Reconstruction of the Late Pleistocene to Late Holocene vegetation transition using packrat midden and pollen evidence from the Central Mojave Desert

The Mojave Desert of the American West is characterized by plant species that reflect a unique mixture of winter precipitation and summer monsoon climate. Currently, the Mojave Desert experiences a strong summer monsoonal pattern with weak winter precipitation. Data from pollen and packrat midden analyses have revealed a history of Mojave Desert vegetation during the transition from the late Pleistocene to late Holocene (~17500 Cal. years B.P. to ~ 1200 Cal. years B.P.) that highlight a summer dominated monsoonal pattern, similar to those in the greater American Southwest. We compare pollen data from a lava tube in the Cima volcanic field, located in south-central region of the Mojave Desert, with plant macrofossil data from several woodrat midden localities in the region. The record for the Cima volcanic field reveals a vegetation history spanning the last ~ 8300 Cal. years B.P., with data from ancient woodrat middens detailing the record from ~17500 Cal. years B.P. to 7,800 Cal. years B.P. A Bryson macro-physical climate model for the transition from the late Pleistocene to the Late Holocene was created and compared to our findings to assess possible relationships between climatic variations and the arrival of diagnostic plant species within the Mojave Desert.


Introduction
The transition from the Pleistocene to Holocene (LP/H) resulted in significant climate-driven change within Mojave Desert plant communities. Plant species exhibited both geographic expansion from the Sonoran Desert and more localized elevational shifts establishing modern vegetation zones. Packrat middens and sediment pollen analyses are some of the most reliable proxies to recreate vegetation history and past climate (Cole 1985). Packrat middens often contain abundant pollen and plant microfossils, encased in crystalized woodrat urine (Cole 1985). A pollen record obtained from sediment within a lava tube of the Cima volcanic field enabled a reconstruction of early Holocene vegetation communities at low elevation. The LP/H transition in southern California (Quade et al 1998;Wigand & Rhode 2002) was quite dramatic east of the Sierra Nevada. Spaulding (1985) and Koehler et al. (2005) indicate that the key elements of Mojave Desert vegetation arrived by 9,500 Cal. years B.P. Creosote bush and white bursage are considered co-dominants, though Wigand's (2003) research just south of the Coso Range at the northwestern edge of the Mojave Desert indicates that with increased winter precipitation, white bursage is favored. Conversely during years exhibiting increased summer precipitation, creosote bush is favored.
Climate at the end of the Pleistocene probably favored white bursage, because periods of increased winter precipitation may still have been common. The impact of climate change on landscape dynamics at the end of the Pleistocene has been previously discussed by Harvey et al. (1999). The authors indicate that alluvial fans in the Mojave Desert during the LP/H transition was very active, exhibiting debris flows feeding onto the fan surfaces, headward fan trenching, and distal progradation. Increased geomorphology activity in the Mojave Desert during the LP/H may have been due primarily to decreased vegetation cover. The replacement of Pygmy juniper and sagebrush at the end of the Pleistocene by creosote bush and white bursage led to a more open discontinuous vegetation cover in the Mojave Desert. Schumm's model (Langbein & Schumm 1958) indicates drier conditions result in reduced vegetation density. When reduced vegetation cover coincided with episodes of high-intensity summer rains, it resulted in increased hillslope debris flow activity in the Mojave Desert. Fine sediments eventually filled the pluvial basins of the region, and as the basins dried up. the fine particles were transported as aeolian sediments onto the Cima volcanic field. In this research, we examine late Quaternary Mojave Desert vegetation history, derived from pollen and packrat midden proxies, to reveal the timing and order of arrival of diagnostic species into the region.

Study area
The Mojave Desert lies between the summer monsoondominated Sonoran Desert to the southeast and the zonal, winter storm-dominated Great Basin Desert to the northwest. It therefore exhibits a combination of the climates of two regions resulting in a fluctuating summer monsoonal pattern originating from the southeast Mojave Desert. Generally, the Mojave Desert is characterized by three climatically distinct regions. These include: a) the western region dominated by cool winters and warm summers; b) the central region, which has generally warmer winters than the eastern and western regions and hot summers; and c) the eastern region, which has cooler and wetter winters with greater precipitation (McKinney 2018).
The Cima volcanic field is located southwest of Las Vegas, Nevada (Fig. 1), and consists of roughly 60 associated lava flows and 40 cinder cones of Pleistocene age covering 150 km 2 of the Ivanpah uplift (Dohrenwend et al. 1984;Turrin et al. 1985;Wilshire et al. 1987;Wood et al. 2005). Potassium-Argon (K-Ar) dating of cinder cones within the Cima volcanic field provide an age range of 0.015 m yr to 1.09 ± 0.08 m yr (Dohrenwend et al. 1986). The lava flows are mostly alkali olivine basalt, hawaiite and basanite, and occur in two flow types, 1) elongate flows with low gradients, and 2) roughly equant flows (of similar length and width) with high gradients (Dohrenwend et al. 1984). The mean annual precipitation of Cima volcanic field is well within the range for arid to semiarid zones, at 12-25 cm and the plant community of the Cima volcanic field is dominated by creosote bush (Larrea tridentata), white bursage (Ambrosia dumosa), brittle bush (Encelia farinosa) and Mormon tea (Ephedra trifurca) though higher elevations (> 1300 masl) contain Joshua tree (Yucca brevifolia) (Brown et al. 1990).

Pollen sampling
Six sediment samples were collected in 2012 from sediments inside one of the Cima lava tubes. Pollen was recovered using sodium polytungstate flotation (Wigand 1987). The processed samples were stained and mounted in silicon oil before being counted at 400 power with a light microscope.

Packrat middens
Plant macrofossils from 18 packrat middens from near the Nevada Test Site were analyzed during this study. The middens were mapped, described, photographed and subsamples taken over a period of several years. Samples were soaked in distilled water for 24 hours, screened through nested screens, and dried. The resultant macrofossil material was then sorted and weighed and entered into a database.

Chronology
Twenty plant macrofossil and organic-rich sediment samples were dated by Beta Analytic Inc, Miami, Florida using accelerator mass spectrometry (AMS). The dates were calibrated using IntCal13 curve 133 (Reimer et al. 2013).

Paleoclimate Simulation
A Bryson MCM synoptic climate model simulation was generated for the central Mojave Desert highlighting the last 15,000 years. The simulation was based on 30 year means from the Las Vegas, Nevada, climate means of 1971 to 2000 (Bryson & Bryson 2000) and compares correlations between predicted climatic variation and our reconstructed vegetation histories.

Results
Chronology AMS dating results are presented in Tables 1 and 2. The three sediment samples from the Cima volcanic field span the last 8,300 Cal. years B.P. Middens ages ranged between 7,853 ± 119 Cal. years B.P. to 17,480 ± 282 Cal. years B.P., covering the entire LP/H transition (Tab. 2).

Pollen assemblages
Ten pollen types were identified, and their abundance analyzed within the Cima volcanic field samples (Fig 2).

Paleoclimate simulation
The MCM climate reconstruction reveals that summer precipitation within the Mojave Desert during the LP/H has the great variability, followed by that of spring precipitation. Fall and winter precipitation were much less variable., though during the LP/H both fall and spring precipitation have episodes of high variability that correspond to those periods when summer precipitation exhibits its highest variability. As will be discussed below, episodes of highest frequency and magnitude in summer precipitation are crucial to the appearance of characteristic Mojave Desert plant species.

Discussion
Preservation is clearly an issue with pollen samples from the Cima volcanic field. The pollen counts (Pollen Sum and Total Pollen) indicate that the number of pollen present declines dramatically below 70 cm (Tab. 4). Calculation of the actual pollen per sample using the Lycopodium spores recovered and the pollen grains counted. indicate that even below the first 20 cm or so preservation declines dramatically. Pollen type diversity indicates that even though some types are rare, it is clear that preservation is much worse below 70 cm as many pollen types disappear from the record. The surface textures of most of the pollen grains has been destroyed, or severely degraded, even in the uppermost sample suggesting either bacterial activity, or oxidation. Despite issues with preservation, the number of Lycopodium recovered for statistical purposes were generally sufficient to provide adequate estimates of pollen populations, except for the lowest sample. Lycopodium values for sample six are low, however estimates of its pollen population are probably close to the abundance of pollen in the sample. Creosote bush pollen is not found in this record. Both Ambrosia dumosa (white bursage) and Larrea tridentate (creosote bush) are common within the Cima volcanic field basalt flows today (Figs. 4, 5). Table 4. Pollen and spore count statistics, and population estimates for the Cima volcanic field samples. Note that the pollen sum (total terrestrial pollen) and pollen total (total terrestrial and aquatic pollen-we did have Cyperaceae and Sedge pollen) decreases rapidly with depth. Also, the total Ambrosia pollen, and total pollen population estimates per sample decreases rapidly with depth as well.   Thompson et al. 1999. The y axis uses precipitation variables, and the x axis uses temperature variables to establish the climatic clustering of plant species. Peter E. Wigand

Sample Number Pollen Sum Total Pollen Total Spores Lycopodium Recovered Mean Ambrosia Population Estimate Total Pollen Population Estimate
In our counts, only Ambrosia dumosa pollen appears in the record. This does not mean that Larrea tridentata was not present at the Cima volcanic field, rather the discrepancy may be due to the nature of creosote bush flowers. Larrea tridentata is insect pollinated with several species of native bees being specially adapted to its pollination (Fig. 6). Because creosote bush is insect pollinated, it also produces much less pollen. On the other hand, Ambrosia dumosa is wind pollinated and has no petals so that the wind may blow the pollen directly off the anthers, and flowers are arranged along a stalk that can intercept the pollen in the wind (Fig. 6). In addition, white bursage produces abundant pollen several magnitudes greater than creosote bush, so there is a much greater statistical probability of it appearing in a poorly preserved pollen record. This issue also characterized the pollen record from Lower Pahranagat Lake north northeast of Las Vegas (Wigand 2017). Creosote bush was the dominant plant in the community surrounding Lower Pahranagat Lake, but it rarely appeared in the pollen record. Though creosote bush and white bursage may appear just as abundant on the landscape their presence in the pollen record can be significantly different. Finally, two other common pollen types recovered from the Cima volcanic field samples are Sphaeralcea (globe mallow) and Eriogonum (buckwheat) (Fig. 6). The globe mallow blooms in spring and is quite common throughout the Mojave Desert. Buckwheat is very common throughout the Mojave Desert and blooms in the spring as well and may represent several different species.
The regional woodrat midden record indicates that some components of the Mojave Desert vegetation community arrived during the late Pleistocene (e.g., Yucca brevifolia). Others appear briefly and disappear and reappear again during the early Holocene. This seems to mirror the high variability of the climate during this period. In the MCM climate reconstruction, summer precipitation has several episodes of highly variable precipitation. Both the short frequency and high magnitude of rainfall during these periods is unique when the climate reconstruction for the last 17,000 years is viewed. The spring and fall precipitation pattern generally follow that of summer, but at much reduced magnitudes (Fig. 7). The first period of high magnitude and high frequency summer (and spring and fall) precipitation occurs between 15,000 and 14,000 Cal. years B.P. It is at this time Yucca brevifolia appears at Little Skull Mountain at the southern end of the Nevada Test Site. The second episode occurs between 13,500 and 11,000 Cal. years B.P. and is characterized by a slight increase in shrub species diversity. It is not until 9,500 Cal. years B.P. that the full array of Mojave Desert shrub species becomes common and wide spread at all lower elevations in the woodrat midden record (Tab. 4). After 10,500 Cal. years B.P. the current range of climate conditions became stabilized and Mojave Desert plant species came to characterize the vegetation assemblages of the region. Until 10,500 Cal. years B.P. the  Creosote bush (the smaller pollen grain) is a thick-walled pollen grain and should survive some degradation. White bursage is the larger thick-walled pollen grain with rough surface structure. Although creosote bush should be there, its pollination ecology does not provide sufficient opportunities for it to be preserved in the record; D. Globe mallow; E. Buckwheat. movement of Mojave Desert plant species into the region may be visualized as waves lapping onto the seashore as the tide rises. Each intrusion of a plant species may have been characterized by species establishment followed by local extinctions as the climate temporarily reverted to less favorable conditions.

Conclusion
The pollen from the Cima volcanic field reveals a record from at least ~8,300 Cal. years B.P. to the present of typical Mojave Desert vegetation. Though preservation is clearly an issue, pollen types offrom plants of the modern Mojave Desert are found throughout the record. Although pollen of creosote bush was not present, studies have shown that this is often an artifact of pollen preservation. This record in combination with the woodrat midden record of the area surrounding Yucca Mountain provides evidence for an earlier Holocene establishment of Mojave Desert vegetation community than previously thought. It also indicates that the formation of the Mojave Desert plant community was characterized by intermittent establishments during episodes of favorable climate and retreats and possible local extinctions during less favorable periods of climate. Some species became established as early as 16,000 years ago during the late Pleistocene.