The lower Colorado River Valley: A Pleistocene desert

QUATERNAR’

RESEARCH

25, 392-400

(1986)

The Lower Colorado River Valley: A Pleistocene Desert L. COLE

KENNETH Indianu

Dunes

National

Lakeshore,

1100 North

Minerul

Springs

Road,

Porter,

Indiana

46304

Received August 5, 1985 A chronological sequence of plant macrofossil assemblages from twenty-five pack rat middens provides a record of desert scrub vegetation for most of the last 13.380 yr B.P. from a hyperarid portion of the lower Colorado River Valley. At the end of the late Wisconsin, and probably during much of the Quaternary. the Picacho Peak area, Imperial County, California, supported a typical Mohave Desert association of Lurreu divuricura (creosote bush), Coleogyne ramosissimu (blackbrush), Yucca brevifofia (Joshua tree), and Y. whipp(ei (Whipple yucca). Recent arrivals of Sonoran Desert plants such as Olneyu tesotu (ironwood) and Fouquieria splendens (ocotillo) suggest that the area supported relatively modern Sonoran desert scrub species for relatively short periods during interglaciations. C 1986 Umversity of Washington.

INTRODUCTION

Taxonomy is consistent with Munz (1974) unless other authorities are noted. In the lower Colorado River Valley, high temperatures combine with high insolation to produce values of pan evaporation exceeding 3650 mm/yr, the highest on the continent (U.S. Geological Survey, 1970). Precipitation is erratic and averages less than 50 mm/yr at low elevations near the Gulf of California. The modern vegetation is a sparse desert scrub dominated by Larrea divaricata Cav. ssp. tridentata Felger and Lowe (creosote bush), Ambrosia dumosa (white bursage), and EnCelia furinosa (brittle bush). Total vegetative cover of shrubs is less than 6% (K. L. Cole, unpublished data). The area has been classified as the Colorado Desert (Yeager, 1957) or the lower Colorado Valley Subdivision of the Sonoran Desert (Shreve,

In the last 20 yr, knowledge of the history of vegetation in the warm deserts of North America has been greatly increased by analyses of plant remains preserved in hard organic deposits left by pack rats (genus Neotoma) in dry caves, rock shelters, and crevices (Spaulding et al., 1983; Wells, 1983). Most of these pack rat middens of late Wisconsin or early Holocene age have provided evidence for xerophytic conifer woodlands which formerly extended into lowlands that now support desert scrub communities. Although Pleistocene fossil records of xerophytic shrub communities lacking conifers have recently been reported from low elevations in the modern Mohave Desert (Spaulding, 1983; Wells and Woodcock, 1985), the Pleistocene location of plant assemblages analogous to modern Mohave or Sonoran desert scrub communities remains unknown. It has been speculated that these communities survived the last ice age further south in Mexico, and subsequently invaded the United States in the Holocene (Wells, 1976). In this paper, I present a 13,000-yr record of pack rat midden assemblages from the lower Colorado River Valley (Fig I), the most arid area in North America.

1964). THE FOSSIL RECORD

Twenty-five pack rat middens were collected from andesite rock shelters between 240 and 300 m elevation near Picacho Peak in Imperial County, California (32”58’N., 114”5O’W.), 50 km north of Yuma, Arizona. Twenty-eight radiocarbon dates on midden materials date the plant macrofossil assem392

0033-5894186

$3.00

Copyright ,Q 1986 by the Universiry of Wshmgton. All rightr of reproduction in any form rexrved.

LATE PLEISTOCENE

PACK RAT MIDDENS

393

FIG. 1. Map showing the location of the study area and mean annual precipitation. The dashed line in the Gulf of California shows the approximate Wisconsin-age sea level (after Bloom, 1983). Pack rat midden sites are labeled as: PP = Picacho Peak (this study), CM = Chemehuevi Mountains, NW = New Water Mountains. WM = Whipple Mountains, WH = Wellton Hills. BM = Butler Mountains. and TM = Tinajas Altas Mountains. Precipitation isohyets from unpublished map by Raymond Turner. U.S.G.S.. Tucson.

blages from 13,380 yr B .P. to modern (Table I). The assemblages contained a total of 68 plant taxa with 9-24 taxa per sample (Table 2). Concentrations of macrofossils of important plants are presented in Figure 2. The middens record a desert scrub vegetation for at least the last 13,000 yr. The oldest deposits contain species typical of the modern Mohave Desert such as Chvysothamnus teretifolius (rabbit brush), Salvia mohavensis (Mohave sage), Coleogyne ramosissima (blackbrush), Yucca brevifolia (Joshua tree), Y. whippfei (Whipple yucca), Eriogonum fasciculatum (woody buckwheat), Larrea divaricata, and Oryzopsis hymenoides (Indian ricegrass). Of these plants, only Larrea divaricata occurs in the area today. A tandem accelerator mass spectrometer radiocarbon date of 12,780 yr B.P. on creosote bush twigs securely establishes the presence of this widespread, modern dominant in the late

Wisconsin desert scrub at Picacho Park. The only previous late Wisconsin record for Larrea divaricata is a sample dated at 11,250 yr B.P. from 240 m elevation in the Butler Mountains, Yuma County, Arizona (80 km SE; Van Devender et al., 1985), and from undated, but most likely, late Wisconsin sloth dung from caves in the Mojave Desert (Laudermilk and Munz, 1934). Coleogyne ramosissima, Yucca folia, and Y. whipplei apparently

brevi-

disap12,000 and

peared from the site between 11,000 B .P. while Chrysothamnus

teretiLycium cooperi (peach thorn wolfberry), Brickelia atractyloides Gray. (= B. arguta Rob.), and Acacia greggii (catclaw) persisted until about 10,000 yr B.P. Lycium cooperi and Acacia greggii are presently restricted at

folius,

Salvia mohavensis,

their lower elevations to riparian habitats along desert washes and do not live on dry, rocky slopes where the middens were col-

394

KENNETH TABLE

L.

COLE

1. RADI~CARBONDATESFROMNEARPICACHOPEAK,IMPERIALCOUNTY,CALIFORNIA

Material dated

Lab no.

Sample

Elev. (m)

Age

17 D2 17 D (between Dl and D2) 17 Dl 17 E 17 E 15 15 15 23 B 17 B 16 5D SD 6B 5B 26 22 3A 1 10 8C 4 9B 17 A 21 18 11 24 25 7

285

12,390 * 340

AA-578”

Larrea di\wricutu twigs

285 285 285 285 275 275 275 280 285 245 270 270 280 270 280 300 245 245 275 275 240 275 285 290 250 275 280 280 270

13,380 k 350 10,850 ? 630 12.730 2 410 9,750 ? 270 12,500 t 360 11,160 2 190 10,640 ? 320 10,540 2 250 10.420 k 110 10,240 t 220 10,090 2 170 9,990 2 610 9.590 2 190 9,380 ” 300 9,090 ?- 150 8.990 i 110 8.720 2 110 8,410 -r- 275 8,330 k 120 8,110 k 160 7,960 k 180 7,650 2 120 7.350 k 90 4.970 k 80 4,800 k 80 630 k 140 110 k 40 Modern debris-Not Modern debris-Not

AA-577” AA-576“ AA-579B” AA-579A” AA-363” A-3573 AA-362” A-3575 A-3371 A-3406 A-2350 A-2172 A-2173 A-2171 A-3374 A-3372 WK-166 A-1876 A-3405 A-3344 A-2170 A-3343 A-3373 A-3404 A-3505 A-3574 A-3404 dated dated

Yuccu whipplei leaf Lurreu divuricutu twigs Lurreu divaricutu twigs Lurreu divuricutu twigs Yuccu brel,ifoliu leaf Neotomu fecal pellets Lurreu divaricata twigs Neotoma fecal pellets Neotoma fecal pellets Neotoma fecal pellets Midden debris Neotomu fecal pellets Neotomu fecal pellets Neotomu fecal pellets Neotomu fecal pellets Neotomu fecal pellets Midden debris Opuntiu busiluriu seeds Neotoma fecal pellets Neotomu fecal pellets Lurrea divaricatu twigs Neotomu fecal pellets Neotoma fecal pellets Neotomu fecal pellets Neotomu fecal pellets Neotomu fecal pellets Neotoma fecal pellets

a Tandem accelerator mass spectrometer date.

lected.

Wats. var. aspera Benson (Mormon tea) persisted in the area until about 8000 yr B.P. The two samples dated at 4970 and 4800 yr BP contain few taxa and seem to indicate severely arid conditions during this interval. Interestingly, Ambrosia dumosa, Fouquieria splendens (ocotillo) and Olneya tesota (ironwood) were only found in three samples less than 700 yr in age, suggesting that their arrival in the area and the final establishment of the modern desert scrub communities occurred sometime after 700 yr B. P. If this result is validated by more extensive late Holocene sequences, then it indicates a Neoglacial amelioration of desert climates similar to that recorded in Ephedra

nevadensis

the Mohave (1985).

Desert

by Cole and Webb

DISCUSSION

The Picacho Peak records are interesting in comparison to other pack rat midden sequences from the lower Colorado Valley. The lower elevational limit for pinyon-juniper woodland in the late Wisconsin prior to 11,000 yr B.P. was as low as 5 10 m in the Whipple Mountains, California (145 km N), and 460 m in the Tinajas Altas Mountains, Arizona (85 km SE; Van Devender et al., in press). A more xeric woodland dominated by Juniperus cf. californica (California juniper) persisted at these sites until about

LATE PLEISTOCENE

PACK RAT MIDDENS

395

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396

KENNETH

L.

COLE

LATE

PLEISTOCENE

PACK

RAT

MIDDENS

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398

KENNETH

L. COLE

LATE PLEISTOCENE

8900 yr B.P. Low-elevation records for juniper are 260 m in the Chemehuevi Mountains, California (175 km N; Wells and Hunziker, 1976), 320 m in the Whipple Mountains (Van Devender, 1977), and 240 m in the Butler Mountains, Arizona (Van Devender et al.. 1985). Today, Jlrniperrrs californicu is found as low as 600 m elevation in both the southern Mohave Desert and on the western edge of the Colorado Desert (K. L. Cole, unpublished data). In contrast to the other records, the Picache Peak area at 245-300 m elevation supported a desert scrub lacking conifers for the last 13.000 yr. The oldest assemblages at the Picacho site are similar to modern vegetation found in the transition between the Mohave and Sonoran deserts, I.50 to 250 km to the northwest of the sites. Because other portions of southwestern deserts with more extensive midden coverage (Spaulding et al., 1983; Van Devender and Spaulding, 1979; Thompson and Van Devender, 1982; Mead and Phillips, 1981; Phillips, 1977) show only small changes in vegetation from full-glacial times (ca. 18,000 yr B.P.) to 12.500 yr B.P., the full-glacial vegetation of the Picache Peak area most likely was a desert scrub similar to the oldest sample. Even paleovegetational models which call for substantial climatic warming in the southwest prior to the end of the Wisconsin (Cole, 19851 show only minor changes in dominant vegetative types between 18,000 and 12,500 yr B.P. The loss of the higher elevation Mojave Desert species, Y~cctr hrevifoliu and Coleogpne rurnosissimu, around 12,000 yr B.P. probably is a result of the first substantial warming of the Pleistocene to Holocene transition. The modern dominants Encelia .furinosu and Perrcephyllwn schottii first appear after 12.000 yr B.P. along with Acacia gregii and Lyciwn cooperi. Lurreu divuricutu, Enceliu furinosu, and Peucephyllum schottii (pygmy cedar) have apparently been continuously codominant for the last 11,000 yr. The arid conditions existing today in the

PACK RAT MIDDENS

39’)

lower Colorado River Valley are largely a result of local physiographic factors which probably kept the area relatively arid in the Pleistocene despite a more southerly position of winter storm tracts. With sea level about 120 m lower (Bloom, 1983), the arid lowlands around the head of the Gulf ot California would have been greatly expanded during glacial periods (Fig. 1). This area has probably been a core desert for xerophytic generalized desert species such as creosote bush throughout the Pleistocene. While Axelrod (1979) takes the roots of the Sonoran Desert flora back to the late Miocene 5 to 8 myr ago, characteristic Sonoran species arrived at their modern distributions late in the present interglaciation. The lower Colorado Desert is best viewed as a refugium for Mohave Desert plant species while ice age woodlands dominated higher elevations. Sonoran Desert species most likely have migrated in and out of the area during interglaciations. ACKNOWLEDGMENTS Funds were provided from NSF Grants DEB 79-23840 to P. S. Martin and BSR G-14939 IO P. 5. Martin and T. R. Van Devender. University of AI,!zona. Thomas R. Van Devender provided extensive aid in the identification of fossils and in the preparation of the manuscript, although I take full re\ponGbility for the results and opinions presented herein. Eight radiocarbon date5 were provided by R. M. Turner. U. S. Geological Survey. A. Long. D. J. Donahue, A. J. T. Jull, L. Toolin. and T. Zabell. NSF l%cility for Radioisotope Analysis. University of Arizona. provided additional radiocarbon dates on the tandem accelerator. T. L. Burgess. 0. K. Davis. R. Hiebert. S. McLaughlin. J. Bowers. P. S. Martin. and R. Turner assisted in the field and provided helpful comments on the manuscript. The figul-e\ \A.ere prrpared by M. Skeds.

REFERENCES Axelrod. D. 1. (1979). “Age and Origin of Sonoran Desert Vegetation.” California Academy 01 Sciences Occasional Papers. Bloom. A. L. (1983). Sea level and coastal morphology of the United States through the late Wisconsin glacial maximum. In “Late-Quaternary Environments of the United States.” Vol. I. “l-he Late Pleistocene” (S. C. Porter. Ed.). pp. 215-219. Univ. of Minnesota Press, Minneapolis.

400

KENNETH

Cole, K. L. (1985). Past rates of change, species richness, and a model of vegetational inertia in the Grand Canyon, Arizona. American Naturalist 125, 289-303. Cole, K. L., and Webb, R. H. (1985). Late Holocene vegetation changes in Greenwater Valley, Mojave Desert, CA. Quaternar?, Research 23, 227-235. Laudermilk, J. I?, and Munz, P. A. (1934). “Plants in the Dung of Nothrotherium from Gypsum Cave, Nevada,” Vol. IV, pp. 31-37. Carnegie Inst.. Washington, Contributions to Paleontology. Mead, I., and Phillips, A. M. (1981). The late Pleistocene and Holocene fauna and flora of Vulture Cave, Grand Canyon, Arizona. Southwestern Naturalist 26, 257-288. Munz, P. A. (1974). “A Flora of Southern California,” University of California. Berkeley. Phillips, A. M. (1977). “Packrats, Plants and the Pleistocene in the Lower Grand Canyon.” Ph.D. dissertation, University of Arizona. Shreve, F. (1964). Vegetation of the Sonoran Desert. In “Vegetation and Flora of the Sonoran Desert” (F. Shreve and I. L. Wiggins, Eds.), pp. 9-186. Stanford Univ. Press, Stanford. Spaulding, W. G. (1983). Late Wisconsin macrofossil records of desert vegetation in the American Southwest. Quaternary Research 19, 256-264. Spaulding, W. G., Leopold, E. B., and Van Devender, T. R. (1983). Late Wisconsin paleoecology of the American southwest. In “The Late Wisconsin of the United States” (S. C. Porter, Ed.), pp. 259-293. Univ. of Minnesota Press, Minneapolis.

L. COLE Thompson, R. S., and Van Devender, T. R. (1982). Late Pleistocene vegetational records from desert grassland in the Santa Catalina Mountains, Arizona. American Quaternary Association Abstracts 7, 167. U.S. Geological Survey (1970). “The National Atlas of the United States of America.” Van Devender, T. R. (1977). Holocene woodlands in the southwestern deserts. Science 198, 189-192. Van Devender, T. R., Martin, P. S., Thompson, R. S., Cole, K. L., Jull, A. J. T.. Long, A., Toolin, L. J.. and Donahue, D. J. (1985). Fossil packrat middens and the tandem accelerator mass spectrometer. Nature, 317, 610-613. Van Devender, T. R., and Spaulding, W. G. (1979). The development of vegetation and climate in the southwestern United States. Science 204, 701-710. Wells, P. V. (1976). Macrofossil analysis of wood rat (Neotoma) middens as a key to the Quaternary vegetation of arid America. Quaternary Research 6, 223-248. Wells, P. V. (1983). Paleobiogeography of montane islands in the Great Basin since the last glaciopluvial. Ecological Monographs 53, 341-382. Wells, P. V., and Hunziker. J. H. (1976). Origin of the creosote bush (Larrea) in the deserts of southwestern North America. Annals ofthe Missouri Botanical Gardens 63, 843-861. Wells, P. V., and Woodcock, D. (1985). Full-glacial vegetation of Death Valley, California: Juniper woodland opening to Yucca semidesert. Madrono 32, 11-23. Yeager, E. C. (1957). “The North American Deserts.” Stanford Univ. Press. Stanford.