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Modern Disturbance of Ancient De


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									                                                       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

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.


        Desert pavements (Engel and Sharp, 1958; Cooke et al, 1993, McFadden et al, 1998) are

smooth, old (~104 y to >105 y), 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 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; 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 d isturbed 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 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 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

       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 pave ment 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 the (light) tra ffic 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 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 CO 2 (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 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 CO 2

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 phenomeno n

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


                                      Acknowledge ments

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.

                                          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 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.

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       d   1m

Figure 1


Figure 2

Figure 3

Figure 4

                     Size Distribution of Upright and Overturned Bipolar Clasts
                              on Disturbed and Undisturbed Pavement

            25                                                 upright-dist

            20                                                 upright-undist




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

                                                    Figure 5

a        b

    Figure 6


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