Geoscience Reference
In-Depth Information
Table 3.3
Methods that have been used to estimate the age of rock varnish formation (after Dorn, 1994).
Method
Theory
Precision level
a
Comments/case Examples
Appearance
Varnish darkens over time
Relative
Controlled by factors other than
time
Thickness
As varnish gets older, it grows
vertically
Relative
Also controlled by
microenvironment
Coverofblacksurface
varnish
Varnish grows laterally away from
nucleation centres
Relative
Derbyshire
et al.
(1984)
Other bottom varnish
growth
As age increases, undersides of clasts
are coated with Fe-clay rock
varnish (Mn poor)
Relative
Derbyshire
et al.
(1984)
Trace element trends
Assumes varnish derived from
underlying rock.
Trace element profiles with depth
reflect time
Relative
Bard (1979)
Metal scavenging
Zn, Cu, Ni and other metals increase
over time as they are scavenged by
Mn-Fe oxides
Relative
Dorn
et al.
(1992b)
Palaeomagnetism
Magnetic field aligned when Fe oxides
precipitate
Correlative
Clayton, Verosub and Harrington
(1990)
Tephra-chronology
Glass fragments from known volcanic
eruptions might be identifiable in
rock varnish
Correlative
Harrington (1988)
Varnish geochemical
layering
Sequences of chemical (e.g. Mn:Fe,
Pb,
δ
Correlative
Dorn (1992)
13
C) and textural changes
correlated from site to site
Stratigraphy
Dating material on or under varnish
constraints varnish age
Correlative and
numerical
Dragovich (1986)
Cation ratio
Mobile cations are leached faster than
immobile cations (K
+
Ca)/Ti
Calibrated
Dorn (1989); Dorn
et al.
(1990)
K-Ar dating
As varnish clays accumulate, they may
undergo a diagenesis that refixes K
or dates K in Mn oxides
Numerical
Dorn (1989); Vasconcelos
et al.
(1992)
Uranium series
Uranium precipitates with Mn oxides
and then decays
Numerical
Knauss and Ku (1980)
Radiocarbon
Accreting varnish can capsulate
underlying organic matter
Numerical
Dorn
et al.
(1992a)
a
Relative, correlative, calibrate,
and
numerical
are Quaternary dating terms recommended by Colman, Pierce and Birkeland (1987); see text for
details.
3.2.6
Rock varnish
nishes in subaerial situations that represent a continuous
deposition are suitable for investigation, while assump-
tions have to be made about rates of deposition and the
correlation and relative dating of microlayer variations to
other palaeoclimate records. In some cases it is possible
to use absolute age controls (Table 3.3), e.g. by using
cosmogenic
36
Cl exposure dating or AMS
14
C dating of
organic material trapped within varnish layers during their
deposition (Watchman, 2000; Dorn, 2007).
It is the manganese component of rock varnish, rep-
resented in the Fe-Mn ratio that is a particularly useful
In some drylands, especially in the American south-
west, valuable palaeoenvironmental information has been
gained from rock varnish studies (see also Chapter
6; Dorn, 1990, 2007). The potential for rock varnish
palaeoenvironmental contributions is considerable: var-
nish is widespread in deserts, can possess a long record
and there is a growing body of evidence that suggests rock
varnish microstratigraphy can provide a detailed environ-
mental record otherwise unavailable from other sources
