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The Las Vegas Formation (LVF) is a well-characterized sequence of groundwater discharge (GWD) deposits exposed in and around the Las Vegas Valley in southern Nevada. Nearly monolithologic bedrock surrounds the valley, which provides an excellent opportunity to test the hypothesis that GWD deposits include an aeolian component. Mineralogical data indicate that the LVF sediments are dominated by carbonate minerals, similar to the local bedrock, but silicate minerals are also present. The median particle size is ~35 μm, consistent with modern dust in the region, and magnetic properties contrast strongly with local bedrock, implying an extralocal origin. By combining geochemical data from the LVF sediments and modern dust, we found that an average of ~25% of the LVF deposits were introduced by aeolian processes. The remainder consists primarily of authigenic groundwater carbonate as well as minor amounts of alluvial material and soil carbonate. Our data also show that the aeolian sediments accumulated in spring ecosystems in the Las Vegas Valley in a manner that was independent of both time and the specific hydrologic environment. These results have broad implications for investigations of GWD deposits located elsewhere in the southwestern U.S. and worldwide.

Recent studies have shown the oxygen isotopic composition (δ18O) of modern terrestrial gastropod shells is determined largely by the δ18O of precipitation. This implies that fossil shells could be used to reconstruct the δ18O of paleo-precipitation as long as the isotopic system, including the hydrologic pathways of the local watershed and the gastropod systematics, is well understood. In this study, we measured the δ18O values of 456 individual gastropod shells collected from paleowetland deposits in the San Pedro Valley, Arizona that range in age from ca. 29.1 to 9.8 ka. Isotopic differences of up to 2‰ were identified among the four taxa analyzed (Succineidae, Pupilla hebes, Gastrocopta tappaniana, and Vallonia gracilicosta), with Succineidae shells yielding the highest values and V. gracilicosta shells exhibiting the lowest values. We used these data to construct a composite isotopic record that incorporates these taxonomic offsets, and found shell δ18O values increased by ~4‰ between the last glacial maximum and early Holocene, which is similar to the magnitude, direction, and rate of isotopic change recorded by speleothems in the region. These results suggest the terrestrial gastropods analyzed here may be used as a proxy for past climate in a manner that is complementary to speleothems, but potentially with much greater spatial coverage.

The Eglington fault is one of several intrabasinal faults in the Las Vegas Valley, Nevada, USA, and is the only one recognized as a source for significant earthquakes. Its broad warp displaces Late Pleistocene spring deposits of the Las Vegas Formation, which record hydrologic fluctuations that occurred in response to millennial- and submillennial-scale climate oscillations throughout the late Quaternary. The sediments allow us to constrain the timing of displacement on the Eglington fault and identify hydrologic changes that are temporally coincident with that event. The fault deforms deposits that represent widespread marshes that filled the valley between ca. 31.7 and 27.6 ka. These marshes desiccated abruptly in response to warming and groundwater lowering during Dansgaard-Oeschger (D-O) events 4 and 3, resulting in the formation of a pervasive, hard carbonate cap by 27.0 ka. Vertical offset by as much as 4.2 m occurred after the cap hardened, and most likely after younger marshes desiccated irreversibly due to a sudden depression of the water table during D-O event 2, beginning at 23.3 ka. The timing of displacement is further constrained to before 19.5 ka as evidenced by undeformed spring deposits that are inset into the incised topography of the warp. Coulomb stress calculations validate the hypothesis that the substantial groundwater decline during D-O event 2 unclamped the fault through unloading of vertical stress of the water column. The synchroneity of this abrupt hydrologic change and displacement of the Eglington fault suggests that climatically modulated tectonics operated in the Las Vegas Valley during the late Quaternary.

Madsen et al . question the reliability of calibrated radiocarbon ages associated with human footprints discovered recently in White Sands National Park, New Mexico, USA. On the basis of the geologic, hydrologic, stratigraphic, and chronologic evidence, we maintain that the ages are robust and conclude that the footprints date to between ~23,000 and 21,000 years ago. Madsen et al . ( 1 ) question the veracity of calibrated radiocarbon ages used to constrain the antiquity of human trackways discovered recently at White Sands National Park (WHSA) Locality 2, New Mexico, USA ( 2 ). The ages were derived from seeds of the aquatic plant Ruppia cirrhosa , which they suggest may suffer from hard-water (or reservoir) effects, making them too old, potentially by thousands of years. We were well aware of this possibility, investigated it, and presented several lines of evidence that argued against such a problem. Here we respond to each of their four primary points.

The Younger Dryas (YD) climate event is the preeminent example of abrupt climate change in the recent geologic past. Climate conditions during the YD were spatially complex, and high-resolution sediment cores in the North Atlantic, western Europe, and East Asia have revealed it unfolded in two distinct stages, including an initial stable climatic period between ~ 12.9 and 12.2 ka associated with a weakened Atlantic Meridional Overturning Circulation (AMOC) and a second phase characterized by variable conditions until 11.7 ka as the AMOC recovered. Decades of investigations into the climate of western North America during the YD have failed to identify this stepped phenomenon. Here we present hydroclimate data from paleospring deposits in Death Valley National Park (California, USA) that demonstrate unequivocal evidence of two-stage partitioning within the YD event. High groundwater levels supported persistent and long-lived spring ecosystems between ~ 13.0 and 12.2 ka, which were immediately replaced by alternating wet and dry environments until ~ 11.8 ka. These results establish the mid-YD climate transition extended into western North America at approximately the same time it was recorded by hydrologic systems elsewhere in the Northern Hemisphere and show that even short-lived changes in the AMOC can have profound consequences for ecosystems worldwide.

Early footsteps in the Americas Despite a plethora of archaeological research over the past century, the timing of human migration into the Americas is still far from resolved. In a study of exposed outcrops of Lake Otero in White Sands National Park in New Mexico, Bennett et al . reveal numerous human footprints dating to about 23,000 to 21,000 years ago. These finds indicate the presence of humans in North America for approximately two millennia during the Last Glacial Maximum south of the migratory barrier created by the ice sheets to the north. This timing coincided with a Northern Hemispheric abrupt warming event, Dansgaard-Oeschger event 2, which drew down lake levels and allowed humans and megafauna to walk on newly exposed surfaces, creating tracks that became preserved in the geologic record. —AMS

Portable luminescence readers are exciting new tools that have the potential to rapidly determine the age structure of late Quaternary stratigraphic columns. This is important because high-resolution age profiling can reveal details about the temporal dynamics of climate cause and ecosystem effect, often while researchers are still in the field. In this paper, we compare new portable luminescence reader measurements of total photon counts with a suite of robust, highly resolved ages from middle to late Pleistocene-age paleowetland deposits in the Las Vegas Valley of southern Nevada. Our results show that total photon counts correlate with age, with a quadratic equation providing the best fit to the data. Significant scatter is present in the data, which is likely the result of dose rate variations, multiple sediment sources, and transport mechanisms that include both eolian and fluvial processes. The observed scatter can be reduced significantly using a simple pretreatment procedure involving a 250 μm sieve and neodymium hand magnet to normalize particle sizes and remove magnetic grains. Following this treatment, age estimates based on the reader measurements have an average error of 30 ± 18% when compared against known ages. These findings confirm that portable reader measurements scale with age in paleowetland deposits, allowing its use in establishing rapid, albeit approximate, chronologies for these deposits throughout the American Southwest.

One of the enduring questions in the field of paleohydrology is how quickly desert wetland ecosystems responded to past episodes of abrupt climate change. Recent investigations in the Las Vegas Valley of southern Nevada have revealed that wetlands expanded and contracted on millennial and sub-millennial timescales in response to changes in climate during the late Quaternary. Here, we evaluate geologic evidence from multiple localities in the Mojave Desert and southern Great Basin that suggests the response of wetland systems to climate change is even faster, occurring at centennial, and possibly decadal, timescales. Paleowetland deposits at Dove Springs Wash, Mesquite Springs, and Little Dixie Wash, California, contain evidence of multiple wet and dry cycles in the form of organic-rich black mats, representing periods of past groundwater discharge and wet conditions, interbedded with colluvial, alluvial, and aeolian sediments, each representing dry conditions. Many of these wet-dry cycles date to within the Younger Dryas (YD) chronozone (12.9–11.7 ka), marking the first timeintra-YD hydrologic variability has been documented in paleowetland deposits. Our results illustrate that desert wetland ecosystems are exceptionally sensitive to climate change and respond to climatic perturbations on timescales that are relevant to human society.

A middle to late Pleistocene sedimentary sequence in the upper Las Vegas Wash, north of Las Vegas, Nevada, has yielded the largest open-site Rancholabrean vertebrate fossil assemblage in the southern Great Basin and Mojave Deserts. Recent paleontologic field studies have led to the discovery of hundreds of fossil localities and specimens, greatly extending the geographic and temporal footprint of original investigations in the early 1960s. The significance of the deposits and their entombed fossils led to the preservation of 22,650 acres of the upper Las Vegas Wash as Tule Springs Fossil Beds National Monument. These discoveries also warrant designation of the assemblage as a local fauna, named for the site of the original paleontologic studies at Tule Springs. The large mammal component of the Tule Springs local fauna is dominated by remains of Mammuthus columbi as well as Camelops hesternus , along with less common remains of Equus (including E. scotti ) and Bison . Large carnivorans including Canis dirus , Smilodon fatalis , and Panthera atrox are also recorded. Micromammals, amphibians, lizards, snakes, birds, invertebrates, plant macrofossils, and pollen also occur in the deposits and provide important and complementary paleoenvironmental information. The fauna occurs within the Las Vegas Formation, an extensive and stratigraphically complex sequence of groundwater discharge deposits that represent a mosaic of desert wetland environments. Radiometric and luminescence dating indicates the sequence spans the last ∼570 ka, and records hydrologic changes in a dynamic and temporally congruent response to northern hemispheric abrupt climatic oscillations. The vertebrate fauna occurs in multiple stratigraphic horizons in this sequence, with ages of the fossils spanning from ∼100 to ∼12.5 ka.

Tule Springs Fossil Beds National Monument (TUSK) preserves 22,650 acres of the upper Las Vegas Wash in the northern Las Vegas Valley (Nevada, USA). TUSK is home to extensive and stratigraphically complex groundwater discharge (GWD) deposits, called the Las Vegas Formation, which represent springs and desert wetlands that covered much of the valley during the late Quaternary. The GWD deposits record hydrologic changes that occurred here in a dynamic and temporally congruent response to abrupt climatic oscillations over the last ~300 ka (thousands of years). The deposits also entomb the Tule Springs Local Fauna (TSLF), one of the most significant late Pleistocene (Rancholabrean) vertebrate assemblages in the American Southwest. The TSLF is both prolific and diverse, and includes a large mammal assemblage dominated by Mammuthus columbi and Camelops hesternus . Two (and possibly three) distinct species of Equus , two species of Bison , Panthera atrox , Smilodon fatalis , Canis dirus , Megalonyx jeffersonii , and Nothrotheriops shastensis are also present, and newly recognized faunal components include micromammals, amphibians, snakes, and birds. Invertebrates, plant macrofossils, and pollen also occur in the deposits and provide important and complementary paleoenvironmental information. This field compendium highlights the faunal assemblage in the classic stratigraphic sequences of the Las Vegas Formation within TUSK, emphasizes the significant hydrologic changes that occurred in the area during the recent geologic past, and examines the subsequent and repeated effect of rapid climate change on the local desert wetland ecosystem.

A new species of mastodon from the Pleistocene of western North America, Mammut pacificus sp. nov. is herein recognized, with specimens identified throughout California and from two localities in southern Idaho. This new taxon differs from the contemporaneous M. americanum in having narrower teeth, most prominently in M3/m3, as well as six sacral vertebrae, femur with a proportionally greater mid-shaft diameter, and no mandibular tusks at any growth stage. All known Pleistocene Mammut remains from California are consistent with our diagnosis of M. pacificus, which indicates that M. americanum was not present in California.

Late Pleistocene groundwater discharge deposits (paleowetlands) in the upper Las Vegas Wash north of Las Vegas, Nevada, have yielded an abundant and diverse vertebrate fossil assemblage, the Tule Springs local fauna (TSLF). The TSLF is the largest open-site vertebrate fossil assemblage dating to the Rancholabrean North American Land Mammal Age in the southern Great Basin and Mojave Desert. Over 600 discrete body fossil localities have been recorded from the wash, including an area that now encompasses Tule Springs Fossil Beds National Monument (TUSK). Paleowetland sediments exposed in TUSK named the Las Vegas Formation span the last 250 ka, with fossiliferous sediments spanning ∼100–13 ka. The recovered fauna is dominated by remains ofCamelopsandMammuthus, and also includes relatively common remains of extinctEquusandBisonas well as abundant vertebrate microfaunal fossils. Large carnivorans are rare, with onlyPuma concolorandPanthera atroxdocumented previously. Postcranial remains assigned to the speciesCanis dirus(dire wolf) andSmilodon fatalis(sabre-toothed cat) represent the first confirmed records of these species from the TSLF, as well as the first documentation ofCanis dirusin Nevada and the only known occurrence ofSmilodonin southern Nevada. The size of the recovered canid fossil precludes assignment to other Pleistocene species ofCanis. The morphology of the felid elements differentiates them from other large predators such asPanthera,Homotherium, andXenosmilus, and the size of the fossils prevents assignment to other species ofSmilodon. The confirmed presence ofS. fatalisin the TSLF is of particular interest, indicating that this species inhabited open habitats. In turn, this suggests that the presumed preference ofS. fatalisfor closed-habitat environments hunting requires further elucidation.

The Diamond Valley Lake local fauna from southwestern Riverside County, California is characterized by a classic suite of well-preserved late Pleistocene (Rancholabrean NALMA) vertebrates, including over 100,000 identifiable fossils representing more than 105 vertebrate, invertebrate and plant taxa from 2646 localities. The fauna is the largest open-environment, non-asphaltic late Pleistocene assemblage known from the American southwest. Located within the northern Peninsular Range physiographic province of southern California, the Diamond and Domenigoni Valleys contain bedded silts and clays intercalated with coarse-grained channel fill representing a braided stream environment. These fluvial sediments, yielding AMS dates from ∼19 ka to ∼13 ka, unconformably truncate older silts, clays and an organic black clay at depth. The clay is lacustrine in origin, with AMS dates from ∼46 ka to ∼41 ka. Numerous diagnostic vertebrate remains occur in both of these sediment packages. The Diamond Valley Lake local fauna constitutes a valuable source of new data on the relative density and diversity of late Pleistocene species from a geographic area where such data are largely absent. The assemblage differs dramatically in preservation and composition from other late Pleistocene coastal and desert localities in southern California.

Desert wetlands are keystone ecosystems in arid environments and are preserved in the geologic record as groundwater discharge (GWD) deposits. GWD deposits are inherently discontinuous and stratigraphically complex, which has limited our understanding of how desert wetlands responded to past episodes of rapid climate change. Previous studies have shown that wetlands responded to climate change on glacial to interglacial timescales, but their sensitivity to short-lived climate perturbations is largely unknown. Here, we show that GWD deposits in the Las Vegas Valley (southern Nevada, United States) provide a detailed and nearly complete record of dynamic hydrologic changes during the past 35 ka (thousands of calibrated (14)C years before present), including cycles of wetland expansion and contraction that correlate tightly with climatic oscillations recorded in the Greenland ice cores. Cessation of discharge associated with rapid warming events resulted in the collapse of entire wetland systems in the Las Vegas Valley at multiple times during the late Quaternary. On average, drought-like conditions, as recorded by widespread erosion and the formation of desert soils, lasted for a few centuries. This record illustrates the vulnerability of desert wetland flora and fauna to abrupt climate change. It also shows that GWD deposits can be used to reconstruct paleohydrologic conditions at millennial to submillennial timescales and informs conservation efforts aimed at protecting these fragile ecosystems in the face of anthropogenic warming.

Analysis of a buried deposit in the Diamond Valley of southern California has revealed well-preserved pollen, wood, and diatom remains. Accelerator mass spectrometry dates of 41,200±2100 and 41,490±1380 14C yr B.P. place this deposit in marine isotope stage 3. Diatoms suggest a shallow lacustrine environment. Pollen data suggest that several plant communities were present near the site, with grassland, scrub, chaparral, forest, and riparian communities represented. Comparison with modern pollen suggests similarities with montane forests in the nearby San Bernardino and San Jacinto ranges, indicating vegetation lowering by at least 900 m elevation and temperatures 4°–5°C cooler than today. An increase in high-elevation conifer pollen documents climatic cooling near the profile top. Early-profile diatoms are typical of warm water with high alkalinity and conductivity, whereas later diatoms suggest a higher flow regime and input of cooler water into the system. We suggest that the sequence is part of the cooling phase of an interstadial Dansgaard–Oeschger cycle. Records of the middle Wisconsin period are rare in southern California, but the Diamond Valley site is similar to records from Tulare Lake in the San Joaquin Valley and the ODP Site 893A record from Santa Barbara Basin. It is probable that the Diamond Valley assemblage is a local expression of a vegetation type widespread in the ranges and basins of southwestern California during the middle Wisconsin.

The changes in the sicula of Early Devonian monograptid graptolite populations show a pattern that is described as punctuated stasis: two longer periods of stasis separated by a transitional population of shorter duration between them. The pattern differs from punctuated equilibrium in that anagenesis is involved rather than cladogenesis. The pattern has been observed in several widely separated areas (Nevada, Thuringia, Poland, Ural Mountains) and is an example of collateral evolution in graptolites. In this case, the collateral morphologic change occurs relatively rapidly in a large, globally distributed taxon. The study permits accurate correlation of the horizon of appearance of Monograptus hercynicus Pemer in the upper delta Zone (conodont zonation) of late Lochkovian age (Devonian) on a global scale. The partial integration of these two biostratigraphies constrains correlations between the shelly, shallow-water biofacies and the deeper graptolitic biofacies. The pattern of punctuated stasis has been discovered in Lower Devonian strata of central Nevada, where 22 horizons in sequence were sampled. Statistical analyses were performed on 13 characters encompassing measurements of the sicula, thecae, and rhabdosome. Most of these characters do not exhibit sustained trends or punctuated changes. However, sicular width exhibits an abrupt increase over a short stratigraphic interval and is used to classify members of the group into four quantitatively defined morphotypes. This natural perturbation in the pattern of change is used to divide the lineage into paleospecies. This makes the segments of the lineage easy to identify and imparts stability to the classification and precision in biostratigraphy. The variation studies enable the ranges of the morphotypes to be used to further subdivide the stratigraphic record of the late Lochkovian. □Graptolites, Devonian, Lochkovian, evolution, punctuated stasis, biostratigraphy, MONOGRAPTUS, phylogeny.

Field work recently completed in the Lake Eyre Basin, South Australia, has resulted in the development of a land mammal (marsupial) biostratigraphy of the Etadunna Formation. Whereas traditional interpretations of the age of this sequence suggest it is about 15 m.y. old, new information indicates that the Etadunna likely is 24–26 m.y. old. In either case, it appears possible to document a four-fold fossil mammal zonation of this rock unit at lakes Palankarinna, Kanunka, Pitikanta, and Ngapakaldi, in a composite section of strata that spans at least 30 m. Magnetostratigraphic data for the same succession are generally consistent with the correlation of the Etadunna Formation sites at Lake Palankarinna with of those at lakes Kanunka, Pitikanta, and Ngapakaldi to the north, as based on paleontological information. The magnetic polarity zonation of these Etadunna Formation strata is consistent with a correlation to the world magnetic polarity time scale at about 24–26 m.y. This is the first fine-scal...

The platform element of the conodontAmydrotaxis praejohnsonin. sp. shows statistically significant morphological changes from the base to the top of thedeltaZone (Lower Devonian). The changes occur particularly in mean denticle number, height and character of the anterior denticles, and shape of the basal cavity. The earliest forms have fewer denticles (= 7.1), an enlarged, high anterior denticle, and a broadly flared posterior basal cavity, whereas late forms have more denticles (= 9.4), equal-sized anterior denticles, and a narrow posterior basal cavity. There is no stasis phase in the species history, but rather a mosaic pattern in which the observed changes in the character states are seemingly independent of each other and proceed at variable rates.