcium carbonate, falling to the lake bottom. This process of the precipitation and deposition of dead micro-organisms out of the water column is restricted to warm months when
productivity (ecology) In ecology, the term productivity refers to the rate of generation of biomass in an ecosystem, usually expressed in units of mass per volume (unit surface) per unit of time, such as grams per square metre per day (g m−2 d−1). The unit of mass ...
is high. The corresponding dark colored layers are composed of organic matter and fine sediment particles transported and deposited during spring freshets as a result of winter snowmelt. The alternation of these two distinct layers allows for high-precision dating of sediment profiles, for each couplet is equivalent to one year. In addition to seasonal variation of sedimentary processes and deposition, varve formation requires the absence of
bioturbation Bioturbation is defined as the reworking of soils and sediments by animals or plants. It includes burrowing, ingestion, and defecation of sediment grains. Bioturbating activities have a profound effect on the environment and are thought to be a ...
. Consequently, varves commonly form under
anoxic Anoxia means a total depletion in the level of oxygen, an extreme form of hypoxia or "low oxygen". The terms anoxia and hypoxia are used in various contexts: * Anoxic waters, sea water, fresh water or groundwater that are depleted of dissolved ox ...
conditions. A well-known marine example of varved sediments are those found in the Santa Barbara basin, off
California California () is a U.S. state, state in the Western United States that lies on the West Coast of the United States, Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and shares Mexico–United States border, an ...
. Another long record of varved sediments is the palaeo-lacustrine record of the Piànico–Sèllere Basin (southern Alps). Here, the detrital layer part of each varve was used as a proxy for 771 palaeofloods which occurred over a period of 9.3 thousand years during an interglacial period in the Pleistocene.


See also

* * *


References


Further reading

* 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. {{Rock type Incremental dating Sedimentary rocks Climatology Sedimentology fi:Lustosavikronologia