Response of eolian geomorphic systems to minor …online.sfsu.edu/jerry/geog810/2005/Rose/lancaster_1997.pdf · Response of eolian geomorphic systems to minor climate change: ex.amples

EISEVIER Geomorphology 19 (1997) 333-347

Response of eolian geomorphic systems to minor climate change: ex.amples from the southern Californian deserts

Nicholas Lancaster *

Desert Research Institute, UCCSN, P.O. Box 60220, Reno, NV, USA

Revised 6 May 1996; accepted 28 August 1996

Abstract

Eolian processes alnd landfonns are sensitive to changes in atmospheric parameters and surface conditions that affect sediment supply and mobility. The response of eolian geomorphic systems to minor climate change can be examined through process-response models based on a combination of relations between short-term changes in climatic variables and eolian activity and the geologic and geomorphic record of Holocene eolian activity.

At both time scales, eolian activity in southern Californian deserts is strongly controlled by variations in precipitation. Wind energy is not a limiting factor in this region. Formation of eolian deposits is a product of climatic changes that increase sediment supply from fluvial and lacustrine sources and may, therefore, be closely tied to periods of channel cutting and geomorphic instability. During intervening periods, eolian deposits migrate away from sediment source areas and are reworked, modified, and degraded. Remobilization of existing dormant dunes is a product of reduced vegetation cover and soil moisture in periods of drier climates. The major control on these processes is decadal to annual changes in rainfall that determine vegetation (cover and soil moisture content.

Keywords: wind transport; eolian features; California; climate

1. Introduction cesses by which these two types of eoliau sediment

Eolian processes involve the mobilization, transport, and deposition of material by the wind. They operate within geomorphic systems that form an interrelated set of processes and landforms in which sediment is transported by the wind from source areas to depositional sinks via transport pathways. Two major components of eolian sediment transport can be identified: (I) material of sand size (> 0.050 pm); and (2) silt- and clay-sized particles (< 0.050 km) or dust. Major differences occur in the pro-

are transported and deposited, although entrainment of all particle sizes is governed by very similar variables. Sources and sinks for sediment are linked by a cascade of energy and materials which can be viewed in terms of sediment inputs and outputs, transfers and storages. Study of eolian processes within a systems framework permits the assessment of geomorphic responses to climate change on differing spatial and temporal scales, as well as focusing attention on the internal and external links between system components.

* Fax: + 1 702 674 71557. E-mail: [email protected]

Because eolian processes and landforms are the result of interactions between the wind and the land

0169-555X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PZI SO169-555X(97)00018-4

334 N. Lancaster/Geomorphology 19 (1997) 333-347

I I I I

Fig. 1. Schematic view of the factors that may influence the response of eolian geomorphic systems to climatic change.

surface, they are sensitive to changes in atmospheric parameters and surface conditions that affect sediment supply and mobility (Fig. 1). Thus, wind action is influenced by changes in wind strength and direction that may be the direct result of global or regional climatic changes as well as by changes in vegetation cover and sediment availability that are an indirect effect of climatic change.

In this paper, minor climatic change is defined as short-term (decades to centuries) change that is on a scale from the Neoglacial and Little Ice Age periods at the most extreme to the limited duration of El Niiio-Southern Oscillation periods, the Sahel drought of the 197Os, and the Dust Bowl of the 1930s at the other extreme. In this context, the changes involved are minor compared to those associated with glacial-interglacial cycles, yet are very significant in regional terms.

Compared to the other fields of geomorphology covered in this volume, eolian geomorphic systems have been rarely studied in the detail that they deserve, and many of the fundamental dynamics on time scales of years to centuries are poorly known. Consequently, this paper begins with a general dis-

cussion of the approaches to the long-term dynamics of eolian systems that seem promising, followed by two case studies from the deserts of southern Cali- fornia that illustrate different approaches.

2. Approaches

There are two main approaches to understanding the response of eolian geomorphic systems to minor climate change. First, the response to short-term climatic variations (e.g., El Niilo) can be examined by developing process-response models based on correlations between climatic variables (e.g., rainfall) and eolian activity. Second, the geologic and geomorphic record of Holocene eolian activity can be correlated with other forms of proxy paleoclimate data to provide a model for responses on time scales of millennia to centuries.

2.1. Relations between minor climatic changes and sand and dust mobilization

Rates of sand and dust emissions are controlled in part by short-term and seasonal changes in wind

o! . 0 10 20 30 40 50 60 70 80 90

Number of dust storm days per year

180

160

20 3 40 50 60 70 80 90

Number of dust storm days per yew

Fig. 2. Relations between dust storm frequency and climatic variables in Mauritania (after Middleton, 1989).

N. L.uncaster/Geomorphology 19 (1997) 333-347 335

strength and variables such as surface roughness, moisture content, and vegetation cover that affect the threshold velocity for sediment mobilization (e.g., Nickling, 1989). In time, feedback mechanisms between the surface and the wind result in the removal of erodible particles and the development of a surface lag or armor of non-erodible particles. These processes tend to make many surfaces self-stabiliz- ing, except where repeated disturbance or re-supply of sediment occurs (Nickling and Gillies, 1989). Periods of geomorphic instability resulting from mi- nor climatic changes may, therefore, affect eolian processes through increased sediment availability as well as changes in vegetation cover and soil moisture.

On a time scale of seasons to years, considerable debate has been generated on the relations between dust emissions and antecedent precipitation. Al-

though intuitively expected, cause and effect relations have proved difficult to establish quantitatively. A complex relation apparently exists between dust emissions and controlling factors such as vegetation cover, surface crusting, and disturbance. In Maurita- nia and Niger, the annual frequency of dust storms is weakly correlated with rainfall from the previous year (r2 = 0.28), but is more strongly correlated (r2 = 0.56) with the average annual rainfall in the previous three years (Fig. 2) (Middleton, 1989). Sim- ilar observations were made by Chepil and Woodruff (1963) for the Great Plains. In the southwestern U.S.A., Braze1 (1989) found poor correlations between annual and seasonal precipitation, the Palmer Drought Severity Index, and frequency of dust events at sites in Arizona and southeastern California (Fig. 3). The clearest signal to emerge was a relation between winter moisture and subsequent annual dust

1 I W Arizona

0 Calibmh

A Nevada

Mean annual precipitation (mm)

0 50 100 150 200 250 300 350 400 500

Mean annual precipitation (mm)

Fig. 3. Relations between dust storm frequency and climatic variables in the American Southwest (after Brazel, 1989).


frequencies (Braze1 and Nickling, 1986). Likewise, MacKinnon et al. (1990) found a statistically significant correlation ( r = - 0.60) between winter precipitation and dust emissions in the following year at Yuma, AZ. They noted a strong reduction in dust emissions in the period 1983-1984 following heavy rainfall resulting from the El Nice event of fall and winter 1982. Relations between the frequency of specific types of dust-generating weather and dust emissions have also proved difficult to establish, partly because of the variability in wind speeds between individual events of the same synoptic type (Braze1 and Nickling, 1986).

2.2. Controls of dune mobility

Sand dune activity (expressed as changes in migration rates and/or variations in the amount of sand movement on the dune itself (Thomas, 1992) is directly proportional to the sand-moving power of the wind, but inversely proportional to the vegetation cover (Ash and Wasson, 1983) and soil moisture content. In turn, vegetation cover and soil moisture are a function of the ratio between annual rainfall (P> and evapotranspiration (El. An index of the sand-moving power of the wind is the percentage of the time the wind is blowing above the threshold velocity for sand transport ( _ 5 m/s) (W ). The ratio between these two terms gives a mobility index: (Lancaster, 1988):

W M=-.--

P/PE

In this index, actual evapotranspiration is replaced by potential evapotranspiration because this term can be estimated from temperature data. Critical values for M, based on field observations of dunes in southern Africa, indicate that dunes are fully active when M exceeds 200; lower slopes and interdune areas are stabilized by vegetation when M lies between 100 and 200; only dune crests are active with M between 100 and 50; and dunes are completely stabilized when M is less than 50. Good relations between modem conditions of dune activity and these climatic parameters have been established in the Sahel region of Africa (Talbot, 19841, in Australia (Ash and Wasson, 19831, the Kalahari (Lancaster,

19881, and the Great Plains of the United States (Muhs and Maat, 1993).

The effects of changes in the intensity and/or frequency of eolian transport events on dune mobility are not well known. In Australia (Bowler and Wasson, 1984) and the Kalahari (Lancaster, 1988) studies indicate that many vegetation-stabilized dunes would be mobile if wind speeds were increased by 30%. In dune fields in the Sonoran Desert, and possibly many other subtropical deserts, dune mobility is apparently reduced by present-day low wind speeds.

Short-term changes in dune mobility have been noted from the Gran Desierto, Sonora, Mexico and the Algodones Dunes. A significant increase in the area of bare sand and the area of small (2-3 m high) crescentic dunes occurred in the northern Gran De- sierto between 1988 and 1990, following a period of low rainfall in this area. Smith (1980) described year-to-year changes in the size of barchan dunes at the Algodones dune field that can be related to variations in the frequency of sand-moving winds as well as the growth of annual or ephemeral plants in interdune areas following rains. A combination of low wind velocities and growth of vegetation in 1978-1979 caused dunes to shrink as sand was trapped in interdune areas.

Observations by early explorers may also provide important information on the past extent of mobile dunes. For example, such accounts indicate that parts of currently vegetation-stabilized dune fields from Nebraska to Texas were active during the 19th century during periods of drought accompanied by higher temperatures (Muhs and Holliday, 1995).

2.3. The Holocene record of eolian activity

Evidence indicating that eolian activity has been both more extensive and/or intense than it is at present is widely distributed in currently arid and semi-arid areas (Lancaster, 1990). Types of evidence include dormant and relict dunes and sand sheets that are stabilized by vegetation, soil development, collu- vial mantles, and deflation lags (Muhs, 1985; Holli- day, 1989; Tchakerian, 1991; Thomas and Shaw, 1991); ventifacts covered by desert varnish (Laity, 1992), and dust mantles in desert margin areas (Tsoar

N. Lancaster/Geomorphology 19 (1997) 333-347 331

and Pye, 1987). Many of these landforms and deposits are the products of major climatic changes in the Late Pleistocene and early Holocene.

Although Thomas and Shaw (1991) have sug- gested that many partially vegetated dunes may not be relicts of past climates, an increasing body of evidence demonstm1es the dynamic nature of many dune systems which display multiple periods of stabilization and reactivation in the late Holocene. For example, the cores of large complex linear dunes in Mauritania consist of sand deposited during the period 20,000-13,000 yr B.P. and stabilized by vegetation and soil formation during a period of increased rainfall 1 l,OOO-4500 yr B.P., further periods of dune formation after 4000 yr B.P. cannibalized existing eolian deposits on the upwind margin of the sand sea and the currently active ‘cap’ of crescentic dunes superimposed on the linear dunes dates to last 30 years (Kocurek et al., 1991). Similar sequences of dune deposits have been recognized in the Sahel (Talbot, 1985) and1 southern Sahara (Viikel and Grunert, 1990). Significant Holocene dune activity has also been recognized in Australia (Nanson et al., 1992).

In North Americ,s, currently vegetation-stabilized dunes on the Great Plains were active after 9000 yr B.P. with maximum eolian erosion and deposition 6000-4500 yr B.P. (Holliday, 1989). Similar chronologies are recognized in Wyoming (Gaylord, 1990) with at least four episodes of enhanced eolian activity after 7500 years ago: maximum aridity occurred 7545-7035 B.P. with a lesser peak 5940-4540 B.P. The Nebraska. Sand Hills have experienced several periods of reactivation and/or dune formation in the Holocene (Ahlbrandt et al., 1983). In many areas of the Great Plains, the most recent dune activity was thought to have occurred between 3500-1500 B.P. (see summary in Muhs and Maat, 1993), but evidence for eolian activity in the past 1000 years is now becoming available (e.g., Madole, 1994). Periods of Holocene dune activity probably occurred during warmer and drier episodes, with prolonged drought between 6500 and 4500 yr B.P.. Periods of dune activity in the southwestern United States are less well constrained, but Stokes and Breed (1993) recognize three periods of dune reactivation 400,2000-3000 and 4700 years ago in northeastern Arizona.

A major problem encountered in studies of Holocene dune stratigraphy and chronology is the difficulty of dating episodes of dune formation and eolian deposition. Reliable dating of periods of dune activity is critical, as many dune fields indicate multiple episodes of dune formation. In some areas, 14C dating of organic materials in paleosols can be used to bracket periods of eolian deposition (e.g., Forman and Maat, 1990; Wells et al., 1990; Kocurek et al., 19911, but many eolian depositional settings do not favor soil formation and/or preservation, or organic matter content in these soils is too low.

Luminescence dating provides a means to establish a chronology of periods of eolian activity and dune formation by measuring the time since burial of sediments and/or stabilization of areas of dunes (Wintle, 1993). The technique has been applied suc- cessfully to develop chronologies of dune formation in Australia (Gardner et al., 1987; Lees et al., 1990) and North America (Forman and Maat, 1990; Singhvi et al., 1982; Edwards, 1993; Stokes and Gaylord, 1993). New developments of the technique that include stimulation of luminescence by photons in infrared (IRSL) and green wavelengths (OSL) appear to provide reliable and precise age estimates for the deposition of eolian sands (Wintle, 1993). The availability of numerical ages for periods of eolian deposition also facilitates correlations with paleoclimatic data from other sources, and leads to more appropri- ate determinations of the response of landforms to climate change.

3. Case studies

3.1. Modem climate variations and dune activity in the Coachella Valley, south-central California

Studies of the response of dunes to climatic variability on the scale of decades are rare. In part this is because records of climate in arid regions are often short, and long-term monitoring of dunes is rarely undertaken. A recent study of dunes in the Coachella Valley of southern California (Lancaster et al., 1993) shows how data on changes in dune morphology and migration rates derived from aerial photographs taken over a fifty-year time span can be correlated with information on rainfall and winds to derive a new


understanding of eolian dynamics on a decadal time scale.

The Coachella Valley of southern California con- tains some of the most active eolian terrains in North America. Sand derived from washes and distal alluvial fans is transported southeast by strong westerly and northwesterly winds that are funneled through the San Gorgonio Pass. Expansion of the surface flow downwind of this constriction results in a rapid decrease of wind energy toward Indio (Sharp and Saunders, 1978) and deposition of sediments in the area east of Palm Springs and the Indio Hills (Fig. 4) where several areas of crescentic, parabolic, and shrub-coppice dunes occur (Beheiry, 1967).

The Indio Hills dune field (Fig. 5) consists of well-defined parabolic and crescentic dune complexes that comprise three linear, NW-SE-trending

areas. These dune complexes are surrounded and separated by regions of undulating sand sheets and blowouts. The upwind (northwest) terminus of significant eolian sand accumulation is characterized by degraded parabolic dunes and sand sheets. South and east of the main dune areas, extensive, continuous sand sheets characterized by mesquite-anchored, shrub-coppice dunes can be found on the early photographs. This area is now occupied by housing developments.

Comparison of aerial photographs taken in 1939, 1953,1972, and 1992 reveals that significant changes in eolian morphology have occurred within the Indio Hills dune field. These changes include alterations in eolian deposit form, location, and size (Fig. 6). For example, the areas characterized by dune ridges increased from 1939 to 1953, and generally declined

Fig. 4. Location map for the India Hills dune field and the Coachella Valley, southern California.

N. Lancaster/ Geomorphology 19 (1997) 333-347 339

thereafter. Both the western and eastern dune areas resulting in increases in the width-length (W/L)

have decreased in length over time (especially since ratios. The area characterized by sand sheets has also 1953), but the width has changed only slightly, declined with the most significant decrease occurring

r-- .-------- .‘~ ~- meters

Dune Ridges

Sand Sheets

?? Vegetated Dune Areas

Agricultural Fields

Mesquite Groves

Fig. 5. The Indio Hills dune field, showing location of main dune complexes. Map shows dune field as it was in 1953.


after 1972. This reflects thinning and disintegration of the sand sheets into isolated regions along the upwind areas of the dune field.

Fig. 7 summarizes changes in dune complex migration rates over time. The margin of the western dune area advanced at an average rate of 27 m/yr between 1939 and 1953. Between 1972 and 1992 the western dune complex encountered wind breaks along the margin of adjacent agricultural areas and the migration rates slowed to only 13 m/yr. The eastern dune complex migrated to the southeast at rates that varied from 26 m/yr during the period from 1939 to 1953 to 39 m/yr between 1953 and 1972. Its migration was also hindered by wind breaks after 1982, the rate of advance decreasing to 31 m/yr. A general decrease in dune migration rates from the period of 1953-1972 to 1972-1992 is evident from the response of the far-eastern dune complex (Fig. 7).

i.O-

0.6 -

! o’6_ 4 0.4

0.2-

A

--s---West Dunes . .+- -EastDunes - A- -FarEastDunes

. +- . _ . . . +.’

. . + . . . . . . ~

t’-- A- -_

Y

o.o- 1930 1940 1950 1960 1970 1980 1990 2000

B

1.8

1.6 4 + . . . -o-Active dunes .-+--3andsheels

1.4 1 ...

3 1.2

5 IO

'+., . . . .

z. I/.'\-

‘+

0.8

0.6

0.4 1

0.2 ] +

I 1 I I I I 1 IQ30 IQ40 1950 1960 1970 1980 IQ90 2000

Fig. 6. (A) Changes in dune complex area, and (B) areas of active dunes and sand sheets in the Indio Hills dune field 1939 to 1992.

45.0

40.0 1

+-W&dune +- Eastdune

- A- - FarEastdune

t .“\

\‘ . . \ . ..+ \

g 35.0-

1 t?

30.0-

3 \ i e \

.z

25'o! '\--;

20.0 \

\ 15.0

10.0 ] L I I I

1939-1953 1953-1972 1972-1992

Fig. 7. Changes in Indio Hills dune migration rates over time, 1939 to 1992.

Unlike the other two dune complexes, however, the far-eastern dune area is unconstrained by wind breaks.

A 20 year record of wind data collected at the Palm Springs airport, reveals that the typical, seasonal patterns of sand transport observed in other parts of California (e.g., Laity, 1987) apply to Coachella Valley. For example, winds capable of moving sand (> 5 m/s) are mainly from two direc- tional sectors: west to northwest (38.15% of all winds) and east and east-southeast (17.69% of all winds). The single most important sand-moving wind direction is west-northwest (23.19% of all winds) and is associated with the late spring to early sum- mer period. The easterly sector is most important (25-29% of the winds) in July through September, reflecting the influence of monsoon air movement from the Gulf of California. Mean wind speed over the year is 5.09 m/s, with the highest values (up to 9.1 m/s hourly average at a height of 10 m) occurring from March to June. The data for dune migration presented above suggest that rates of eolian activity and sediment transport have varied over periods of years to decades. These relations can be assessed using the dune mobility index (Lancaster, 1988). Dune mobility indices for the period 1973- 1991 were calculated using climatic data collected at Palm Springs (Fig. 8). Unfortunately, wind data pre- dating 1973 were not available for Palm Springs, prohibiting the calculation of the dune mobility index for the earlier part of the period covered by the aerial photographs.


50

1 A

30,~, , ; , , , , , , , , , , , , , 197419701978198019821984198819881990

B

P 250

3 200

150

100

50

0

rrrrrrrrrrr--r-r-.-.-

2500 C

o+ll-rrt I t t t t I I t I t t t t tl 197419761978198019821984198619881990

Fig. 8. Variations in percentage of time: winds blowing in excess of 5 m/s, annual precipitation, and the dune mobility index for Palm Springs, CA, 1973 to 1991.

A sensitivity analysis was also performed to determine the primary factor(s) that influenced sand transport during this 18 year period. Comparison of the variability in climatic parameters that may influence the mobility index shows that the percentage of the time the wind was above threshold at Palm Springs varied by 12.70% and potential evapotranspiration varied by only 4.61%, whereas the coeffi-

cient of variability for precipitation was 66.69%. This suggests that the variability in precipitation is the main factor affecting changes in the mobility index from year to year. Fig. 8 also suggests that periods of higher rainfall tend to be less windy than relatively dry spells. It is also possible to assess the importance of total annual precipitation on sand transport by comparing totals of yearly rainfall with the rates of dune complex migration for the periods of 1939-1953, 1953-1972, and 1972-1992. Except for the west dune, the comparisons (Fig. 9) show that the wet intervals of the late 1930s and 1940s as well as those following 1976 generally exhibit relatively lower rates of dune complex migration.

The question of why wind strength does not have more of an influence on the variability of eolian sediment transport can be explained in terms of limiting conditions. For the time scales considered here (one-year intervals>, wind velocities capable of sediment transport occur frequently enough every year (an average of 36.85% of the time during the period 1973-1992) to transport the available sediment and, therefore, the movement of eolian sand is not limited by wind duration and velocity, but by other parameters such as sediment moisture content, sediment availability, and vegetation cover. Sedi- ment moisture content and vegetation cover may be directly controlled by annual precipitation, whereas sediment availability may be determined by longer- period variations in precipitation and stream channel dynamics.

The probable source areas of sediment for the Indio Hills dune field are the modem washes on the

-u- Awagemigmtiinrate .+. -Precipitatiin

35.07 r*O

H 30.0- -13

15.0’10 1939-1953 1953-1972 1972-1992

Fig. 9. Relations between average annual migration rate for the Indio Hills dune field and mean annual precipitation (at Palm Springs).


alluvial fans derived from the nearby Indio Hills. These washes feed sand streaks that extend to the dune field. It appears, however, that sediment supply has been depleted over the period since 1939: the area covered by dunes and sand sheets has declined and the trailing margins of the dune complexes are moving away from the alluvial fan areas and disinte- grate. Some of the largest flood events in the last century have occurred during the time since 1939, yet significant episodes of dune formation have not taken place. One plausible hypothesis is that phases of sediment input to dune fields and dune building in this area (and possibly in many other parts of the southwestern United States) are related to episodes of valley floor dissection. Periods of valley aggrada- tion and entrenchment are well-documented throughout the region (for a review see Miller and Kochel, 1997). Periods of entrenchment appear to be associated with relatively minor increases in total annual precipitation and the frequency of intense storms (e.g., Hereford, 1984; Balling and Wells, 1990). Dissection of valley fill in response to these changes provides sediment to distal alluvial fan areas, where it can be redistributed by the wind to dune areas. As

channels in the valleys reach some form of equilib- rium, the quantity of sediment delivered to the alluvial fans decreases and the eolian geomorphic system changes from a period dominated by dune accumulation to one dominated by dune migration, modification, and degradation. Because precipitation is a predominant control on dune mobility, periods of more rapid dune migration and modification will be associated with drier intervals.

3.2. Holocene eolian activity in the east-central Mo- jaue Desert

The area between Afton Canyon and the Provi- dence Mountains in the eastern Mojave Desert (Figs. 10 and 11) provides an excellent example of an eolian sediment transport system that has been af- fected by direct and indirect effects of climatic changes on a variety of temporal scales. The fan-de- lta of the Mojave River as it exits Afton Canyon is the principal source of sediment for sand that has been transported southeastwards for u 50 km to the depositional sink of Kelso Dunes (Sharp, 1966). The Devil’s Playground, an area of sand sheets, climbing and falling dunes, and small crescentic dunes, forms

km

Fig. 10. Location map for the Kelso eolian eolian sediment transport system in the east-central Mojave Desert.


Fig. 11. Landsat TM (band 5) image of the Kelso dunes eolian sediment transport system.

the corridor through which sand is transported from the Mojave River sink to Kelso Dunes. Currently, mobile crescentic dunes and active sand transport in the region are restricted to the western parts of the Devil’s Playground (Smith, 1984) and the highest parts of the Kelso Dunes (Paisley et al., 19911, and no sand from the Mojave River reaches the main dune mass. A well-developed vegetation cover of grasses (mainly galleta grass) and shrubs (principally creosote bush) occurs on the inactive and relict sand surfaces.

Evidence indicating that eolian activity has been both more extensive and more intense than it is at present occurs throughout the Kelso eolian sediment transport system and includes: vegetation-stabilized dune systems and sand sheets, relict climbing and falling dunes (sand ramps), and eolian sand deposits intercalated with alluvial fan sequences (McDonald

and McFadden, 1994). Other evidence includes eolian sand mantles on beach ridges at Silver Lake (Wells and McFadden, 1987) and ventifacts covered by desert varnish (Laity, 1992). Studies of stratigraphy and luminescence dating indicate that a major change occurred in the eolian depositional environ- ment in the Mojave Desert during the early Holocene. Prior to this time sediment supply from fluctuating lakes was apparently sufficient to promote the accumulation of large climbing and falling dunes, even in relatively mesic climates compared to today (Rendell et al., 1994). After the desiccation of the region in the latest Pleistocene and early Holocene, no further sediment was available from lacustrine sources, because the available sand-sized material had been deflated. Many of the sand ramps ceased to accumu- late after around 8 ka, and were then stabilized by a talus cover and later trenched by ephemeral streams.


Holocene eolian activity was apparently restricted to the major dune fields and immediately adjacent areas (Edwards, 1993; Clarke, 1994) and to areas of active sediment supply (e.g., adjacent to the Mojave River; Laity, 1992; Clarke et al., 1996).

Kelso Dunes form the main depositional sink for eolian sand in the region. The dune field covers an area of approximately 100 km2 at an elevation of 500-900 m on the Piedmont alluvial deposits of the Granite and Providence Mountains (Sharp, 1966; Lancaster, 1993) and consists of a 40 km2 area of active dunes surrounded on the west, north, and east sides by areas of lower vegetation-stabilized dunes. Luminescence dating of eolian sediments in alluvial fan sequences southeast of the main dune field suggests that the Kelso Dunes are at least 20,000 yr old (Clarke, 1994). A major period of dune formation also occurred 8400-10,400 yr ago. Luminescence ages from areas of crescentic dunes on the northern, eastern, and southwest margins of the dune field (Edwards, 1993) indicate a later period of dune formation prior to 4000 and again around 1500 yr ago, with reworking of dunes between 800 and 400 yr ago (Wintle et al., 1994). Eolian sand also accu- mulated east of the main dune field in aggrading alluvial fan deposits around 4250 yr ago (Clarke, 1994).

channel bifurcates, one channel flowing east to Soda Lake, the other flowing northward along the base of Cave Mountain into the East Cronese Basin. Seven paleoshorelines have been identified from the Cronese Basins, and pits and cores throughout the East Cronese Basin show alternating lake and playa conditions with periodic Mojave River floods (Brown, 1989). A narrow spillway connects the East Cronese Basin to the West Cronese Basin with water flowing between the two basins once the lake level in the East Cronese Basin reaches a minimum elevation of 2 m (Brown, 1989). Eolian activity in the Cronese Basin probably reflects deflation of sediment deposited by lakes fed by high floods in the Mojave River.

Results of luminescence dating from the West Cronese Basin and the Devil’s Playground (Rendell and Sheffer, 1996; Clarke et al., 1996) indicate that dune formation has occurred episodically throughout the Holocene (Fig. 12). Significant periods of eolian deposition occurred approximately 6800-5500, 1400-2300, and 200 years ago. Material from a 1 m-high yardang at the south side of the West Cronese Basin near the spillway, gave an age of 250 + 75 B.P., showing that large-scale erosion and transport of sand from the West Cronese Basin has occurred since that time (Clarke et al., 1996).

The Cronese Basins, located north of the Mojave Early-mid-Holocene eolian activity in the region River Wash, are fed by flow in the ephemeral Mo- was probably a response to enhanced sediment sup- jave River as a response to increased rainfall in the ply from former paleolakes and geomorphic instabil- San Bernardino Mountains (Enzel et al., 1992). ity resulting from the desiccation of the region at this Downstream of Afton Canyon the Mojave River time (Wells and McFadden, 19871, together with a

WEST CRONESE BASIN

SALCH

OLD DAD MOUNTAINS

KELSO DUNES

Fig. 12. Timing of major periods of Holocene eolian activity in the Kelso region, compiled from luminescence dating data in Wintle et al. (19941, Clarke et al. (1996) and Rendell and Sheffer (1996).


period of dry climates 6800-5060 yr ago (Spaulding, 1991). The stabilization of dunes at Kelso and else- where in the region after 4000 yr B.P. was probably the result of cooler and wetter regional climates that resulted in increased vegetation cover. The period 4000-3000 yr ago appears to have been cooler and wetter in many parts of the southwestern U.S.A. with lowering of the woodland-desert shrub boundary (Spaulding, 1985; Spaulding et al., 1994), increased groundwater recharge in southern Nevada, and shallow lakes in Death Valley, Searles Lake, and the Silver Lake Basin (Enzel et al., 1992). The latter was probably fed by increased winter rainfall in the San Bernardino Mountains (Enzel et al., 1989).

Limited paleobotanical and dendroclimatological evidence from southern California suggest that relatively dry conditions persisted at around 1450 yr B.P. (Spaulding et al., 19941, and from 850 to 450 yr B.P. (Fritts and Gordon, 1982). A radiocarbon-dated beach ridge with an age of 390 f 90 yr B.P. indi- cates that a permanent lake again developed in the Silver Lake basin at this time as a result of cooler and wetter conditions and increased rainfall coeval with the Little Ice Age in Europe (Enzel et al., 19921, giving rise to’ renewed stabilization of dunes at Kelso. Eolian activity prior to 200 years ago in the West Cronese Basin may reflect deflation of sediment deposited by shallow water bodies in this wetter period.

In summary, luminescence-dated evidence for Holocene eolian actjvity in the east-central Mojave suggests multiple periods of dune activation and stabilization as climates in this region crossed and recrossed mobility thresholds. Accumulation of new areas of dunes appears to have been promoted by increased sediment supply from fluvial and lacustrine sources.

4. Conclusions

Studies of dune dynamics in southern California on two widely differing temporal scales provide the basis for a tentative model for the response of eolian geomorphic systems to minor climate changes. At both the Holocene ;and decadal time scales, eolian activity is strongly controlled by variations in precipitation. Wind energy is apparently not a limiting

factor in this region as winds are above threshold speeds for transport for as much as 30% of the time. Formation of eolian deposits is a product of climatic changes that increase sediment supply from fluvial and locally lacustrine sources. Dune formation may, therefore, be closely tied to periods of channel cutting and geomorphic instability and is therefore highly episodic. During intervening periods, eolian deposits migrate away from sediment sources and are reworked, modified, and degraded. Remobiliza- tion of existing dormant dunes is a product of reduced vegetation cover and soil moisture in periods of drier climates. The major control on these processes is decadal to annual changes in rainfall that determine vegetation cover and soil moisture content. Vegetation cover is probably the single most important determinant of eolian activity at this scale and acts through direct protection of the surface and absorption of momentum from the wind (Wolfe and Nickling, 1993) to increase the threshold wind shear velocity for transport.

Acknowledgements

Research on aspects of eolian dynamics in south- em California has been supported by the Department of Energy, National Geographic Society, National Science Foundation (EAR 92046481, NATO Collab- orative Research Grants Program, and the Nature Conservancy. I thank my collaborators in these pro- jects: especially A.G. Wintle and M.L. Clarke (In- stitute for Earth Studies, University of Wales, Aberystwyth) and Helen Rendell (University of Sus- sex> for luminescence age determinations, A.J. Braze1 and A. Bach (Arizona State University) for climatic data, and J.R. Miller and Lynn Zonge (Desert Re- search Institute) for assistance with aerial photogra- phy and climatic data analysis, interpretation, and mapping.

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