Dust, Drying, and Rock Art

While Western windstorms have always deposited dust on mountains, some recent research suggests that the trend is worsening, likely due to a continuing drought but also to land use in the West that is exposing more bare soil to the wind. -Wall Street Journal, January 7, 2014

This is the grass that grows wherever the land is and the water is,   This is the common air that bathes the globe.  –Walt Whitman, from Song of Myself.

Rapid climate change and potential catastrophic impacts are of major concern as they may severely challenge the ability of societies to respond and adapt due to environmental, economic, cultural and geopolitical constrains. [1]

Human activities and population growth accelerate the oscillations of climate patterns. One result: increasing airborne dust from soil exposure and desiccation of the earth’s surface.  Three billion tons of dust release into the atmosphere every year - with complex effects.

Dilemma of dust.  In a few millennia (sooner?) a few hardy observers and explorers will probe through sedimented layers of dust at the base of boulders and rims.  Luckily they will discover petroglyphs preserved though most of humankind will have disappeared along with many plants and animals from the desert west as we know it.  Some rock art will live on - a faint glimmer of trust in an unforeseeable future.

Flip side of dust.  Dust offers one key to unraveling the past of rock art production. Cycles of airborne dust result in differential deposition. Layered markers worthy of study.  Findings help understand how changing climate shapes cultural sequences as peoples come and go, determined in great part by availability of the essential:  Water.

In the Kimberley region of northwest Australia, a 1500-year mega-drought beginning about 5000 years BP coincided with the boundary between “the fine featured anthropomorphic figures of the Gwion Gwion or Bradshaw paintings, and broad stroke Wandjina figures.”  The dates were arrived at in part by studying the chemistry of airborne dusts deposited in the sediments of a mound spring. [1]

In the arid western US, Varnish Microlamination (VLM) studies, refined over many years by Ronald Dorn and Tanzhuo Liu, have enabled more fine-grained dating for rock art researchers, David Whitley in particular. Rock varnish is a rock coating we usually refer to as patina.  Airborne dust in the form of fine clay minerals comprises the bulk of the sediments that form rock varnish on exposed rock. Notably, Manganese and Iron contribute staining and color - less so during arid times. Hence, discernible lamellae.  Whitley has integrated these studies with his extensive research in the Mojave Desert. The result is an important recent paper, far ranging, and provocative. I find Whitley convincing when he says, “The most conservative interpretation indicates that petroglyph production began in the western Great Basin at least 11,100 years ago and that it continued into the last 300 years.”  [2]

NOTES. Happily, both articles are open access:
[1]  McGowan, Hamish, et al. 2012. Evidence of ENSO mega‐drought triggered collapse of prehistory Aboriginal society in northwest Australia Geophysical Research Letters 39, no. 22. ENSO?  El Niño-Southern Oscillation. 

[2]  David S.Whitley. 2013. Rock Art Dating and the Peopling of the Americas.  Journal of Archaeology, Volume 2013, Article ID 713159, 15 pages.

An article that doesn't mention rock art – or dust – nonetheless a fascinating complement to the topic (PDF): Possible impacts of early-11th-, middle-12th-, and late-13th-century droughts on western Native Americans and the Mississippian Cahokians.  2007.  Notably, Senior author Larry Benson also led the team which recently established an early Holocene age of the Winnemucca Lake petroglyphs.