Editing Varve

{{short description|Annual layer of sediment or sedimentary rock}}
[[File:Pleistocene Glacial Lake Varves at Sandend Bay in Scotland - geograph 4175519.jpg|thumb|upright=1.35|[[Pleistocene]] glacial lake varves at [[Sandend|Sandend Bay]] in Scotland]]
A '''varve''' is an annual layer of [[sediment]] or [[sedimentary rock]].

The word 'varve' derives from the [[Swedish language|Swedish]] word ''varv'' whose meanings and connotations include 'revolution', 'in layers', and 'circle'.

Of the many [[rhythmite]]s in the geological record, varves are one of the most important and illuminating in studies of past [[Climate change (general concept)|climate change]]. Varves are amongst the smallest-scale events recognised in [[stratigraphy]].

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 water|fresh]] or [[brackish]] water, because the high levels of salt in normal sea water [[Coagulation (water treatment)|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]] occurs at high [[ionic strength]] due to the collapse of the clay [[electrical double layer]] (EDL), which decreases the [[electrostatic repulsion]] between negatively charged clay particles.{{Citation needed|date=April 2017}}

==Etymology==
The word 'varve' derives from the [[Swedish language|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.<ref>{{Cite journal  |pages  = 3105–3114  |last = Zolitschka |first = B. |title = Varved lake sediments |journal = Encyclopedia of Quaternary Science |date = 2007  |url = http://www.climategeology.ethz.ch/education/limnogeology/Zolitschka_2007.pdf |access-date  = 2014-03-19 |archive-url = https://web.archive.org/web/20150922035619/http://www.climategeology.ethz.ch/education/limnogeology/Zolitschka_2007.pdf |archive-date = 2015-09-22  |url-status = dead}}</ref>
Initially, "varve" referred to each of the separate components comprising a single annual layer in [[glacial lake]] [[sediment]]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.<ref>De Geer, G. (1912). A geochronology of the last 12,000 years. Proceedings of the International Geological Congress Stockholm (1910),1, 241–257.</ref> More recently introduced terms such as 'annually laminated' are synonymous with varve.

== History of varve research ==
[[File:Parque Geologico do Varvito - Paredao.jpg|thumb|upright=1.35|Varves in a [[Geopark|geological park]] in [[Itu, São Paulo|Itu]], [[Brazil]]]]
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]] suspected laminated sediment in North America could be seasonal, and in 1884 [[Warren Upham]] 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 [[dendrochronology|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 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]], 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]]. 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]], they were considered "too uncertain" for use on a long-term timescale.<ref>{{cite journal | doi = 10.1111/j.1475-4754.2008.00394.x| author = Ramsey | pages = 249–275 | first = C. B. | issue = 2 | year = 2008 | title = Radiocarbon dating: revolutions in understanding | journal = [[Archaeometry (journal)|Archaeometry]] | volume = 50}}</ref> 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.<ref>
{{cite journal|last=Reimer|first=P.J.|year=2009|title=IntCal09 and Marine09 Radiocarbon Age Calibration Curves, 0–50,000 Years cal BP|url=http://researchcommons.waikato.ac.nz/bitstream/10289/3622/1/Hogg%20Intcal09%20and%20Marine09.pdf|journal=Radiocarbon|volume=51|issue=4|pages=1111–1150|doi=10.1017/S0033822200034202|s2cid=12608574|display-authors=etal}}
</ref><ref>
{{cite web|url=http://www.eurekalert.org/pub_releases/2012-10/aaft-jlr101212.php|title=Japanese lake record improves radiocarbon dating|publisher=AAAS|date=18 Oct 2012|access-date=18 Oct 2012}}
</ref>

==Formation==
[[File:Fahrrad-ostern-2006-bluefish-01.jpg|thumb|upright=1.35|Varves formed during the [[Weichselian glaciation]], near [[Eberswalde]], Germany]]
Varves form in a variety of [[marine (ocean)|marine]] and [[Lake|lacustrine]] [[sedimentary depositional environment|depositional environments]] from [[season]]al variation in [[clastic]], biological, and chemical sedimentary processes.

The classic varve [[archetype]] 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]], 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)]] is high. The corresponding dark colored layers are composed of organic matter and fine sediment particles transported and deposited during spring [[Freshet|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]]. Consequently, varves commonly form under [[anoxic sea water|anoxic]] conditions.

A well-known marine example of varved sediments are those found in the Santa Barbara basin, off [[California]].<ref>{{cite journal
 | author1 = Thunell, R.C.|author-link1=Robert Thunell
 |author2=Tappa, E. |author3=Anderson, D.M. 
 | 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
 | access-date = 2007-04-27
|bibcode = 1995Geo....23.1083T | url-access = subscription
 }}</ref> Another long record of varved sediments is the palaeo-lacustrine record of the Piànico–Sèllere Basin (southern Alps).<ref>{{Cite journal|date=2000-11-01|title=A long lacustrine record from the Piànico-Sèllere Basin (Middle-Late Pleistocene, Northern Italy)|url=https://www.sciencedirect.com/science/article/abs/pii/S1040618200000641|journal=Quaternary International|language=en|volume=73-74|pages=47–68|doi=10.1016/S1040-6182(00)00064-1|issn=1040-6182|last1=Moscariello|first1=Andrea|last2=Ravazzi|first2=Cesare|last3=Brauer|first3=Achim|last4=Mangili|first4=Clara|last5=Chiesa|first5=Sergio|last6=Rossi|first6=Sabina|last7=De Beaulieu|first7=Jacques-Louis|last8=Reille|first8=Maurice|issue=1|bibcode=2000QuInt..73...47M|url-access=subscription}}</ref> 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.<ref>{{Cite journal|last1=Witt|first1=A|last2=Malamud|first2=BD |last3=Mangili|first3=C|last4=Brauer|first4=A|date=2017-11-14|title=Analysis and modelling of a 9.3 kyr palaeoflood record: correlations, clustering, and cycles|url=https://hess.copernicus.org/articles/21/5547/2017/|journal=Hydrology and Earth System Sciences|language=English|volume=21|issue=11|pages=5547–5581|doi=10.5194/hess-21-5547-2017|issn=1027-5606|doi-access=free}}</ref><ref>{{Cite journal|last1=Mangili|first1=C|last2=Witt|first2=A|last3=Malamud|first3=B|last4=Brauer|first4=A|date=2017-08-21|title=Detrital layer frequency in a 9336 year varved sequence within the Pianico-Sellere Palaeolake (Southern Alps, Italy) [Downloadable Data]|publisher=PANGAEA - Data Publisher for Earth & Environmental Science|doi=10.1594/PANGAEA.879779|url=https://doi.pangaea.de/10.1594/PANGAEA.879779}}</ref>

==See also==

* {{annotated link|Dendrochronology}}
* {{annotated link|Dendroclimatology}}
* {{annotated link|Rhythmite}}

== References ==
{{reflist}}

{{refbegin}}

==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.
* [[Barbara Wohlfarth|Wohlfarth, B.]] (1996), The chronology of the Last Termination: A review of radiocarbon-dated, high-resolution terrestrial stratigraphies. ''Quaternary Science Reviews'' '''15''' pp.&nbsp;267–284.
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{{Wikibooks |Historical Geology|Varves}}
{{commons category|Varved clay}}

{{Rock type}}

[[Category:Incremental dating]]
[[Category:Sedimentary rocks]]
[[Category:Climatology]]
[[Category:Sedimentology]]

[[fi:Lustosavikronologia]]