Revision 3/20/01

Desert Pavement: an Environmental Canary?

________________________________

P. K. Haff Division of Earth and Ocean Sciences

Nicholas School of the Environment and Earth Sciences Duke University

Durham, NC 27708

2

Abstract

Ongoing disruption of ancient, varnished desert pavement surfaces near Death Valley

National Park is inferred to be the result of unusually intense animal foraging activity.

Increased levels of bioturbation are associated with enhanced vegetation growth

stimulated by recent El Nino precipitation. The occurrence of abundant, recently

overturned, varnished clasts suggests that the pavement disturbances reported here are

rare on the millennial time scale of desert varnish formation. These observations suggest

the possibility that changes in desert pavement surfaces may provide early hints of future

changes in desert ecology and environment.

3

Introduction

Desert pavements (Engel and Sharp, 1958; Cooke et al, 1993, McFadden et al,

1998) are smooth, old (~104 y to >105y), stony surfaces that commonly occur on

abandoned alluvial units in arid terrain, Fig. 1. On desert pavement, a monolayer of

pebbles, often platy and coated with desert varnish, overlies a stone-poor to stone-free

matrix (the Av layer) of silt, clay and fine sand, derived principally from wind-blown dust

Wells, et al, 1985; McFadden, et al, 1987). Colluviation, clast displacement, clast

fracture, matrix creep or flow, and the processes associated with growth of the fine-

grained matrix lead to smoothing of the original depositional surface. Mature pavements

(late Pleistocene) show essentially no sign of original constructional features such as bar

and swale topography or debris flow lobes and levees. Younger pavements retain

remnants of original topography, while on the most immature, Holocene, pavements

many clasts are still in original depositional positions (Bull, 1991).

Desert pavement surfaces are typically mechanically weak. Most surface clasts on

well-developed pavements lie in edge-to-edge contact with their neighbors, somewhat

like the mosaic on a tiled floor. Clasts are seated in the underlying fine-grained matrix,

but they are not strongly cemented to each other or to the matrix. Pavement stones are

often easily dislodged by a footstep. Long-term pavement stability is a function of

isolation from disruptive forces, not of strength of the pavement itself. This type of

stability may be termed “environmental stability” to distinguish it from a stability gained

from inherent mechanical resistance to physical disruption such as characterizes

duricrusts. The importance of recognizing environmentally stable systems lies in their

potential role as detectors of environmental change, since the longevity of their present

4

state is due to relative stability of the local environment. I report here a non-

anthropogenic destabilization of some of these surfaces that may be related to changing

desert environmental conditions. The example and study site are local, but the purpose of

the discussion is general – to call attention to desert pavement surfaces as potential

environmental “canaries”, i.e., sensitive indicators of environmental change.

Pavement disturbances. The study area in which the reported pavement

disturbances were observed is located east of Death Valley National Park, in Greenwater

Valley, California. Here smooth and varnished expanses of desert pavement with closely

packed surface clasts, Fig. 1, flank the main valley wash. Clast lithology is variable, but

in the study area clasts are principally of volcanic origin. A set of fluvial terraces of

increasing elevation and age border the modern wash, separated from the wash and from

one another by risers a few meters in height. Desert pavement covers the surface of these

terraces, with the most darkly varnished pavement tending to occur on the highest terrace.

On each set of these terraces widespread clast displacement has occurred recently,

disaggregating the otherwise intact, two-dimensional packing of surface stones, Fig. 2.

The disturbed areas, indicated by the arrows in Fig. 1, are somewhat darker than the

ambient undisturbed pavement. The apparent darker coloration is due to the increased

surface roughness resulting from disturbance. Transitions between disturbed and

undisturbed patches are abrupt, as shown in Fig. 2 and in the lower inset of Fig. 1. Most

stones smaller than about 4 cm in the disturbed areas appear to have been displaced. In

the disturbed areas up to 50% of all clasts several centimeters in diameter and larger have

also been detectably overturned. Detection of overturns depends on differences in

coloration of the desert varnish coatings (Engel and Sharp, 1958; Cooke et al, 1993;

5

Oberlander, 1994) that form on exposed versus buried portions of many desert pavement

clasts. Manganese-rich subaerial varnish is brownish, Fig. 3, while the buried surface of

the same clast is often stained orange by iron oxides, Fig. 4. Geomorphic and

archaeological evidence suggests that varnish coatings on desert pavement clasts take on

the order of a thousand years to form (Hunt and Mabey, 1966; Dorn, 1988; Bull, 1991;

McFadden, et al., 1989; Oberlander, 1994). It has been argued as well that varnish

formation has been minimal or absent during much of the late Holocene (Hunt and

Mabey, 1966; Elvidge and Iverson, 1983). Studies by McFadden et al. (1989) indicate

that some varnishing and subsurface reddening occurs on Mojave desert piedmonts where

surface clasts are of middle or early Holocene age; clasts younger than about 2000 y

show only minimal varnishing, and no subsurface reddening. Orientational stability of

clasts in the intact pavement is also indicated by the apparent preferential etching or

dissolution of vesicular silica on buried orange surfaces of some rhyolitic clasts (see also

McFadden et al., 1998). Thus, clasts with brown varnish on their upper surfaces and

buried orange undersides (called “bipolar” clasts here) may have been seated in their

present orientation for a time span measured in millennia. If disturbances of the sort

described above had occurred frequently during that time interval, then at a given

location a large fraction of bipolar surface clasts would be overturned (unless

regeneration of Mn-rich brown varnish is exceptionally quick on newly exposed Fe-rich

clast surfaces). In general, except in recently disturbed areas, bipolar clasts on intact

pavement tend to lie predominantly “right-side up”, with brown surfaces exposed. Thus

the disturbances described, and the inferred animal responses to El Nino-enhanced

6

vegetation growth discussed below, are thought to be rare over a time on the order of, or

greater than, the varnishing time scale.

Causes of disturbance. On many pavements, isolated overturns of unknown

origin can be found by casual inspection. Many of these overturns are now re-integrated

into the intact pavement. Such overturns, occasionally a decimeter or more in diameter,

might result from the action of animals. Experience shows that human passage across a

pavement can also result in inadvertent overturns of decimeter-sized clasts. These point-

disturbances, whose origin remains speculative, are distinct from the areally extensive

disturbances described here. Seismic shaking can cause widespread unseating of

pavement clasts (Haff, Hector Mine earthquake, California, unpublished study), but no

significant seismic activity occurred near the study site during the time of inferred surface

disruption.

Observations at the study site between 1994 and 1999 showed widespread areas

of dislodgment and overturning of previously seated pavement stones up to 8 cm in

diameter. Stone displacements and overturnings resulted in destruction of the otherwise

characteristic mosaic pattern of the local pavement surface. Disturbed areas ranged from

a few square decimeters up to patches as large as 0.25 ha. On one set of sampled plots

about 40% of bipolar stones on recently disturbed surfaces were in an overturned state,

compared to about 10% overturns on nearby surfaces, Fig. 5. Disturbances were judged

to be recent (months to a few years old) based on the presence of precariously perched

clasts that could be displaced with the slight tap of a pen. Mature pavement surfaces are

usually characterized by a smooth layer of adjacent, flat-lying clasts, whereas the recently

disturbed areas contain many up-ended and double-stacked or overlapping stones. Over

7

time, a disturbed pavement surface “heals” as small inputs of energy from rainsplash and

other processes gently jostle clasts back into their flat-lying preferred state of minimum

energy. In experiments in Panamint Valley, California (Haff and Werner, 1996), small

patches of pavement 5 cm across were cleared by pushing pebbles to one side. After

about 14 years, some of these cleared areas have become re-surfaced, and are

indistinguishable to the eye in smoothness and surface coverage from adjacent

undisturbed pavement. These observations are consistent with the conclusion that the

Greenwater pavement disturbances are at most a few years old.

Distinction from anthropogenic disturbances. The disturbances reported here

are distinct from pavement disruptions characteristic of modern human activity. There are

many examples of human disturbance of once pristine pavements in the US Southwest,

variously due to military and recreational activities and urbanization of the desert floor.

World War II desert training maneuvers of Gen. George Patton impacted large areas of

pavement in the California and Arizona deserts (Prose, 1985). Off-road recreational

vehicle use at the classic pavement study area of Engel and Sharp (1958), near Stoddard

Wells, California, has resulted in complete destruction of the pavement fabric (P. K. Haff,

unpublished). Vehicle tracks, fracture and abrasion of clasts, overturning of large cobbles

and small boulders, indentation and compression of sub-pavement soils, bulldozer blade

scrapes, campfire rings, prospect pits, shell impact craters, and exposure of expanses of

soil matrix denuded of its original pebble cover are all signs of modern anthropogenic

pavement disturbance. The pavement disturbances discussed here lack all of these signs

of human causation, and are clearly not due to direct human impact

8

Pavement vegetation. Relatively dense coverings of Oligomeris, Plantago, and

other annuals (Hickman, 1993) were observed on some pavement surfaces in response to

heavier than average precipitation associated with the El Nino events of 1991-94 and

1997-98. Disturbed areas were spatially correlated with the presence of annual

vegetation. In April 1998, normally barren pavements were in places covered with a

carpet of small annuals (few cm to a decimeter in height), including grasses and

wildflowers, Fig. 6a. Living annual vegetation was nearly absent from the same surfaces

in April 1999, Fig. 6b. Disturbed pavements in the study area supported up to 160

annuals, or clumps of annuals, per square meter. Plant densities on undisturbed pavement

adjacent to disturbed areas ranged from near zero up to about 100 individual annuals per

square meter, with little clumping. The clumping resulted in significantly greater biomass

on the disturbed sites. Vegetation patches were frequently localized, with sharp

boundaries separating areas of relatively dense vegetation from vegetation-free pavement.

The edges of many patches of disturbed pavement were coterminous with the edges of

patches of annual vegetation. In some cases the pavement on the vegetated side of these

boundaries had a slightly (but detectably) greater mean clast diameter than the pavement

on the unvegetated side of the boundary, perhaps reflecting the edge of an alluvial

constructional feature whose original topographic expression has been erased over time.

Bioturbation of pavement surfaces. The growing blades and branches of most

annuals are too weak to effect major displacement of any but the smallest clasts. Clast

displacement is inferred to be a secondary effect associated with foraging by rodents

and/or birds. Kangaroo rats (Dipodomys), hares (Lepus) and ravens (Corvus) are

commonly seen in areas of desert pavement in the study area. Under normal conditions

9

the (light) traffic of animals across the pavement is responsible for occasional

displacement of small pebbles (Haff and Werner, 1996). With enhanced vegetative

growth, normally transient animal species may be attracted to the vegetation as a food

source, and animal populations themselves may burgeon. A rodent or bird would have no

trouble overturning clasts several centimeters in diameter as it searched for seeds or roots.

On one occasion in April 1999, flocks composed of more than 100 birds, probably horned

larks (Eremophila) or pipits (Anthus), were observed foraging vigorously on desert

pavement among dead Oligomeris. The pavement surface was subsequently found to be

significantly disturbed. At the study site Oligomeris often tends to grow in a series of

straight-line segments that form a polygonal network on the pavement. The polygons,

with linear dimensions on the order of a decimeter, correspond to hairline cracks in the

stone-free silty matrix that underlies the pavement stones. Water percolates preferentially

down these fractures (P. K. Haff, unpublished observations), providing a preferential site

for plant establishment. The smooth undisturbed pavement surface does not reflect the

underlying soil cracks. At the site visited by the flocks of birds, clast displacement was

localized along the edges of the vegetation polygons, with the centers of the polygons

often remaining undisturbed. There seems to be little doubt that the vegetation attracts

animals that disturb the pavement.

Not all pavements were subject to enhanced vegetation growth or suffered

disturbance during the time period of the present study. In nearby Death Valley and

Panamint Valley, California, many pavements remained relatively free of annual

vegetation. Disturbances of the type found in Greenwater Valley were not observed in

these localities. One reason may be spatial variability of salt content of pavement

10

subsoils: high salt concentrations have been correlated with lack of vegetation on some

desert pavement surfaces (Musick, 1975). Also, many vegetated pavement surfaces in

Greenwater Valley were not subject to bioturbation, the surfaces remaining intact beneath

the vegetative cover. However, some areas as large as 0.25 ha that were vegetated but

undisturbed in spring 1998 had been disrupted by the spring of 1999. Such pavement

disturbances had not been observed by the author during earlier work in the present study

locality in the mid-1980’s.

Possible role of El Nino. A significant response of vegetation to climatic

fluctuations such as the El Nino events of the 1990’s is to be expected. Bursts of annual

vegetation frequently follow years of significantly higher than average winter

precipitation in the study region (Went and Westergaard, 1949; Beatley, 1974). However,

clast disturbances of the kind described above, although apparently associated with recent

El Ninos, have not historically been caused by enhanced precipitation – otherwise

randomly oriented bipolar clasts would be the rule rather than the exception. (El Nino

events typically recur every 4-5 years with “super” El Nino events like 1997-1998

occurring every 30-40 years (Lau, 1985).) The disturbances reported here might reflect a

synergistic interaction between episodes of higher than average precipitation, the effects

of increasing atmospheric CO2 (Smith et al., 2000), or other (unknown) variables related

to shifts in the overall environment (e.g., to climate change), or they might be related

directly to changes in El Nino patterns themselves. Thus the 1990-1995 El Nino-Southern

Oscillation event is the longest on record (Trenberth and Hoar, 1996). An example of a

new and unexpected side-effect of recent (1992-1993) El Nino precipitation was the

widely publicized outbreak of the often fatal hantavirus pulmonary syndrome (HPS) in

11

New Mexico and elsewhere (Engelthaler, et al., 1999). HPS was traced to an explosion in

the deer mouse (Peromyscus) population, itself a function of enhanced growth of

vegetation. While there is of course no direct connection between disease and desert

pavement disturbances, the HPS phenomenon is offered as an example of how an

unexpected and indirect effect associated with large variations in an apparently unrelated

variable - rainfall - has recently appeared in an ecosystem in the US Southwest.

Conclusions . Recent computer simulations (Neilson and Drapes, 1998; Brown,

1998) of potential vegetative changes in the US Southwest resulting from increased

atmospheric CO2 levels suggest that the present-day desert climate may become more

humid, with increasing abundance of grasses. The stability of modern desert pavements

depends to a large extent on suppression of vegetation by low water availability and high

summer temperatures. The data presented above suggests that the observed pavement

disturbance may be a new phenomenon associated with changes in temporal and spatial

vegetation patterns and animal response to those patterns. It seems worthwhile to

consider, then, whether desert pavement may represent a kind of environmental canary,

with observed pavement disruption being an initial signal of change in the millennial

stability of desert pavement surfaces. Recognition of further changes in the integrity of

these surfaces may provide useful insights into (and warnings of) overall changes in the

desert environment.

12

Acknowledgements

I would like to thank Fred Landau and Dave Charlet for their assistance with

identification of plant species, Stan Smith for providing information on Mojave Desert

ecosystems, Tonya Haff for her help with bird identification, and Bill Meurer for

petrographic analysis. This work was supported in part by the US National Science

Foundation Grant No. EAR-9814276 and the US Army Research Office Grants Nos.

DAAH04-94-G-0067 and DAAD19-99-1-0191.

13

Figure Captions

Fig. 1. Desert pavement surface, located in Greenwater Valley (asterisk on inset map),

California. The stones are mostly of volcanic origin. The darker pavement areas (arrows)

represents patches of extensive overturning and jostling of pavement stones. The lower

inset (not to scale of the photo) shows the occurrence of disturbed (“d”) and undisturbed

(“u”) patches along a 19 m pavement transect.

Fig. 2. Close-up view of sharp boundary between disturbed pavement (to left of lens cap

(4.5cm)) and intact pavement (to right). The arrow points to an example of an overturned

clast with original lighter underside now oriented upwards.

Fig. 3. View of exposed, darker (brownish) surface of varnished pavement clast

(rhyolite); scale is set to 4 cm.

Fig. 4. View of previously buried surface of clast shown in Fig. 3. Below the soil

surface, the presence of Fe oxides and the absence of Mn oxides lend an lighter (orange)

appearance to the clast. Although varnish age is difficult to establish quantitatively,

correlation of the degree of subaerial varnish darkening with alluvial terrace sequences,

with occurrence of clast fracture, with smoothing and eradication of primary depositional

features, and with the occurrence of native American artifacts, suggests that millennia are

14

required for significant varnish accumulation on desert pavement clasts (Hunt and

Mabey, 1966; Bull, 1991; McFadden et al., 1989; Oberlander, 1994).

Fig. 5. Size distribution of upright bipolar clasts (“upright-dist”) compared to that of

overturned bipolar clasts (“overturned-dist”) measured on one disturbed surface; also

shown for adjacent undisturbed surface is size distribution of upright bipolar clasts

(“upright-undist”) compared to that of overturned bipolar clasts (“overturned-undist”).

Fig. 6. (a) Desert pavement in Greenwater Valley, California, showing abundant growth

of vegetation in response to El Nino rains of 1997-8; photographed in April, 1998. (b)

Same scene as (a); photographed in April, 1999, showing normal barrenness of desert

pavement. Nonetheless, foraging on and disturbance of desert pavement continued in

1999 on these recently vegetated surfaces.

15

References Cited

Beatley, J. C., 1974, Phenological events and their environmental triggers in Mojave

Desert ecosystems, Ecology, v. 55, p. 856-863.

Bull, W. B., 1991, Geomorphic Responses to Climate Change, Oxford Univ. Press, New

York, 326 pp.

Brown, K. S., 1998, Green Thumb for the Southwest, (News Focus) Science, v. 281, p.

1275.

Cooke, R., Warren, A., and Goudie, A., 1993, Desert Geomorphology, UCL Press,

London, 526 pp.

Dorn, R. I., 1988, A rock varnish interpretation of alluvial-fan development in Death

Valley, California, National Geographic Research, v. 4, 56-73.

Engel, C. G. and Sharp, R. P., 1958, Chemical data on desert varnish, Bulletin of the

Geological Society of America, v. 142, p 487-518.

D. M. Engelthaler, D. G. Mosley, J. E. Cheek, C. E. Levy, K. K. Komatsu, P. Ettestad, T.

Davis, D. T. Tanda, L. Miller, J. W. Frampton, R. Porter, and R. T. Bryan, 1999, Climatic

and environmental patterns associated with hantavirus pulmonary syndrome, Four

16

Corners Region, United States, Emerging Infectious Diseases [serial online] v. 5, at

http://www.cdc.gov/ncidod/EID/vol5no1/engelthaler.htm.

Haff, P. K. and Werner, B. T., 1996, Dynamical processes on desert pavements and the

healing of surficial disturbances, Quaternary Research, v. 45, p 38-46.

Hickman, J. C., ed., 1993, The Jepson Manual, Higher Plants of California, Univ.

California Press, Berkeley, 1400 pp.

Hunt, C. B. and Mabey, D. R., 1966, General geology of Death Valley, California, USGS

Professional Paper No. 494-A .

Lau, K-M., 1985, Elements of a stochastic-dynamical theory of the long-term variability

of the El Nino/Southern Oscillation, Journal of the Atmospheric Sciences, v. 42, p 1552-

1558.

McFadden, L. D., Wells, S. G., and Jercinovich, M. J., 1987, Influences of eolian and

pedogenic processes on the origin and evolution of desert pavements, Geology, v. 15, p

504-509.

McFadden, L. D., Ritter, J. B. and Wells, S. G., 1989, Use of multiparameter relative-age

methods for age estimation and correlation of alluvial fan surfaces on a desert piedmont,

Eastern Mojave Desert, California, Quaternary Research, v. 32, p 276-290.

17

McFadden, L. D., McDonald, E. V., Wells, S. G., Anderson, K., Quade, J., and Forman,

S. L., 1998, The vesicular layer and carbonate collars of desert soils and pavements, age

and relation to climate change, Geomorphology, v. 24, p. 101-145.

Musick, H. B., 1975, Barrenness of desert pavement in Yuma County, Arizona, Journal

of the Arizona Academy of Sciences, v. 10, p 24-28.

Oberlander, T. M., 1994, Rock varnish in deserts, p 106-119 in Geomorphology of Desert

Environments, eds., Abrahams, A. D. and Parsons, A. J., Chapman and Hall, London, 674

pp.

Neilson, R. P. and Drapes, R. J., 1998, Potentially complex biosphere responses to

transient global warming, Global Change Biology, v. 4, p 505-521.

Prose, D. V., 1985, Persisting effects of armored military maneuvers on some soils of the

Mojave Desert, Environmental Geology and Water Sciences, v. 7, p 163-170.

Smith, S. D., Huxman, T. E., Zitzer, S. F., Charlet, T. N., Housman, D. C., Coleman, J.

S., Fenstermaker, L. K., Seemann, J. R., and Nowak, R. S., 2000, Elevated CO2 increases

productivity and invasive species success in an arid ecosystem, Nature, v. 408, p. 79-82.

18

Trenberth, K. E. and Hoar, T. J., 1996, The 1990-1995 El Nino – Southern Oscillation

event: longest on record, Geophysical Research Letters, v. 23, p 57-60.

Wells, S.G., Dohrenwend, J. C., McFadden, L. D., Turrin, B. D. and Mahrer, K. D., 1985,

Late Cenozoic landscape evolution on lava surfaces of the Cima volcanic field, Mojave

Desert, California, Geological Society of American Bulletin, v. 96, p 1518-1529.

Went, F. W. and Westergaard, M., 1949, Ecology of desert plants III. Development of

plants in the Death Valley National Monument, California, Ecology, v. 30, p 26-38.

19

Figure 1

d u

1 m

20

Figure 2

overturn

21

Figure 3

22

Figure 4

23

Size Distribution of Upright and Overturned Bipolar Clastson Disturbed and Undisturbed Pavement

0

5

10

15

20

25

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

stone size, mm

freq

uen

cy

upright-dist

overturned-dist

upright-undist

overturned-undist

Figure 5

24

Figure 6

a b