Desert Pavement: an Environmental Canary?

Revision 3/20/01

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P. K. Haff Division of Earth and Ocean Sciences Nicholas School of the Environment and Earth Sciences Duke University Durham, NC 27708

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.

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

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 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

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 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

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

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

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.

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.

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 1 m u

Figure 1

overturn

Figure 2

Figure 3

Figure 4

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

25 upright-dist overturned-dist 20 upright-undist overturned-undist

10 frequency

0 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