Between sleeping and awakening. We tend to think of the world’s dune fields as fearsomely awake, threatening and encroaching, creating vast sand and dust storms – and this is indeed the case for great stretches of our planet’s sand seas, the ergs. But far larger dune terrains are asleep, stable and vegetated. Or they are at least dozing, waiting to be re-activated, and understanding exactly what are the circumstances under which previously dormant dunes can spring back to life is critical to the livelihoods of the people who live in arid lands. Furthermore, the implications for atmospheric dust load and climate change are profound.
The image above is from the Great Sandy desert of Western Australia, where single linear dunes stretch for tens of kilometres. Yes, as the outback winds blow, the sand moves, as we can see from the ripples on the dune, but the scrawny (though remarkably diverse) vegetation is sufficient to stabilise the dunes themselves. The extraordinary morphology of these dunes is shown in this Google Earth image, the dune in the photograph above being the central one in the satellite view.
But it is only a snapshot and things can change remarkably quickly. As climate changes, what are the mechanisms by which slumbering dunes can be stirred into wakefulness? It’s clearly more complex than simply changing winds and precipitation, but how can we answer this question and make any predictions about the future? I discussed this intriguing topic a little in The Desert book (whose publication is now, by the way, delayed until February – not through any fault of mine), so here is a taster from Chapter 6, Ancient and Modern, Boom and Bust:
The deserts of today contain long histories of aridity (probably more than 20 million years in the Atacama, more than 7 million years in the Sahara). Of particular interest is that those histories are not ones of sustained levels of aridity, but rather a history of fluctuation, of ebb and flow between semi- and hyper-arid as the climate changed. The movement of active dunes was prevented as vegetation grew, the winds died down or precipitation increased, even slightly. When the climate dried out, the dunes resumed their march and these episodes of activity and stabilization are recorded in the sands. These kinds of changes can even be seen in historical times: the largest dune field in the western hemisphere can be found in the prairies of Nebraska, appropriately, part of the ‘Great American Desert’. These dunes are vegetated and immobile today, but a thousand years ago they were on the move and they were periodically resuscitated by the droughts before and during the pursuit of Manifest Destiny, causing problems for the settlers’ wagon trains. Navajo oral histories provide first-hand evidence of changing vegetation and fluctuations in dune activity. Dunes cover around five per cent of the global land surface today, but most are stable. [The map above] shows the current distribution of active and ‘relict’, immobile, ergs, a snapshot in time. The map would have looked very different a few thousand years ago and will change significantly in the future.
In order to decipher the history of phases of erg activity, a means of determining the age of a particular layer of sand is required. For a long time this was simply not possible: the desert is far from an ideal environment in which to preserve the mineral and fossil materials that provide the geological clocks, and, even should carbonaceous remains be available, carbon dating only works for ages younger than 60,000 years. However, over the last couple of decades two remarkable clocks have been developed that have revolutionized our ability to decipher the stories of the deserts and transformed our understanding of how they work. Once again, isotopes are the key. Once a sand grain, typically quartz, is buried, it is bombarded from its surroundings by natural radiation from isotopes of potassium, thorium and uranium. The radiation strips away electrons from the mineral atoms but those electrons remain trapped in the crystal structure of the sand grain, a store of energy that can be released simply by shining light on to the grain. In doing so, the energy of the trapped electrons is released as light and the grain glows, it luminesces. The longer the grain has remained buried, away from sunlight, the more electrons are trapped and the more energy will be released. Since isotopes are predictable, we know the rate at which electrons are produced and so we have a clock. Optically stimulated luminescence dating (OSL) gives us a measure of how long our sand grain has been in the dark, for when it re-emerges into the sunlight the clock is re-set.
However, once the grain is back at the surface, it is bombarded by extraterrestrial cosmic rays which themselves re-organize some atoms into new isotopes, and the longer the grain remains at the surface, the more isotopes accumulate: now we have a clock that measures how long a grain has been exposed on the surface. As with all such measurements, data from one sand grain is far from sufficient, and huge numbers of analyses are required to come to a statistically acceptable conclusion. And nature creates all kinds of complications in the story of an individual sand grain that can cause ambiguities and complications in interpreting the data, but these methods have given us a means of quantifying episodes of desert activity and have provided extraordinary insights into the dynamics of arid lands.
For example, detailed studies of dunes from the Kalahari and the Namib Deserts have produced chronologies of periods of movement and stability over the last 80,000 years or so. There are some significant correlations of events between different types of dunes from widely different locations, in general correlating with the global history of pulses of glaciation. However, the data from different areas can show significant conflicts, and correlation of the timings of dune movements with proxies for temperature, wind and oceanic upwelling from the continuous archives of neighbouring ocean sediments raises some interesting and challenging questions. It seems that different parts of the same sand sea can be active at different times, and that dune movement is more a measure of changing wind strength than a reliable indicator of aridity: ironically, the same burial ages coming from large numbers of sand grains must reflect accumulation and therefore a reduction in the strength of the winds.
It’s a complex environment, the desert, but while we are only beginning to understand it, other forms of life have known it well for close to 500 million years…
The processes whereby dune fields can become hypnopompic (or, indeed, hypnagogic) remain elusive, but a great deal of work has been done in the sands of the Kalahari. The crucial role of vegetation – and the influence of the activities of homo sapiens – has recently been reported by a group of researchers from the universities of Virginia, California, and New Mexico: Sleeping sands of the Kalahari awaken after more than 10,000 years. Most of the Kalahari dunes had been stabilised by vegetation that survived in balance with the methods of the resident pastoralists. Until water wells were drilled. The dramatically increased water supply resulted in a complete change in the agricultural intensity, a growth of livestock numbers far in excess of the carrying capacity of the land, and, consequently, a severe loss of vegetation, crucially the stabilising grasses. The dunes began to stir.
It's unclear, say [the paper’s authors], whether the Kalahari's dunes hang on the edge of a tipping point between their current state--"vegetated fixed linear dunes"--or have moved to what researchers call a degraded state, "barren and active dunes."
Yes, it’s complex, and the work examines recovery of grasses after grazing has been halted. However, exactly what stimulates an “irreversible shift to a stable degraded state” – permanently awake and active dunes – has yet to be understood. The rate at which change from stable to active can take place is clearly revealed by spending a little time on Google Earth and its wondrous historical imagery tool. Here are three images from one the sites in Botswana that the project describes – incredible, visible fluctuations over a period of less than ten years.
The full report can be read at the Ecological Society of America’s Ecosphere journal website.
And I feel that I should warn readers that “hypnopompic dunes” is not standard terminology, but rather a flight of my sometimes-overactive imagination. But then again, it does have a certain ring to it…
[Map of active and relict ergs from: Peter G. Fookes and E. Mark Lee, ‘Desert environments: landscapes and stratigraphy’, Geology Today, 25/5 (2009).]