Geology Reference
In-Depth Information
100
Dating Using Carbonate Coatings
1.5
Dating Using Lichen Diameters
1570?
1650
1710
80
Lost River Valley, Idaho
1.0
60
1780
0.5
40
1860
1890
20
0.0
Swedish Lappland
Lichenometry
1920
0
5
10
15
20
Age (ka)
AB
0
1950
1850
1750
1650
12
Number of Moraines
10
8
6
4
2
0
Fig. 3.4
Carbonate coatings as a function of deposit
age, from soils in the Lost River Valley, Idaho.
Although rates vary by a factor of more than 2,
a long-term average rate of about 0.6 mm/10 kyr appears
relevant for this setting. Modified after Vincent
et al
.
(1994).
Years A.D.
Fig. 3.5
Lichen diameter as a function of age.
A. Lichen growth curve calibrated using surfaces of
known age in the Swedish Lappland. Note nonlinear
growth, with the most rapid growth for youngest
surfaces. B. Histogram of the number of moraines with
a given maximum lichen diameter and, hence, of a given
age inferred from the calibration curve. Modified after
Denton and Karlen (1973).
of 2 and perhaps 3 over the last few tens of
thousands of years (Fig. 3.4). The mean rate over
the last 20 kyr has been about 65
μ
m/kyr. Those
clasts from surfaces well over 100 ka commonly
have coatings on the order of 10 mm thick. We
note that, whereas in this particular region of
Idaho one might be able to determine the age
of a surface by documenting the thickness of
these coatings, the technique requires a local
calibration, which in turn necessitates having
some surfaces of known absolute age. Uncer-
tainties, therefore, creep in, not only from the
variability of the coating thicknesses themselves,
but from the errors in the dating methods used
to obtain the absolute ages of the surfaces used
in the calibration.
to record the largest lichen found from the
surfaces of commonly hundreds of boulders on
the surface to be dated, or to take the mean of the
five largest lichens. Again, local calibration is
necessary, as climate dictates the rate of lichen
growth. Typically, the calibration of the growth-
rate curve comes from surfaces that have been
exposed for a known period of time, most com-
monly from man-made structures. One clever
approach uses lichens on tombstones, which of
course have the age stamped on them! Licheno-
metry has been used effectively to date moraines
in glacial settings (Denton and Karlen, 1973;
Porter, 1981) (Fig. 3.5) and major rockfalls in
rugged landscapes (Porter and Orombelli, 1981).
This technique has seen a resurgence recently
through the work of Bill Bull (Bull, 1996; Bull
and Brandon, 1998; Bull
et al.
, 1994). He argues
that the lichenometric method is much more
robust if one records the maximum diameters
of lichens on many hundreds to thousands of
rocks on the feature to be dated (he measures
using a digital micrometer linked to a
minicomputer in his backpack). A major
problem with the previous practice was that
one relied heavily upon the regularity of the
statistics in the population of lichen diameters,
Lichenometry
A technique that has long enjoyed use in
geomorphic studies is lichenometry, in which the
diameter of a specific type of lichen is used as a
proxy for the time a rock has been exposed at the
surface (e.g., Benedict, 1967; Innes, 1984, 1985;
Locke
et al.
, 1979; Porter, 1981). The lichen most
commonly employed is
Rhizocarpon
(one or
another subgenus), which is crudely circular in
shape and whose diameter is thought to increase
linearly with time. Until recently, the practice was
