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