Deep-rooted and completely erroneous preconceptions of our planet’s arid lands as sterile bit-players in the great game of the earth’s dynamic systems have long inhibited our scientific enthusiasm for, and understanding of, the desert. We are now beginning to catch up – take, for example, this recent headline from the American Geophysical Union:
The world's deserts may be storing some of the climate-changing carbon dioxide emitted by human activities, a new study suggests. Massive aquifers underneath deserts could hold more carbon than all the plants on land, according to the new research.
As described in a summary of this research on Science Daily:
Humans add carbon dioxide to the atmosphere through fossil fuel combustion and deforestation. About 40 percent of this carbon stays in the atmosphere and roughly 30 percent enters the ocean, according to the University Corporation for Atmospheric Research. Scientists thought the remaining carbon was taken up by plants on land, but measurements show plants don't absorb all of the leftover carbon. Scientists have been searching for a place on land where the additional carbon is being stored--the so-called "missing carbon sink."
The lead author of the report in the AGU publication, Geophysical Research Letters, is Yan Li, a desert biogeochemist with the Chinese Academy of Sciences in Urumqi, Xinjiang; he and his team examined the character of groundwaters in the gigantic closed system of the arid Tarim Basin, and came up with some fascinating – and provocative – results. Runoff waters from the surrounding mountains pick up, as a normal part of the carbon cycle, some CO2 dissolved from the rocks and soils through which the rivers flow. However, by the time that water ends up in aquifers, the underground reservoirs beneath the desert, it contains substantial amounts of DIC, dissolved inorganic carbon.
Being able to date the carbon, Li and his colleagues could distinguish between old carbon originating from the rivers and very young carbon added to the water as it seeped through the soils of the irrigated oases along the desert margins. These are poor soils, not in themselves sources of much CO2 - it originates from the respiration of the roots of crops and microbes in the soil. And because these crops are irrigated almost constantly, not only to keep them growing but to wash out the salts that, as in all desert agriculture, accumulate in the soil, most of the CO2 is transported downward into the groundwater moving out below the desert to be trapped in the deep aquifers. Importantly, because of the salts, these waters are saline and alkaline and the solubility of CO2 in saline/alkaline water is much higher than in pure or acidic water – the desert groundwater is a very significant CO2 sink.
Because of their ability to date the carbon dissolved in the waters, the researchers were able to establish that the levels jumped substantially in historical times as the development of the Silk Road enabled the beginnings of oasis agriculture. Man’s activities – irrigation and over-irrigation – have augmented the efficiency of this carbon sink by, it is estimated, a factor of twelve:
Based on the various rates that carbon entered the desert throughout history, the study's authors estimate 20 billion metric tons (22 billion U.S. tons) of carbon is stored underneath the Tarim Basin desert, dissolved in an aquifer that contains roughly 10 times the amount of water held in the North American Great Lakes.
The study's authors approximate the world's desert aquifers contain roughly 1 trillion metric tons (1 trillion U.S. tons) of carbon--about a quarter more than the amount stored in living plants on land.
And because this is a saline and alkaline aquifer, the water is completely unsuitable for agriculture – it will likely remain below the desert as essentially permanent carbon storage - undoubtedly not the only missing sink, but a hitherto unidentified one. As Li remarks: “The fact that such a huge carbon pool and active sink has been unstudied for so long may simply be because it is remote and hidden under deserts: out of sight, out of mind.”
[Image at the head of this post is of agriculture and dunes along the northern edge of the Tarim Basin, Google Earth; diagram of the process of carbon storage from Yan Li, Yu-Gang Wang, R. A. Houghton, Li-Song Tang. Hidden carbon sink beneath desert. Geophysical Research Letters, 2015; DOI:10.1002/2015GL064222]
Yet again, thank you NASA. As announced last month, their latest extraordinary earth-monitoring system is in orbit, commissioned and providing data: SMAP, the Soil Moisture Active Passive mission provides a high-resolution view of continuing changes in soil moisture across our world and allows understanding, analysis and planning in an unprecedented and fascinating way.
The map above is one of several already available:
These maps of global soil moisture were created using data from the radiometer instrument on NASA's Soil Moisture Active Passive (SMAP) observatory. Each image is a composite of three days of SMAP radiometer data, centered on April 15, 18 and 22, 2015. The images show the volumetric water content in the top 2 inches (5 centimeters) of soil. Wetter areas are blue and drier areas are yellow. White areas indicate snow, ice or frozen ground.
The soil moisture scale is in cm3/cm3 and demonstrates dramatically the often-forgotten fact that drylands, home to a third of our planet’s population, comprise over 40% of the land area.
As usual, the technology is extraordinary:
For an introductory description of how the radar and radiometer instruments conspire, see http://smap.jpl.nasa.gov/observatory/overview/. The NASA press release summarises the mission and the ways in which the data will be used:
Launched Jan. 31 on a minimum three-year mission, SMAP will help scientists understand links among Earth's water, energy and carbon cycles; reduce uncertainties in predicting climate; and enhance our ability to monitor and predict natural hazards like floods and droughts. SMAP data have additional practical applications, including improved weather forecasting and crop yield predictions.
A first global view of SMAP's flagship product, a combined active-passive soil moisture map with a spatial resolution of 5.6 miles (9 kilometers), is available at:
During SMAP's first three months in orbit, referred to as SMAP's "commissioning" phase, the observatory was first exposed to the space environment, its solar array and reflector boom assembly containing SMAP's 20-foot (6-meter) reflector antenna were deployed, and the antenna and instruments were spun up to their full speed, enabling global measurements every two to three days.
The commissioning phase also was used to ensure that SMAP science data reliably flow from its instruments to science data processing facilities at NASA's Jet Propulsion Laboratory in Pasadena, California, and the agency's Goddard Space Flight Center in Greenbelt, Maryland.
"Fourteen years after the concept for a NASA mission to map global soil moisture was first proposed, SMAP now has formally transitioned to routine science operations," said Kent Kellogg, SMAP project manager at JPL. "SMAP's science team can now begin the important task of calibrating the observatory's science data products to ensure SMAP is meeting its requirements for measurement accuracy."
Together, SMAP's two instruments, which share a common antenna, produce the highest-resolution, most accurate soil moisture maps ever obtained from space. The spacecraft's radar transmits microwave pulses to the ground and measures the strength of the signals that bounce back from Earth, whereas its radiometer measures microwaves that are naturally emitted from Earth's surface.
"SMAP data will eventually reveal how soil moisture conditions are changing over time in response to climate and how this impacts regional water availability," said Dara Entekhabi, SMAP science team leader at the Massachusetts Institute of Technology in Cambridge. "SMAP data will be combined with data from other missions like NASA's Global Precipitation Measurement, Aquarius and Gravity Recovery and Climate Experiment to reveal deeper insights into how the water cycle is evolving at global and regional scales."
It is, to me, wondrous that even a simple examination of the three successive images from which the one at the head of this post was taken reveals clear changes over the course of a few days – I can only begin to imagine what analysis of the real data will reveal.
The UN’s map and classification of global drylands drives home their importance to the way our planet works. The new SMAP data bring this to life in a dramatic way.
[SMAP image credit: NASA/JPL-Caltech/GSFC]
Where does sand come from? A simple question, not always easily answered, and certainly not in some isolated tropical island environments. The Maldives Archipelago consists of 22 coral atolls, each containing a multitude of individual reefs, many of which surround a small island rimmed with glistening white sand. Those islands are vital, not only for the inhabitants and the economy, but for biodiversity and the health of the Maldives ecosystems. But they are vulnerable landforms, consisting simply of piles of unconsolidated sand, rarely more than 3m above sea level – and that sea level is rising. Understanding how the islands are maintained, the sources and movement of the sand that builds them, is an important, and, until now, poorly understood process. However, recent work by a team from the University of Exeter, in the UK, and collaborators from New Zealand and Australia, has revealed the details of reef island sustenance – and it all comes down to huge quantities of parrot fish excrement.
The study focused on the small island of Vakkaru (shown above), partly cultivated, partly vegetated, and completely surrounded by its white beaches, lagoon and reef. There is no source of sand other than that reef, for the Maldives are, after all, in the middle of the Indian Ocean and the nearest rivers flowing down to the shore, hills being weathered and eroded, are a very long way away. The whole sedimentary system is entirely biogenic, being run by the teeming ecosystem of the atolls. Exeter’s Chris Perry and his colleagues have meticulously quantified the major sediment-generating habitats, the abundance of different sediment-producing critters in each, and the rates of production. Vakkaru and its reef are little more than a maximum of a kilometre across, and the area of the island itself is less than 0.2 square kilometres, yet every year nearly 700 tons of new sediment is produced. Perhaps 10% of this comes from the broken-up calcareous segments that halimeda, a genus of macroalgae (or seaweed), produces as part of its structure, but more than 85% of the sand generated is comprised of parrotfish faeces.
A variety of parrotfish, particularly the excavator species, Chlorurus sordidus and Chlorurus strongylocephalus, and the scraper species Scarus niger, Scarus frenatus, and Scarus rubroviolaceus, chew up coral in order to extract nutrition from algae, and then excrete the indigestible stuff – as sand-sized grains of calcium carbonate. And significant populations of parrotfish do so in prodigious quantities. It is reported that the native Hawaiian name for the female redlip parrotfish translates to “loose bowels,” and this video is a striking illustration:
All of this sediment manufacture takes place around the reef itself. Some sediment gets flushed out into the deeper ocean, but much is transported, particularly during the monsoon season, into and across the lagoon and up onto the island. During this process, many of the halimeda fragments are further broken up and the dominant sand of the island is parrotfish poop. This illustration from the paper published last month in Geology summarises how the system works:
Many varieties of parrotfish are endangered, but it is clear that their role extends beyond key participants in biodiversity. As the report concludes:
While the need to protect parrotfish populations is commonly based on the need to sustain benthic ecological interactions, this study demonstrates their further critical beneficial role as producers of carbonate sediment and thus as key biogeoengineering species that can sustain local landform maintenance.
It has long been known that parrotfish manufacture sand – although rarely featured in tourism brochures, fish excrement is responsible for many of Hawaii’s gleaming white beaches – but this fascinating new analysis reveals the scale on which these bioengineers work.
[Parrotfish image by Chris Perry from the Science News and University of Exeter reports. Movie by Matthew Duncan. Paper: Linking reef ecology to island building: Parrotfish identified as major producers of island-building sediment in the Maldives, C.T. Perry, P.S. Kench, M.J. O'Leary, K.M. Morgan and F. Januchowski-Hartley, Geology, first published online April 27, 2015]
Well, sometimes you just have to throw modesty to the dusty winds and shamelessly take on a little self-promotion. The desert book was just reviewed for The Geological Society by Andrew Goudie, Emeritus Professor in Geography at Oxford, a leading international authority on arid lands. The review is available online (it will be published in Geoscientist in a couple of months), but here it is:
This handsome book is informative, well-illustrated, broad-ranging, and clever. The author, a geologist and professional writer, who in 2009 wrote a well-received book, ‘Sand: A Journey through Science and the Imagination’, has managed to weave together a whole array of different strands that serve to make deserts what they are.
Using some of his own field experiences, coupled with a wide reading of the literature, he has succeeded in covering the science of deserts (including climate, geomorphology, and wildlife), while at the same time discussing the human inhabitants of deserts, art and literature, and some of the arresting characters who risked their lives in discovering and traversing the world’s dryands.
It aims, as the author explains, to ‘provide an evocation, a celebration, a consideration of our response to the desert, the idea of the desert’, for ‘deserts are landscapes of the mind as much as physical realities, places of metaphor and myth.’ Using examples from central Australia, the Namib, the Gobi, the Sahara, the Mojave and the Atacama, it examines such landscapes in the context of their place in history, as birthplaces of civilizations, evolutionary adaptations, art, ideology and philosophy. To be sure, it does not cover everything relating to this vast topic, but it provides a superb introduction to what makes deserts so fascinating and alluring.
To give an example of how different material is cleverly combined, consider his treatment of flash floods. The climatic and geomorphological conditions that produce them are described, there are some graphic descriptions from the literature, but there is also a description of an explorer who was killed by a flash flood in the Algerian Sahara, Isabelle Eberhardt. We learn that she probably had syphilis, was illegitimate, was a habitual user of drugs, was highly promiscuous, and cut her hair like a man.
Equally, some pervasive surface features - stone pavements - are explained scientifically, but are also placed in the context of the disturbance of desert surfaces in the Libyan Deserts by the narrow tyres of the Model T Fords used by great desert explorers like Ralph Alger Bagnold. Similarly, dust storms are introduced by a consideration of the life and writings of Mildred Cable and her colleagues in the Gobi, but this is followed seamlessly by a discussion of how the global importance of dust storms has been revealed by the latest satellite-borne sensors.
Lovers of deserts will love this book and will also learn much from it.
Reviewed by Andrew Goudie, University of Oxford.
[Image from NASA: A dust storm was blowing large quantities of dust out over the Persian Gulf and Arabian Sea on Saturday, December 13, 2003. In this true-color composite scene, acquired by the Terra and AquaModerate Resolution Imaging Spectroradiometer (MODIS) instruments, the dust storm (light brown pixels) can be seen extending from the Arabian Peninsula (left) eastward over the Persian Gulf and the Gulf of Oman toward the Arabian Sea. Parts of southern Afghanistan and much of Pakistan are also covered by airborne dust.]
In large areas of Australia there are probably several hundred tons of termites in every square kilometre.
From The Desert, Lands of Lost Borders, Chapter 6:
The most ubiquitous (and irritating) vegetation in the Australian outback is spinifex, strictly Triodia. This coarse, tough grass grows in landscape-smothering tussocks, and its spiky leaf tips contain small shards of silica that have a habit of embedding themselves in the skin of passing animals, including humans.
Spinifex performs an important function in terms of dune stabilization and is a key participant in the fire ecology of the desert, but it is essentially inedible for animals and would smother the land and clog the ecosystem if left unchecked. In other climates, plant debris is cleared by wood-decaying fungi, but the desert is too dry for them. However, crucially, termites eat spinifex and there are a lot of them. Spinifex may be an archetypal feature of the landscapes of the outback, but so are termite mounds. They come in a bewildering array of shapes and sizes, each one extending far below the surface for water supply and providing a complex climate-controlled home to a community of hundreds of thousands of individuals. The termites consume the spinifex (along with a vast variety of other organic matter) and keep it under control, but they cannot digest it. For that, through a remarkable example of symbiosis, they require the specialized microbes in their gut that convert the cellulose to acetate, a kind of vinegar that then feeds the termites. Termite mounds provide safe havens for a variety of other creatures (some lizards lay their eggs in them) and the process of their construction moderates the desert soils, influencing water infiltration and evaporation, changing the structure and permeability. This, in turn, promotes plant growth and diversity, the entire vertebrate and invertebrate burrowing ecology and the food chain as a whole. Termite mounds in the Sahara and the Sahel are referred to as ‘houses of the devil’, but without this ‘keystone species’ arid lands would be very different — it has been estimated that most or all of the biomass produced in the Chihuahuan Desert is consumed by termites.
As a recent press release from Princeton University observed, “Termites might not top the list of humanity's favorite insects”, but it went on to highlight our on-going and emerging understanding of the critical role that they play in the arid ecosystems of the world’s arid lands:
new research suggests that their large dirt mounds are crucial to stopping the spread of deserts into semi-arid ecosystems and agricultural lands. The results not only suggest that termite mounds could make these areas more resilient to climate change than previously thought, but could also inspire a change in how scientists determine the possible effects of climate change on ecosystems.
In the parched grasslands and savannas, or drylands, of Africa, South America and Asia, termite mounds store nutrients and moisture, and — via internal tunnels — allow water to better penetrate the soil. As a result, vegetation flourishes on and near termite mounds in ecosystems that are otherwise highly vulnerable to "desertification," or the environment's collapse into desert.
Princeton University researchers report in the journal Science that termites slow the spread of deserts into drylands by providing a moist refuge for vegetation on and around their mounds. They report that drylands with termite mounds can survive on significantly less rain than those without termite mounds. The research was inspired by fungus-growing termites of the genus Odontotermes, but the theoretical results apply to all types of termites that increase resource availability on and/or around their nests.
This research is fascinating, but it would not be entirely surprising to farmers in the Sahel who are resurrecting – very successfully – the traditional methods of water management for new trees and for new crops. Digging a planting pit through the hardened surface and adding organic matter creates not only a water-conserving environment for plant growth, but attracts termites that process the organic material for use by the plants and aerate the soil through their tunnelling. And those tunnels are extraordinary. The internal structure of a termite mound (a complex ecosystem in its own right) has been dramatically demonstrated by the work of Scott Turner at The State University of New York College of Environmental Science and Forestry. Through taking plaster casts, he reveals the architectural skills of the ecosystem engineers as strange and compelling sculpture:
The tunnelling continues far below the surface (termite mound materials that bring minerals from the subsurface have been used for gold prospecting in Australia) and for many metres beyond the mound. The scale of this landscape management activity is staggering: look carefully at this photo of just a small area of the Australian desert, and you will see hundreds of termite mounds.
No, termites may not be our favourite insects, but our planet would be a different – and, arguably, worse – place without them.
[Image, “Crater termites of the worker and soldier castes attending to damage of their nest, Schanskop, Pretoria”, GNU Free Documentation License, Author JMK]
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).]
"Since October 1998, the American Geosciences Institute has organized this national and international event to help the public gain a better understanding and appreciation for the Earth Sciences and to encourage stewardship of the Earth. This year's Earth Science Week will be held from October 12-18 and will celebrate the theme Earth's Connected Systems."
I have celebrated this annual event in previous posts (although I seem to have overlooked last year) with a specific focus on the admirable Earth Science Literacy Initiative and their "Big Ideas" summary. This captures the fundamentals of our understanding of how our planet works and our relationship with it - the basics of our engagement with geology.
In addition to what we know, what keeps science alive and fascinating are all the things that we don't. This year, for readers who have not come across them already, I would like to draw attention to the superb series of recent posts on GeoLog, the official blog of the European Earth Sciences Union. Titled "The known unknowns - the outstanding 49 questions in Earth sciences", these summarise the basic questions that continue to vex our profession and stir controversy and debate, together with valuable links to appropriate resources. The link is to the third in the series, and I have so far counted 25 questions, so there is clearly more intrigue to come. As the introduction to the series states:
Science is about asking questions, as much as it is about finding answers. Most of the time spent by scientists doing research is used to constrain and clarify what exactly is unknown – what does not yet form part of the consensus among the scientific community. Researchers all over the globe are working tirelessly to answer the unresolved questions about the inner workings of our planet, but inevitably new answers only lead to new questions. What are the main questions that will keep Earth scientists busy for many years to come?
What I would like to think is that these kinds of initiatives will provide accessible and compelling materials that will stimulate young folk to become geologists and not-so-young folk to enquire further.
Meanwhile, a note about my absence for the last few weeks - I have been traveling. The image at the head of this post might provide a clue as to where, as might the photo below. More will, inevitably, be revealed in the near future.
These images are from the November 1968 edition of the now sadly defunct Desert Magazine, a special on Death Valley. The “Riddle of the Racetrack” refers to the enduring mystery of the “sailing stones” of Racetrack Playa, the remote dry lake in the northern part of the valley, and begins:
OFF the beaten path in the northwest corner of Death Valley National Monument lies a hidden valley—and a mystery. The valley contains a dry lake approximately one-and-a-quarter miles wide and three miles long. The Racetrack Playa at first glance appears like any other of hundreds of such dry lakes in the southwest.
It has one different and mystifying feature; rocks and other objects on its surface have been known to shift, move and skate about! No one has actually seen any of these objects move but the tracks left from such movement are obvious.
There are many theories explaining the phenomena. Some say it has to do with the earth's magnetism, while others claim it is related to the sunspots. Still others suspect the gravitational pull of the moon producing an effect similar to the ocean's tides. Under scientific examination, however, most of these theories can be dismissed.
Sunspots and magnetism are only a couple of the conjectures for the motivation of the sailing stones - the supernatural, aliens playing chequers (sorry, ‘alien tractor beams’) and teenage pranksters have also featured. But, while the debate has continued ever since the mystery was first observed a century ago, the preferred explanations are entirely natural. However, that debate is now – largely – settled, thanks to the work of a team led by Scripps Institution of Oceanography/UC San Diego palaeobiologist Richard Norris in collaboration with Ralph Lorenz, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory.
In a video accompanying the description of the work, Norris relates how they conducted what one of his colleagues described as “probably the most boring experiment ever.” Now, this is surely open to discussion. Experiments set up in the 1930s and 40s to record the fall of a drop of pitch (average wait eight years, and winner of the Ig Nobel Prize for physics in 2005) must vie for the honour. But it’s certainly true that the Racetrack Playa project required patience and modern technology. The Desert Magazine article commented that
It appears that the mystery will remain unsolved until some hardy soul camps at the playa's edge all winter, and waits with a movie camera for the action to begin. Any volunteers?
The Scripps team had the advantage of sophisticated weather stations, telemetry, time-lapse cameras, and GPS, so they could get things set up and leave, returning periodically to see what might – or might not – have happened. It took six years.
The winter season was clearly critical: decades of occasional observations recorded rocks being in different places from where they had been before the winter, all apparently having ploughed up the distinctive furrows in the mud of the lake bed, and some being truly huge.
The mysterious winter activity had long stimulated theories of the moving force being ice. This may be the same location of searing summer desert heat, but the playa lies at an elevation of over 1100 metres, and the winter nights are cold: any water that accumulates will freeze and, perhaps, the ice will shove the rocks around. The potential role of ice had been treated seriously over decades of research, including an important study published in the Bulletin of the Geological Society of America in 1955 (I have a PDF if any reader is interested). The author, George M. Stanley, meticulously measured the tracks of multiple stones (including an 11 kg example that had travelled 265 metres), together with the movement of ice. Although he did not directly witness any activity, his conclusions were as follows:
(1) Certain groups of stone trails on Racetrack Playa have identical signatures over areas nearly 500 feet across. They must have been formed by blown ice floes which held numerous small stones or scribers in generally fixed positions, and which rotated during travel.
(2) In several cases a single, moving ice floe released a small stone and picked it up again after a few feet of travel.
(3) Many tracks favor origin by ice floes blown across the playa by wind rather than by wind blowing the lone objects.
(4) Mathematical treatment applied to stone trails within a signature group demonstrated that the scribers moved in accordance with trigonometric relationships between separated loci in a rotating, planar body.
(5) There are ice ramparts and other signs of ice-floe action on Racetrack Playa, aside from the stone trails.
In other words, he saw ice as the motivating force, but most likely picking up the stones, blowing across the playa, and dumping them on melting There are tracks with no stone at the end, and this would seem a reasonable explanation, but Stanley noted that even lightweight objects such as the droppings of wild jackasses also created tracks and that such biodegradable materials might leave no trace. Stanley suggested that the high altitude of Racetrack Playa, and therefore the common occurrence of winter ice, explained why this phenomenon was so much more common there than on other, lower, desert playas. But he did note other, isolated examples, and a further fascinating piece of evidence in support of the power of ice on desert lakes:
On December 3, 1952, the central transcontinental telephone line east of Reno, Nevada, was suddenly disconnected for unknown causes… and a repair crew was sent out to find the difficulty. After following the line eastward from Reno the crew came to Toulon Lake, a part of Carson Sink covered with a year-round body of shallow water…
At the lake, partly ice-covered, the repair crew discovered that 3000 feet of line was missing; this included 20 poles loaded with 4 arms and 40 wires on 150-foot spans. Some poles had been set in caissons of corrugated metal 10 feet in diameter and 4 feet high, filled with boulders to protect the poles against waves on the lake.
Ice had formed for 3000 feet from the west shore and 2 or 3 miles along it, with open water to the east; the ice was 4 inches or more thick and floated on about 2 feet of water. Emergency repairs were made by dragging an insulated cable across the ice with a propeller-driven ice boat. The uprooted poles and remnants of wrecked caissons were found 300 feet south of the original position of the line. After two days the wind reversed direction, and the ice started to move north, tearing out the emergency repairs; it continued to move north under the influence of a 30-mile-an-hour wind a total distance of 700 feet and left the wrecked poles and caissons 400 feet north of the original line. Ice was pushed up several feet on the east shore.
This is really quite entertaining: another instance in which the severing of telephone cables revealed geological phenomena. It was the sequential cutting of the transatlantic submarine cables on the Grand Banks off Newfoundland in 1929 that led to our understanding of the processes and power of turbidity currents…
Investigations at Racetrack Playa continued, and, in the 1970s, Robert Sharp, then a geologist at the California Institute of Technology, and Dwight Carey, a geology student at the University of California, Los Angeles, labelled large numbers of rocks with, for some reason or another, women’s names and tracked their positions over the years. As the National Geographic reported:
Hortense (R) moved 820 feet (250 meters) in one winter. Karen (J), a 700-pound (320 kilograms) rock at the end of a 570-foot-long (174 meters) track, didn't move at all during their seven-year study and disappeared years later. Karen showed up again in 1996, when Paula Messina, a geologist at San José State University who had been mapping the paths of all the sliders on the Racetrack, found her far north of where Sharp had last seen her. "When I told him I had positively identified several of his original rocks, his reaction was a little like one would expect from a man who was just told I found his children."
But still, while plenty of witnesses had reported the before and after positions of many of the stones, nobody had actually caught them in the act – until now. Early this year, the Norris team’s patience was finally rewarded. As Lorenz reports in a New Scientist article this week:
On a Sunday night in January, a park ranger forwarded an email reporting that a tourist had seen the rocks moving. I dropped everything and went to Death Valley. There I met the Norris team, who had also seen movement. We drove up to the playa, which was partly flooded and frozen over, to retrieve our instruments…
We were standing on the cliffs, watching the morning sun melt the thin floating ice sheet, when it happened. A gust of wind, no more than 4 metres per second or so, picked up. And then we heard the crack, and saw the ice sheet slowly glide, bulldozing some rocks along and leaving others.
The action – sedate and sporadic, lasting a total of around 18 seconds – was captured in a series of time-lapse images (blue arrows, stationary rocks for reference, red arrow, the moving rock; darker areas are ice):
The team had developed small GPS devices which they embedded in a selection of rocks at the beginning of the experiment, and so the long-term data could be collected – “The largest observed rock movement involved 60 rocks on December 20, 2013 and some instrumented rocks moved up to 224 m between December 2013 and January 2014 in multiple move events.”
But one of the remarkable things about all this is that the rocks are moved by thin ‘windowpane’ ice, unlike the thick stuff that caused the destructive events on Lake Toulon. Nevertheless, some rocks achieved velocities of ten centimetres per second and travelled 60 metres. The results have been compiled into this helpful graphic in the New Scientist article:
The work is reported in Plos One, is open-access and freely available online – it makes for a fascinating read. Herewith, a captivating image from that paper, together with the description:
View from the ‘source hill’ on the south shore of Racetrack Playa. View is looking north on December 20, 2013 at 3:15 pm. Steady, light wind, 4–5 m/s has blown water to the northeast exposing newly formed rock trails. Lower image shows overlay of lines to emphasize the congruent shape of adjacent rock trails as well as the proximity of rock trails to rocks that did not move. Image has not been enhanced.
[Thanks to Hans Begrich and Walter Vogelsberg for giving me the ‘heads-up’ on this work. It has been widely reported elsewhere (linked in the above), including on other geo-blogs, but I thought that a little of the historical background would be of interest. Wikipedia also has a good overview of the ‘sailing stones’ mystery, and, for fascinating videos etc. of this new project, the Scripps site is well-worth visiting. Image of boulder and track, James Gordon, Attribution-NonCommercial 2.0 Generic (CC BY-NC 2.0), https://www.flickr.com/photos/james_gordon_losangeles/8440177460/in/photostream/]
In his classic and wonderful book Desert Solitaire, the original eco-warrior, Edward Abbey, described the landscapes of Utah’s Arches National Monument:
… here all is exposed and naked, dominated by the monolithic formations of sandstone which stand above the ground and extend for miles, sometimes level, sometimes tilted or warped by pressures from below, carved by erosion and weathering into an intricate maze of glens, grottoes, fissures, passageways, and deep narrow canyons.
At first look it all seems like a geologic chaos, but there is a method at work here, method of a fanatic order and perseverance…
But exactly what the method actually was that sculpted the arches and other bizarre examples of natural land art has long been a topic of debate. Native Americans saw their origins in the work of the Great Sky Father, early settlers thought they were prehistoric native carvings, and conventional wisdom has ascribed these landforms in a general way to the erosional work of wind, water and salt. Fine, but why arches? Clearly, anisotropy within the body of the sandstone must be part of the equation, perhaps related to fractures and differential stress distribution; an interesting paper along these lines was published in the Geological Society of America Bulletin twenty years ago, and the pre-publication version is freely available online.
Just last month, a fascinating piece of work titled “Sandstone landforms shaped by negative feedback between stress and erosion” was published in Nature Geoscience and described clearly in an article from The Smithsonian. Jiri Bruthans and his colleagues in Prague use a series of physical experiments (see the fascinating video on the Smithsonian site), mathematical modelling and ‘ground-truthing’ to demonstrate that the gravitational load on a pile of sandstone can cause a differential stress distribution within the rock and the consequent ‘locking’ of sand particles along trajectories that, as a result, are stronger and more resistant to weathering and erosion. Furthermore, once the looser grains are removed the remaining locked volumes become even stronger, hence the feedback component. By introducing imperfections in the sandstone (small cuts, fractures and so on) they can reproduce an impressive variety of exotic landforms – including arches:
(illustration from the Nature Geoscience paper)
This is intriguing in itself, but what I couldn’t help wondering is that surely there must be a connection between this work and the strange world of granular physics. This is an inexhaustible topic that first fascinated me as I was researching the Sand book and has had me in its grip ever since. Explaining the behaviours of the ‘simple’ material, sand – dry, wet, damp – poses challenges to cutting-edge physics and engineering research worldwide. Among these strange behaviours is jamming, the sudden locking up of otherwise free-flowing grains. The (apparently) simplest example is sand, or any other granular material, flowing through a funnel: periodically, unpredictably, and frustratingly the grains will lock and the flow stops. A critical part of the design of a sandglass is the size and shape of the aperture in relation to the sizes and shapes of the sand grains – continuous flow without jamming has to be guaranteed. And then, among the myriad challenges presented by the requirements of handling industrial granular materials, is the flow of grain from a silo through a hopper – something that turns out not to be at all simple.
Silos of grain occasionally, and without warning, explode, despite design specifications that theoretically far exceed the stresses of the contained material (a tragedy when, as has happened in Scotland, the grain should have been used in the production of single malt whisky). Research to explain such events has demonstrated that the distribution of stress within a pile of grains is far from uniform, that particularly high stress is carried along very specific trajectories – ‘force chains’ – within the pile and those forces can spontaneously structure themselves to act against the walls of the container: the silo collapses.
The ever-shifting and re-forming force chains that are typical of granular flow through a hopper have been cleverly modelled. The illustration at left is by Dennis C Rapaport of the Department of Physics at Bar-Ilan University in Tel Aviv and the animation can be seen on his website. And here from a YouTube video by George Lesica:
In both, the shape-shifting force chains can be clearly seen, and it is these that, if they happen to organise themselves properly, contribute to the material jamming. Junyao Tang and Bob Behringer of the physics department at Duke University have made a compelling video of this happening:
So, differential stresses, force chains and jamming in granular materials seem to me to be intimately related to the work published in Nature Geoscience, and the sculpting of bizarre landforms. For these phenomena are not restricted to flowing granular materials, but occur in sand piles – I find this illustration from the work of Radoslaw Michalowski at the University of Michigan intriguing:
While the term arching has been accepted in the geotechnical literature, the concept does not relate to a formation of a physical arch (as seen in karst formations), but rather a re-distribution of stress (or variation in the stress field), where stiffer components of the system attract more loads. The description is still elusive, and research toward prediction of arching is carried out. Arching in a model of a sand heap is illustrated in the figure, producing a stress ‘dip’ at the center of the base… While the appearance of the stress dip may be a curiosity problem, arching associated with it is a phenomenon of interest and importance in geotechnical engineering.
Ring any bells, this fanatic order?
[Image at the head of this post "Double arch" by Hustvedt - Own work. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons. For an excellent introductory video of some of the wonders and challenges of granular materials I suggest this from the National Science Foundation.]