Varnish Microlamination (VML) Dating

We provide rock varnish-based age estimates for surficial geological

and geoarchaeological features in the western US deserts.


VML Dating: An Introduction

       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 microstratigraphy.

       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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