Rock varnish is a dark coating on subaerially exposed rock surfaces. It is probably
the world's slowest-accumulating sedimentary deposit, growing at only a few to tens of microns per
a thousand years (Liu and Broecker, 2000). Its thickness ranges from <5 µm to 600 µm, with a
typical thickness of about 100 µm. Although found in all terrestrial environments, rock varnish is
mostly developed and well preserved in arid to semiarid deserts of the world (Figure 1). It is composed of about 30% manganese and iron oxides, up to 70% clay minerals, and
over a dozen trace and rare earth elements (Potter and Rossman, 1977, 1979). The building blocks of rock varnish
are largely blown in as airborne dust (Fleisher et al., 1999; Moore et al., 2001; Thiagarajan and Lee, 2004).
Microlaminations in rock varnish were first reported by Perry and Adams (1978), who recognized their
potential as a paleoenvironmental indicator in drylands. Microlaminations can be observed when the
varnish is shaved thin enough (5-10 µm) to see through in ultra-thin section with a light
microscope (Figure 2). Electron microprobe chemical mapping reveals that dark layers in varnish thin
section are rich in Mn and Ba, but poor in Si and Al, while orange and yellow layers are poor in
Mn and Ba, but rich in Si and Al (Figure 3). These two types of layers are intercalated to form a distinct
A growing body of evidence indicates that varnish microstratigraphy carries a climate record (Dorn, 1984;
Dorn, 1990; Cremaschi, 1996; Liu and Dorn, 1996; Liu et al., 2000; Broecker and Liu, 2001; Lee and Bland, 2003).
In the drylands of western USA (Figure 4), Mn-poor yellow layers (usually containing 5-15% MnO) formed during dry periods of
the Holocene and the last interglacial, while Mn-rich black layers (usually containing 25-45% MnO) deposited
during wet periods of the last glacial time; Mn-intermediate orange layers (usually containing 15-25% MnO)
formed during periods of climatic transition between extremely dry and extremely wet condition (Broecker and
Liu, 2001) (Figure 5). Furthermore, the glacial-age black layers in varnish microstratigraphy appear to correlate in
time with the cold episodes of the Younger Dryas and Heinrich Events (Broecker, 1994; Bond et al., 1993)
in the North Atlantic region (Liu and Dorn, 1996;
Liu et al., 2000; Liu, 2003) (Figure 6 and Figure 7).
Varnish microlamination (VML) as a correlative dating technique (cf. terminology in Colman et al., 1987) is
relatively new and different in principle and independent of both cation-ratio and AMS 14C methods
(Dorn, 1983; Dorn et al., 1989). It was first used by Dorn (1988) to study the chronostratigraphy of
alluvial-fan deposits in Death Valley of California. Subsequent studies by others (Liu, 1994; Liu and
Dorn, 1996; Liu, 2003) have greatly improved the usefulness of the technique. The basic assumption in this
dating approach is that the formation of varnish microstratigraphy is largely influenced by regional climatic
variations. Since climatic signals recorded in varnish have been proven to be regionally contemporaneous (Liu and
Dorn, 1996; Liu et al., 2000), varnish microstratigraphy has the potential use as a tephrachronology-like
dating tool. A rigorous blind test of this method has been conducted on late Quaternary
lava flows in the Mojave Desert, California (Liu, 2003; Phillips, 2003). The test results demonstrate that
varnish microstratigraphy is a valid dating tool to provide surface exposure age for late Pleistocene (i.e., 12 - 85 ka) surficial
geomorphic features in the Great Basin of western USA (Marston, 2003) (Figure 8). New
radiometric age calibration and climatic correlation of varnish microstratigraphy with the SPECMAP record have extended the utility of the VML method to surficial
geomorphic and geoarchaeological features of late Quateranry (i.e., 0 - 300 ka) (Liu and Broecker, 2007) (Figure 9).
As a unique dating technique, the VML method has great chronometric applications in earth science and geoarchaeology. Without
age calibration, it can be used as a regional correlation and mapping tool (Liu and Dorn, 1996). Once radiometrically
calibrated, it can yield minimum-limiting surface exposure ages for various geomorphic features (e.g., alluvial-fan
surfaces, desert pavements, hillslope deposits, lava flows, debris flows, fault scarps, meteor crater) and
geoarchaeological features (e.g., stone tools, petroglyphs, geoglyphs) (Figure 10) in the
western USA drylands where other dating techniques such as radiocarbon and cosmogenic radionuclide methods are either not applicable or difficult to use. Since
microstratigraphy has been observed in varnish from other deserts of the world (e.g., Negev Desert of Israel, Patagonia
Desert of Argentina, Coastal Desert of Peru, Gürbantongüt Desert of western China, the Indus River Valley of Pakistan, and Strzelecki Desert of South
Australia) (Figure 11), the VML method may also have potential chronometric applications in those desert regions of the world.
(For further understanding of the VML dating technique and its chronometric applications in earth science and geoarchaeology, please go to
Selected Publications; for more information about rock varnish in general and rockart sites in the western USA dryalnds, please go to
Links at VML Dating Lab's website.)
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