Biological Dating Methods

Biological dating methods generally use the size of an individual species of plant as an index of the age of the substrate on which it is growing. They may be used to provide minimum age estimates only, as there is inevitably a delay between the time a substrate is exposed and the time it is colonized by plants, particularly if the surface is unstable (e.g., in an ice-cored moraine). Fortunately this delay may be short and not significant, particularly if the objective is simply establishing a relative age.

4.3.1 Lichenometry

Lichens are made up of algal and fungal communities living together symbiotically. The algae provide carbohydrates via photosynthesis and the fungi provide a protective environment in which the algal cells can function. Morphologically, lichens range from those with small bush-like thalli (foliose lichens) to flat disc-like forms, which grow so close to a rock surface as to be inseparable from it. These crustose lichens commonly increase in size radially as they grow and this is the basis of lichenometry, the use of lichen size as an indicator of substrate age (Locke et al., 1979). Lichenometry has been most widely used in dating glacial deposits in tundra environments where lichens often form the major vegetation cover and other types of dating methods are inapplicable (Beschel, 1961; Benedict, 1967). The technique may also be used to date lake-level (and perhaps even sea-level) changes, glacial out-wash, and trim-lines, rockfalls, talus stabilization, and the former extent of permanent or very persistent snow cover.

4.3.1.1 Principles of Lichenometry

Lichenometry is based on the assumption that the largest lichen growing on a rock substrate is the oldest individual. If the growth rate of the particular species is known, the maximum lichen size will give a minimum age for the substrate, because all other thalli must be either late colonizers or slower growing individuals (i.e., those growing in less than optimum conditions). Lichen size dates the time at which the freshly deposited rocks become stable, because an unstable substrate will prevent uninterrupted lichen growth. Growth rates can be obtained by measuring maximum lichen sizes on substrates of known age, such as gravestones, historic or prehistoric rock buildings, or moraines of known age (perhaps dated independently by historical records or radiocarbon). It is also possible to measure growth directly by photographing or tracing lichens of varying sizes every few years on identifiable rock surfaces (Miller and Andrews, 1973; Ten Brink, 1973). Generally, the maximum diameter of the lichen thallus is measured on individuals that have shown fairly uniform radial growth.

Growth rates vary from one region to another so it is necessary to calibrate the technique for each study site, but the general form of the growth curve is now fairly well established. After initial colonization of the rock surface, growth is quite rapid (known as the great period); growth then slows to a more or less constant rate (Fig. 4.14; Beschel, 1950). Different lichens grow at different rates and indeed some species may approach senescence while other species are still in their great period of growth. The black foliose lichen Alectoria minuscula, for example, rarely exceeds 160 mm in diameter on rock surfaces in Baffin Island; lichens of this size represent a substrate age of -500-600 yr B.P. By contrast, Rbizocarpon geographicum has only just entered its period of linear growth by this time (at -30 mm diameter) and will continue to grow at a nearly constant rate for thousands of years after that. In fact, it has been estimated that a 280 mm thallus of Rh. geographicum on eastern Baffin Island dates its substrate at -9500 ± 1500 yr B.P. (Miller and Andrews, 1973). Similarly, a 480 mm Rh. alpicola thallus in the Sarek mountains of Swedish Lapland is thought to have begun its growth following deglaciation of the region -9000 yr B.P. (Denton and Karlen, 1973b). Different lichens may thus be selected to provide optimum dating resolution over different timescales. However, in view of its ubiquity, ease of recognition, and useful size variation over the last several thousand years, the lichen Rh. geographicum has been most commonly used in lichenometrical studies (Fig. 4.15; Locke et al., 1979). Once a growth curve for the species in question has been established, measurements of maximum lichen sizes on moraines and other geomorphological features can be used to estimate substrate age (Fig. 4.16).

4.3.1.2 Problems of Lichenometry

There are three general areas of uncertainty in lichenometry, relating to biological, environmental, and sampling factors (Jochimsen, 1973).

(a) Biological Factors

Lichens are exceedingly difficult to identify to species level in the field and most users of lichenometry have no training in lichen taxonomy. Indeed, lichen taxonomy is itself a contentious subject, which compounds the users' difficulties. Rhizocarpon geographicum is exceedingly similar to Rh. superficial and Rh. alpicola (King and Lehmann, 1973) and doubtless many investigations have been based on a mixture of observations (Denton and Karlen, 1973b). This presents no problem, of course, providing that the different species grow at similar rates, but generally such factors are not well known. What evidence there is suggests that growth rates may vary between species (Calkin and Ellis, 1980; Innes, 1982). Lichen dispersal and propagation rates

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150-

100-

Brooks Range, Alaska (30;-14)

St. Elias and Wrangeil Mountains, Alaska (36;-5.5)

Cordillera Blanca,

Peru

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100-

St. Elias and Wrangeil Mountains, Alaska (36;-5.5)

Rhizocarpon Geographicum Growth Rate

Cordillera Blanca,

Peru

Front Range, Colorado (85;-1)

Rhizocarpon Geographicum Growth Curve

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Approximate Age of Substrate (yr B.R) Radiocarbon Years since Substrate Stabilization

FIGURE 4.14 Growth rates of Rhizocarpon geographicum in different areas of the world for the Holocene (left; Rodbell, 1992) and for the last 1600 yr (right; Calkin and Ellis, 1980). Numbers in parentheses on left-hand diagram show mean annual precipitation (mm) and mean annual temperature (°C) in each region. Error bars on each curve are probably ± 15-20%.

Lichen Dating

growth rate of the lichen is known.This specimen, growing on pyroxene-granulite gneiss in the glacier foreland of Hogvaglbreen (Jotunheimen, southern Norway) is growing at >1.1 mm a"1 (based on measurements from 1981-1996). The calipers are open at 10 cm (photograph kindly provided by J. Matthews, University of Wales, Swansea).

growth rate of the lichen is known.This specimen, growing on pyroxene-granulite gneiss in the glacier foreland of Hogvaglbreen (Jotunheimen, southern Norway) is growing at >1.1 mm a"1 (based on measurements from 1981-1996). The calipers are open at 10 cm (photograph kindly provided by J. Matthews, University of Wales, Swansea).

are also inadequately understood. Many lichens propagate their algal and fungal cells independently so that it may take some time for two individuals to find each other and form a new symbiotic union. In other cases, lichens are propagated when part of the parent breaks off the rock substrate and is blown or washed away to a new site. In either case, there may be a significant delay between the exposure of a fresh rock surface and colonization by lichens. Furthermore, even when lichen cells become established, decades may elapse before the thallus becomes visible to the naked eye. As time passes, rock surfaces may become virtually covered in lichens and inevitably this results in competition between individuals; indeed some lichens appear to secrete a chemical that inhibits growth in their immediate vicinity (Ten Brink, 1973). Such factors seem likely to reduce growth rates as rocks become heavily lichen-covered and this may give the erroneous impression of a relatively young age for the substrate.

Finally, as lichens become very old, growth rates may decline. Little information is available on senescence in lichens and unfortunately this corresponds to the part of the growth curve where there is the least dating control. Often growth rates beyond a certain age (i.e., the final dated control point) are assumed to continue at a constant rate, whereas in all probability the rate declines with increasing lichen age. This will lead to (possibly large) underestimates of substrate age; such errors can be avoided if extrapolation of growth rates is not attempted.

Lichen measurements on surfaces of known age in the Kebnekaise - Sarek region

O Glacier drift □ Open mine A Railroad excavation ■ Building

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Williams Senescence Curve

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Responses

  • Semira Dahlak
    How lichenometry is used in dating glacial deposit in tundra environment?
    2 years ago

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