Geology and Cyclicity

1842, 5 years after Agassiz's "Discourse of Neuchatel", the French mathematician Joseph Alphonse Adhémar elaborated a hypothesis to explain a cyclic occurring of ice ages. He calculated the variations of the "direction" and declination of earth axis and the "movements" of earth around the sun during the geological past.
These cyclic factors influence the time and the energy density of solar radiation that reach earth from sun, causing cyclic climatic change. Adhémar proposed that in a period of 11.000 years the hemisphere that experiences a longer winter, resulting from these three astronomical factors, would develop in an ice age.
But in 1852 Alexander von Humboldt noted that Adhémar didn't consider an important factor in his calculations, even if one hemisphere experience lower radiation, the opposite hemisphere experience an increase, so in the end the total sum remains more or less identical.

Nevertheless the idea of the French mathematician was intriguing, and would influence later researchers.

In 1833, James Croll (1821-1890), son of a poor stonecutter of Perthshire, purchased a copy of the "Penny Magazine", a magazine for children education. He was fascinated and began extensively to read, and some time later acquired he's first books dealing with natural science; "At first I was totally confused, but then the beauty and simplicity of the ideas provided me with delight and surprise, and I began seriously to study the matter."

Croll had no easy living in the next 20 years; he travelled the country, most time working as casual labourer, and in 1850 managed (for a brief time) the only Scottish pub were no alcohol was allowed.
He then found work as a maintenance supervisor of the Andersonian College in Glasgow, where he had access to the library and the hosted scientific works, a knowledge resource that he grateful exploited.
"At that time, the question of what could have triggered the ice age was much discussed among geologists. So in the spring of 1864, I directed my attention to this topic."
From 1864, Croll corresponded with Sir Charles Lyell, on links between ice ages and variations in the Earth's orbit. This led to a position in the Edinburgh office of the Geological Survey of Scotland, as keeper of maps and correspondence, where the director, Sir Archibald Geikie, encouraged his research. He also corresponded with Charles Darwin on erosion by rivers.

Croll, based on observations of the astronom Urbain Jean Josef Leverrier, used in his calculations an important factor that Adhémar did not know, the "movements" of the perihelion and aphelion on earth's ecliptic (precession of the equinoxes). He published his research in the book "Climate and Time, in Their Geological Relations" in 1875.

Fig.1. Glacial and interglacial conditions when eccentricity is at its superior limit, from CROLL1875, frontispiece (from FLEMING 2006).

Fig.2. Variations in the earth's orbit for three million years before 1800 A.D. and one million years after it, from CROLL 1875, following p. 312 (from FLEMING 2006).

Geikie wrote about the work of Croll: "The astronomical theory seems to me the best solution to the present ice age riddle. It bears in it all the decisive factors for the occurrence of alternating cold and warm periods, and accounts for the peculiar character of glacial and interglacial climates."

But there was a problem, even if dating methods at these times were only approximate, geological evidences supported a very young age of glacial deposits, but after Croll´s theory the last glacial period had ended 80.000 years ago. When Croll died, highly respected, geologists considered his theory wrong.
Geikie resumed: "It may well be, that with certain modifications of his views; one day we will solve the secret. But for now we must be continue to work and wait."

The modifications as hoped by Geikie will come only years later, and a glass wine will be the first step to solve the problem of the cyclicity of glacial periods: Geology and Cyclicity: Milankovitch´s idea.

Fig.3. Orbitally forced cyclic sedimentation in the Trubi Formation of Zanclean age at Scala dei Turchi, in the Rossello composite section (Sicily), that represents the template for the Pliocene Series.

Fig.4. Orbitally forced cyclic sedimentation as expressed in the uppermost part of the Trubi Formation and in the overlying Monte Narbone Formation at Punta Piccola (Sicily), where the Piacenzian GSSP has been defined.

References:

CHORLTON, W. (ed) (1985): Ice Ages (Planet Earth). Time-Life Books: 176
CROLL, J. (1875): Climate and Time, in their Geological Relations. A theory of secular changes of the Earth's Climate. D. Appleton and Company, New York: 630
FLEMING, J.R. (2006): James Croll in Context: The Encounter between Climate Dynamics and Geology in the Second Half of the Nineteenth Century. History of Meterology 3: 43 - 54

Dating catastrophes

Methods for dating mass-movement events earthquakes, volcanic eruptions and other geological hazards comprise a wide range of methods, historic, radiometric, stratigraphic and biological. For example Carbon-14 dating, applicable when the moving mass incorporated vegetation, or other radioactive elements that decay with time, surface dating of freshly exposed boulders by cosmogenic isotopes, dating of sediments by thermoluminescence, dendrochronology or lichenometry and for historic times written records or witness accounts.

Historical references are the most reliable sources for reconstructing the temporal distribution of catastrophes, especially for the last few centuries. Going further back in time, medieval time and antiquity ,the record gets poorer and more inexact. And for prehistoric time we miss descriptions of this kinds, or did we?

Considering (pre-)historic events, the oral tradition and legends all over world maybe represent first efforts to record and explain such phenomena, even if we have to be cautious, myths in geology can only be a supplementary help, not a fact, and much is left by the interpretation of stories by the compiler. Nevertheless knowing some old stories about the landscape can not be so bad for a geologist.

In the area of Seattle, Washington, at least five sites with landslide deposits or large boulders are known by local legends of the Duwamish people to be haunted by a´yahos. A´yahos are spirits with the body of a serpent and the antlers and forelegs of a deer. Old folks used to say not to look directly to an a´yahos because it could shake the ground or turn people to stone.

Non-Salish Cascadia Native representation of two -headed snakes, likely to represent spirits comparable with a´yahous. Quileute ceremonial representation of t´abale, a vicious guardian spirit on the northwestern Washington coast (from LUDWIN et al. 2007).

In 1990s geophysical investigations revealed that the area of Seattle is passed by a fault system and at least 1100 years ago an earthquake hit the entire zone- triggering mass movement all over the landscape.

Reconstructed "events" by means of oral tradition and mythological conventions.
Date range estimates used the following assumptions: a 'generation' is no fewer than 15 and no more than 40 years, events before age 5 are not remembered. the maximum lifespan is 100 years, flood survivors were 'old' when seen. and an 'old' person is at least 40. (from LUDWIN et al. 2007).

The bay of Lituya situated in Alaska is a narrow, only 2 kilometres broad, but 11 long bay open to the Pacific Ocean. The native Tlingit Indians tell that in a cavern, deep underground, lives a demon, similar in appearance to a great toad or frog. If someone dares to disturb the tranquillity of the bay, the demon will rip apart the sea and the earth and catch the intruder and transmute him to a bear.

Behind the anger of demons maybe an exceptional (yet undated) geological event hides, as a recent example shows:
On the 9th July 1958 an earthquake produced by a fault nearby triggered a rockfall, with an estimated volume of 40 million cubic metres and a weight of 90 million tons that felt from a height of 1000m in the water. The resulting wave reached a height of 524, the highest ever (human-) documented wave.

The Yurok Indians, once native in the Cascade Range, tell about the creation of the world by "earthquake" and "thunder":

And from there, earthquakes and thunder went south. They first went to the south and let fall the ground. In rapid succession, there was an earthquake and another earthquake, and then the water filled the place. "This is what brings people to live", said earthquake. "They would have no food, if there were not place for the creatures of the sea, to live in it. From here, they will obtain what they need to live, where prairie has become water."

During excavation in today's marshes along the coast of the region a series of sand and peat layers was discovered. Such deposits are formed when the coast is flooded by a tsunami, and sand is deposited, overlying an ancient soil. The discovery proves that this area in the past has been repeatedly devastated by tsunamis.

References:

FRITZ, H.M., HAGER, W.H., MINOR, H-E. (2001): Lituya Bay Case: Rockslide impact and wave run-up. - Science of Tsunami Hazards 19(1), 3-38.
LUDWIN, R.S. & SMITS, G.J. (2007): Folklore and earthquakes: Native American oral traditions from Cascadia compared with written traditions from Japan. From PICCARDI, L. & MASSE, W.B: (eds): Myth and Geology. Geological Society, London, Special Publications, 273: 67-94
LANG, A.; MOYA, J.; COROMINAS, J.; SCHROTT, L. & DIKAU, R. (1999): Classic and new dating methods for assessing the temporal occurrence of mass movements. Geomorphology 30:33-52

Extinctions & Excrements

"dal letame nascono i fior
dai diamanti non nasce niente"
From dung flowers are born
From diamonds nothing comes
"Via del Campo", Fabrizio de André (Italian poet-musician)

Until 20.000 years ago North America showed a biodiversity of large mammals comparable with modern Africa, if not greater. 10.000 years later 34 genera with animal-species weighing more than a ton were extinct.

The extinction of the Pleistocene Megafauna is still an unsolved mystery. The proposed hypothesis range from overkill by human hunters to a meteor impact and climate change at the end of the last glacial maximum. Geologically speaking it happened suddenly, but a new study now maybe can date more precisely the extinction pattern and duration, using an unusual data source - fossil excrements and the inhabitants of this "biotope".

In 2005 and 2006 sediment cores with a complessive length of 11,7m were taken from Appleman Lake and compared with other cores of lakes in the U.S. State of Indiana.
Thirteen wood, pollen and charcoal samples, recovered from the lacustrine sediments, were dated by radiocarbon method on ages between 7.000 and 14.000 yr BP and used interpolate an age-depth model of the core.

The fungus-genus Sporomiella lives on animal dung and the spores have to pass the digestive tract of large herbivores to germinate. The spores can also accumulate in sediments along with other micro- and macrofossils like pollen and charcoal, so the presence of the fossil spores in sediments correlates with the amount of excrements - "Lots of dung means lots of spores" (JOHNSON 2009), and the amount of dung can give a hind to extrapolate the size of the population of herbivorous animals like mastodon or mammoth.

The timing of the Sporomiella decline and the first major charcoal peak are well constrained by two dates between 14.6 and 14.7ka. The wood pollen (Quercus and Pinus) increases between 10.7 and 12.2 ka.

Figure from GILL et al. 2009: Appleman Lake time series for (A to F) percent pollen abundances of selected taxa (NAP, nonarboreal pollen), (G) Sporormiella and (I) charcoal counts.

Applying this method, Gill et al. found that the amount of spores first decreases slowly, and only in 14.800 years old sediments the number of spores decreases significantly. To old for the proposed impact, and also to old for a climatic or environmental change - vegetation change, interpolated from the pollen assemblage, namely happens only after the faunal demise, and is more probable caused by the extinction of large herbivore, then the cause of extinction.

The greatest impact of humans - in form of the Clovis Culture - on the Pleistocene landscape in North American was supposed in a time interval between 13.330 and 12.900 years ago. The new data predates the Clovis, nevertheless archaeological findings support a lesser tool specialised pre-Clovis culture in the time interval of the Megafauna collapse, so human influence could not be ruled completely out.

Figure from JOHNSON 2009.

The changing environment after the Megafauna collapse, from an open savanna with scattered trees to a spruce-broadleaf woodland, was the result of ceased pasture of shrubs and trees by Mammoth and Co. The expansion of woodlands is also supported by a larger amount of charcoal in the sediments, from time to time the woodlands caught fire, and the ash was eroded, transported and finally deposited in the examined lakes.

Even if the new method con not give us the definitive answer, at least it's provide some new data to better understand the temporal progress and the environmental change of the late Pleistocene extinction event.

References:

GILL et al. (2009): Pleistocene Megafaunal Collapse, Novel Plant Communities, and Enhanced Fire Regimes in North America. Science 326: 1100-1103 http://www.sciencemag.org/cgi/content/abstract/326/5956/1100

GILL et al. (2009): Supporting Online Material for Pleistocene Megafaunal Collapse, Novel Plant Communities, and Enhanced Fire Regimes in North America. Science 326. www.sciencemag.org/cgi/content/full/326/5956/1100/DC1

JOHNSON (2009): Megafaunal Decline and Fall. Science 326: 1072 - 1073. http://www.sciencemag.org/cgi/content/short/326/5956/1072

Interview to Dr. Jacquelyn Gill by the Canadian Broadcast: mp3 (4MB)

--------- Thanks to Ole Nielson for linking to the post----------

Caves and the dream of long term records

Within the Alps, long term climate records, ranging to the last or even to the penultimate interglacial are exceptionally rare. We are simply lacking sediments of these periods - sediments of interglacial's were systematically eroded by the (re)advance of glaciers, lay hidden under younger sediments (mostly postglacial alluvional river deposits) or are simply not yet recognised.

In cave systems we are not (so) affected by the destructive power of glaciers, so there maybe can be found sediments ranging much deeper in time, with a record much completer then in the outside world.

Speleothem growth depends strongly from temperature and water chemistry, and water in liquid form depends strongly by temperature of the environment outside the cave. During a cold period, water will be trapped in form of ice on the surface, and water percolation in the underground will be very restricted or completely missing - the speleothem will stop to growth. During warming, and melting of ice, again water is available, and the speleothem goes on growing. So just the presence of speleothems can provide a first clue to reconstruct past climates. But even better - the isotopic composition of the precipitation changes with the amount of water trapped in ice shields - so measuring the relationship between the two oxygen isotopes (the "light" 16O and the "heavy" 18O) in the- from the water deposited - carbonates, can give a direct information's of extend of ice shields, and so climate, during the past. And the carbon isotopes 13C and 12C, also found in the carbonate, give hinds on vegetation and soil cover of the catchment area of the cave. Plants assimilate preferred the lighter isotope - a high values of delta13C indicate low or even negligible input of soil-derived organic carbon into the dripwater.

Spannagel cave is a large high altitude (entrance to the cave 2.531m, extending down in two main branches to ca. 2.200m a.s.l.) cave network with approximately 10km of length in the Zillertal Alps of Austria.

The "Hintertux glacier" with the morain of the last highstand (1850). The entrance of the Spannagel caves - and also the Spannagel hut- lies on the upper end of the left morain.

It is the largest out of a series of more than 30 caves that developed within the Jurassic marble that covers the "Zentralgneiss" - the tectonic uplifted gneiss core of the "Tauern window", and is itself overlie by the phengitic gneisses.

The area above the cave today is ice free, but it was covered partially by the Hintertux glacier until 1850, and covered entirely by up to 250 thick ice during the past glacial. U/Th dates in the cave showed that deposition of the speleothems occurred repeatedly during the past few hundred thousand years, and is still ongoing, thanks of the constant temperature (1-2°C) in the cave slightly over the freezing point of water.


A stalagmite in the cave, composed of dense, columnar calcite, apparently grew without significant interruption for ca. 50 ka, albeit at a very slow rate, during the penultimate interglacial. The oxygen isotope record shows three prominent maxima, representing three warm phases, separated by a long earlier and a shorter later cold period. The mid points of the transitions into the three warm phases occurred at 240 ± 3 (correlated subsequently with the MarineIsotopicStadium 7.5), 215 ± 2 (MIS 7.3) and 200 ± 3 ka (MIS 7.1) and the end of the interglacial (MIS 7/6 transition) was dated to 190 ± 3 ka.

During full glacial periods no speleothem growth could be found. During the transition of interglacial to glacial conditions the sudden drop of the isotopes values suggest a cooling, but speleothems growth continues. Comparing the curve of the oxygen isotopes with the carbon isotopes shows a remarkable pattern - both curves appear very similar, relatively high delta18O values indicating warm atmospheric conditions coincide with high delta13C values. This suggests very little if any vegetation at this cave during the warm periods, and so less favourable conditions than today - were at least a thin soil developed.
The later interglacial (MIS 5), and also the Holocene show data with higher values of delta18O, and a low delta13C value during the "high stand" of oxygen - high temperature and thick soil and vegetation cover.

Continuous stable isotope record of speleothem growth during the Marine Isotope Stage 7 at the high-alpine Spannagel Cave, Central Alps. SPÖTL et al. (2008): Spannagel Cave, Austria MIS 7 Speleothem Stable Isotope Data. Age is given in kyr BP, isotope ratio per mil VPDB).
Relatively high (reaching -8 to -9 0/00) delta18O values indicating warm atmospheric conditions, these values during the observed interglacial coincide with relative high delta13C values (>2 0/00). The 13C isotope derives mostly of anorganic sources (p.e. dissolving limestone), so high values are sign of lacking vegetation or organic rich soil cover. During "cold periods" complete lack of soil and vegetation produces the high peak between 230-220kyrs.

These facts let conclude that the penultimate interglacial posess three major climatic phases, with warmer periods separated by cooler periods. On average this interglacial was less warmer then the last interglacial or the Holocene in this altitude, with consequent lower equilibrium lines for glaciers. The catchment area of the Spannagel cave must be covered by ice, but the ongoing growths of speleothems demonstrate the presence of water - this implies warm based conditions beneath the ice. Only during full glacial conditions no speleothem deposit occurred.

References:

SPÖTL, C.; MANGINI, A.; BURNS, S.J., FRANK, N. & PAVUZA, R. (2002): Speleothems from the high-alpine Spannagel cave, Zillertal Alps (Austria). In (ed.) SASOWSKY & MYLROIE: Studies of Cave Sediments: Physical and Chemical Records of Paleoclimate
SPÖTL et al.(2006): The last and the Penultimate Interglacial as Recorded by Speleothems From a Climatically Sensitive High-Elevation Cave Site in the Alps. In SIROCKO, F. et al. (ed): The climate of past interglacial. Developments in Quaternary Science 7.
VOLLWEILER, N.; MANGINI, A.; SPÖTL, C.; SCHOLZ, D. & MÜHLINGHAUS, C. (2009: Stalagmites from Spannagel cave (Austria) and holocene climate. Geophysical Research Abstracts. Vol.11

The Ehringsdorf-Formation: or travertine trouble

Even if the first descriptions of the travertine* deposits from Weimar can be traced back to J.C.W.Voigt (1781), and later Goethe draw the first stratigraphic section, only in the years after 1965 they were studied intensively and with modern methods.

STEINER (1983) described three facies in the travertine body, that in 2007 was defined as Ehringsdorf-Formation.

Spring near or marginal facies: characterized by a low topographic gradient and slow running water. In the developing shallow ponds Chara “lawn” developed and fine chalk sand or lake marls were deposited

Slope facies: strong topographic gradient with riffle-pool sequence, the water flows fast and loosing his dissolved carbonate deposits compact travertine layers, the calcification of organic material is very strong.

Valley facies: the slope changes in the broad alluvial plain of the Paleo – Ilm River, the travertine interfingers with loamy and pebbly river sediments. Then follows an alternation of lake marls, an travertine with imprints of reed.

The three-dimensional standard section of the Pleistocene travertine near Weimar is distinguished by three facies ranges following one after another in the direction of the flow of the karst spring waters which are characterized by characteristics or type rocks showing typical structural marks. From the results of the investigation the conclusion is that other occurrences of travertine probably have a similar facial, and, by this, stratigraphically complicated division. This must be taken in consideration much more than until now in all further discussions of stratigraphic questions, also in the discussion of the dating and the absolute age and coordination of paleontological finds in geological sections.

Idealized cross section trough the travertine deposits of Ehringsdorf (after STEINER 1979). White= travertine, black= "Pariser". Notable the coal bearing horizonts in the lower travertine, showing human presence.

The age of the travertine of Ehringsdorf is highly controversial. The position between sediments deposited in cold environments (the fluvial conglomerate represents a braided river system, the uppermost loess layers show ice wedges and cryoturbation) let conclude an interglacial age.

The faunal assemblage supports in part an interglacial position, with the upper part belonging to the Eemian (ca. 130.000y), and the lower part, or at least the base, dating back much further to an Intrasaalian age (OIS7 - 200.000y). The presence of Cricetus major in the soil of the Pariser seems in part to support an Eemian or older date for the upper part of the formation. The malacology on the other side seems to support older ages for both travertine bodies, typical Eemian species are lacking.
The study of human "Präneanderthalian"artefacts, with wedge like utensils and scrapers/spires showed similarities with artefacts found in Eemian to early Würmian (or in this case better Weichsel) ages sites.

The radiometric dating in 2000 using the U/Th method resulted in ages of 236+-13ka for the lower, and 198+-10ka BP for the upper travertine. Unfortunately these results are not universally accepted, the travertine is not a closed system, and water can easily enter the rock and falsifying the isotopic composition.

So still the age remains a trouble.

References:

KATZSCHMANN (2007): The Ehringsdorf Formation. In LithoLex [Online-Datenbank]. Hannover: BGR. Last updated 30.11.2007. [cited 20.06.2009]. Record No. 1000002

*Unfortunately the term “travertine” is somehow vague in different languages. Travertine limestone in English is referred to form around hot springs or by inorganic processes. Calcareous tufa forms by precipitation of calcium carbonate from “cool” springs and river waters, also improved by organic processes. To remain as near as possible to the meaning and use of the word “Travertin” in German, where the distinction is not so clear, here also travertine is used in sense of calcareous tufa.

Tree-rings

Tree-rings of Picea, showing the annual growth rings with the brigth, and wider earlywood, and the darker latewood.

Dendrochronology deals with the dating and the study of the annual growth layers in woody trees and shrubs - appropriately claimed tree-rings. In temperate climates, with a growth period (spring-summer, the produced wood is the so called earlywood) and a rest phase (autumn-winter, the so called latewood) with no or very low growth, trees produce annual rings. Counting this annual rings, the age of the studied plant can be determinate.

Comparing the wide of the rings, claims about annual growth factors affecting the plant can be argued, like temperature, precipitations or competition. In the early fifteenth century, Leonardo da Vinci noted tree rings as annual patterns, and recognized a relationship between tree ring widths and precipitation.
In the following centuries the anatomy and ecology of tree rings were studied in detail. In 1904, the American astronomer Andrew Ellicott Douglass (he later founded the Laboratory of Tree Ring Research, University of Arizona), developed a tree ring chronology for the south-western United States, using the narrow tree rings found in Ponderosa pine trees, caused by droughts, to corelating different, individual trees. By 1914, he showed in a 500-year record a positive correlation between ring width and precipitation. He then used this chronology to date wood samples found in the Indian pueblo sites, affirming dendrology as dating method.
Joined in his studies by Edmund Schulman, they tried to improve and lengthened the tree ring record, discovering the (until then) oldest known trees of the world, the over 4.000 year old bristlecone pines in the Californian White Mountains. The bristlecone pine chronology, and so the climatic reconstruction, now dates back to 8.500 years. This methusalems were also used to calibrate the 14C curve (influenced by the atmospheric relationships of the carbon-isotopes).

Bristlecone pine (Pinus longaeva)

References:

SMITH & LEWIS (2007): Dendrochronology. in ELIAS (ed.) (2007): Encyclopedia of Quaternary science: 459-465

Lichenometry

Lichens are a symbiotic live community between algae and fungi. The alga furnishes nutrients for the fungus, the fungus provide moisture and shelter for the alga. This partnership enables the two partners to colonize habitats, which a single organism couldn’t colonize by itself, and they colonize an extraordinary variety of habitats and surfaces.

Lichens can be found in the death zone of mountains (up to 7400m a.s.l. in the Himalayan), in rainforests, deserts, temperate regions and on the coast of the sea.

The body of lichen that is visible is formed by the fungus, and is called “thallus”. Lichens can divided into three broad groups based on the shape of the thallus: the fruiticose type consists of small tubules and branches, the foliose type which have a leaf-like plant body, and the flattened crustose type. This last group comprises the most common members of the lichen family, and can be found extensively on hard surfaces, including rock outcrops, boulders, tree bark, buildings and gravestones.

Lichens after HAECKEL 1904 "Kunstformen der Natur"

The first study related to Lichenometry, the dating method that use the growth rate of lichens for dating the surface that they colonize, was carried out by the Austrian scientist Roland Beschel in 1950.
Given similar rocks and climatic conditions, the larger the lichen colony, the longer will be the time passed since the growth surface becomes exposed.
Estimating the absolute age of a material from the lichen growing on its exposed surface first requires the determination of the growth rate of the area. After measuring lichen on surfaces of known age (for example by comparing lichens on historic buildings or geomorphic features with known age) it is possible to plot a growth curve that relates lichen diameters to time.
Then is it possible to compare lichen on a surface of unknown age, within the same area, with the grow curve to determinate the surface’s age.

The growth of a lichen proceeds in three different phases:

1) rapid, logarithmic growth

2) linear growth

3) slow growth phase, where lichen growth gradually declines until death.

Only lichen species with a gradual and progressive growth can be used for dating purpose. In a study only the same lichen species can be used. The growth is influenced by local, and regional environmental factors, such as temperature, day length and snow cover.

The photosynthetic productivity is compared to high plants low, only ca. 25% comparing same areas with lichens and plants. This low productivity implies a low growth rate and a great longevity. Some lichen species (like Rhizocarpon geographicum) are estimated to reach (under favourable conditions like in the cold and dry conditions of western Greenland) 5.000 to 9.000 years.

Rhizocarpon geographicum

But because lichens colonies eventually grow together, and can no longer be measured individually, lichenometry as dating tool is used in a range less than 500 years. Under optimal circumstances lichenometry can provide a dating tool accurate to +-5 years over the last 200 years. So these organisms provide accurate dates for young glacial deposits, rockfalls and mudflows – all events that expose new rock surfaces on which lichen can grow.

Brodoa intestiniformis

References

BENEDICT (1990): Experiments on lichen growth. 1 Seasonal patterns and environmental controls. Arctic and Alpine Research 22:244-253

BESCHEL (1961): Dating rock surfaces by lichen growth and its application to glaciology and physiography (lichenometry). In Raasch (ed.) Geology of the Arctic vol.2. Univ. of Toronto Press: 1044-62

BESCHEL (1973): Lichens as a measure of the age of recent moraines. Arctic and Alpine Research 5:303-309

INNES (1982): Lichenometric use of an aggregated Rhizocarpon “species”. Boreas 11:53-57

INNES (1983): Use of an aggregated Rhizocarpon “species” in lichenometry an evolution. Boreas 12:183-190

INNES (1983): Size frequency distributions as a lichenometric technique: an assessment. Arctic and Alpine Research 15:285-294

INNES (1985): An examination of some factors affecting the largest lichens on a substrate. Arctic and Alpine Research 17:99-106

HEUBERGER (1966): Gletschergeschichte. Untersuchungen in den Zentralalpen zwischen Sellrain- und Ötztal. Wissenschaftliche Alpenvereinshefte 20:125

KONRAD & CLARK (1998): Evidence for an early Neoglacial glacier advance from rockglaciers and lake sediments in the Sierra Nevada, California USA. Arctic and Alpine Research 30:272-284

MATTHEWS (1992): The ecology of recently deglaciated terrain. Cambridge University Press

O´NEAL & SCHOENENBERGER (2003): A Rhizocarpon geographicum growth curve for the Cascade Range of Washington and Northern Oregon, USA. Quaternary Research 60:233-241

WALKER (2005): Quaternary dating methods. Wiley Press