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The rapid ecological expansion of grasses with C4 photosynthesis at the end of the Neogene (8-2 Ma) is well documented in the fossil record of stable carbon isotopes. As one of the most profound vegetation changes to occur in recent geologic time, it paved the way for modern tropical grassland ecosystems. Changes in CO2 levels, seasonality, aridity, herbivory, and fire regime have all been suggested as potential triggers for this broadly synchronous change, long after the evolutionary origin of the C4 pathway in grasses. To date, these hypotheses have suffered from a lack of direct evidence for floral composition and structure during this important transition. This study aimed to remedy the problem by providing the first direct, relatively continuous record of vegetation change for the Great Plains of North America for the critical interval (ca. 12-2 Ma) using plant silica (phytolith) assemblages. Phytoliths were extracted from late Miocene–Pliocene paleosols in Nebraska and Kansas. Quantitative phytolith analysis of the 14 best-preserved assemblages indicates that habitats varied substantially in openness during the middle to late Miocene but became more uniformly open, corresponding to relatively open grassland or savanna, during the late Miocene and early Pliocene. Phytolith data also point to a marked increase of grass short cells typical of chloridoid and other potentially C4 grasses of the PACMAD clade between 8 and 5 Ma; these data suggest that the proportion of these grasses reached up to ∼50–60% of grasses, resulting in mixed C3-C4 and highly heterogeneous grassland communities by 5.5 Ma. This scenario is consistent with interpretations of isotopic records from paleosol carbonates and ungulate tooth enamel. The rise in abundance of chloridoids, which were present in the central Great Plains since the early Miocene, demonstrates that the “globally” observed lag between C4 grass evolution/taxonomic diversification and ecological expansion occurred at the regional scale. These patterns of vegetation alteration imply that environmental change during the late Miocene–Pliocene played a major role in the C3-C4 shift in the Great Plains. Specifically, the importance of chloridoids as well as a decline in the relative abundance of forest indicator taxa, including palms, point to climatic drying as a key trigger for C4 dominance.

Colorado's High Plains stand at anomalously high elevations (∼1300-2100 m) for their continental interior setting, but when and why this region became elevated is poorly understood. The Cenozoic history of the High Plains is also likely linked with that of the Rocky Mountains, where the timing and cause(s) of uplift are similarly debated. We present apatite (U-Th)/He (AHe) data for 10 samples from Tertiary intrusives along a ∼200 km west-to-east transect across the High Plains of southeastern Colorado to constrain the timing of exhumation and to gain insight into when and why regional elevation gain occurred. Mean sample AHe dates for the ∼24-22 Ma East Spanish Peak pluton and associated radial dikes from the westernmost High Plains are 18.8 ± 1.4 to 14.1 ± 1.7 Ma, recording substantial postemplacement erosion. AHe results for the mafic to ultramafic Apishapa Dikes (oldest ∼37 Ma, youngest ∼14 Ma) located ∼20-40 km farther north and east on the High Plains range from 12.0 ± 1.4 to 6.2 ± 1.9 Ma, documenting continued exhumation on the western High Plains during the ∼12-5 Ma deposition of the Ogallala Formation farther east and suggesting that the western limit of Ogallala deposition was east of the Apishapa Dikes. In far southeastern Colorado, the Two Buttes lamprophyre was emplaced at 36.8 ± 0.4 Ma and yields a Late Oligocene AHe date of 27.1 ± 4 Ma. Here, the Ogallala Formation unconformably overlies Two Buttes, indicating that the regional ∼12 Ma age for the base of the Ogallala is a minimum age for the exposure of the pluton at the surface. The AHe data presented here document that kilometer-scale erosion affected all of the southeastern Colorado High Plains in Oligo-Miocene time. While exhumation can have multiple possible causes, we favor contemporaneous surface uplift capable of elevating the region to modern heights.

C4 grasses form the foundation of warm-climate grasslands and savannas and provide important food crops such as corn, but their Neogene rise to dominance is still not fully understood. Carbon isotope ratios of tooth enamel, soil carbonate, carbonate cements, and plant lipids indicate a late Miocene–Pliocene (8-2 Ma) transition from C3 vegetation to dominantly C4 grasses at many sites around the world. However, these isotopic proxies cannot identify whether the C4 grasses replaced woody vegetation (trees and shrubs) or C3 grasses. Here we propose a method for reconstructing the carbon isotope ratio of Neogene grasses using the carbon isotope ratio of organic matter trapped in plant silica bodies (phytoliths). Although a wide range of plants produce phytoliths, we hypothesize that in grass-dominated ecosystems the majority of phytoliths will be derived from grasses, and will yield a grass carbon isotope signature. Phytolith extracts can be contaminated by non-phytolith silica (e.g., volcanic ash). To test the feasibility of the method given these potential problems, we examined sample purity (phytolith versus non-phytolith silica), abundance of grass versus non-grass phytoliths, and carbon isotope ratios of phytolith extracts from late Miocene–Pliocene paleosols of the central Great Plains. Isotope results from the purest samples are compared with phytolith assemblage analysis of these same extracts. The dual record spans the interval of focus (ca. 12-2 Ma), allowing us, for the first time, to investigate how isotopic shifts correlate with floral change. We found that many samples contained high abundances of non-biogenic silica; therefore, only a small subset of “pure” samples (>50% of phytoliths by volume) with good preservation were considered to provide reliable carbon isotope ratios. All phytolith assemblages contained high proportions (on average 85%) of grass phytoliths, supporting our hypothesis for grass-dominated communities. Therefore, the carbon isotope ratio of pure, well-preserved samples that are dominated by grass biosilica is considered a reliable measure of the proportion of C3 and C4 grasses in the Neogene. The carbon isotope ratios of the pure fossil phytolith samples indicate a transition from predominantly C3 grasses to mixed C3-C4 grasses by 5.5 Ma and then a shift to more than 80% C4 grasses by 3-2 Ma. With the exception of the Pliocene sample, these isotopic data are broadly concordant with phytolith assemblages that show a general increase in C4 grasses in the late Miocene. However, phytolith assemblage analysis indicates lower relative abundance of C4 grasses in overall vegetation than do the carbon isotopes from the same phytolith assemblages. The discrepancy may relate to either (1) incomplete identification of (C4) PACMAD phytoliths, (2) higher production of non-diagnostic phytoliths in C4 grasses compared to C3 grasses, or (3) biases in the isotope record toward grasses rather than overall vegetation. The impact of potential incomplete characterization of (C4) PACMAD phytoliths on assemblage estimates of proportion of C4, though important, cannot reconcile discrepancies between the methods. We explore hypothesis (2) by analyzing a previously published data set of silica content in grasses and a small data set of modern grass leaf assemblage composition using analysis of variance, independent contrasts, and sign tests. These tests suggest that C4 grasses do not have more silica than C3 grasses; there is also no difference with regard to production of non-diagnostic phytoliths. Thus, it is most likely that the discrepancy between phytolith assemblages and isotope ratios is a consequence of hypothesis (3), that the isotope signature is influenced by the contribution of non-diagnostic grass phytoliths, whereas the assemblage composition is not. Assemblage-based estimates of % C4 within grasses, rather than overall vegetation, are in considerably better agreement with the isotope-based estimates. These results support the idea that, in grass-dominated assemblages, the phytolith carbon isotope method predominantly records shifts in dominant photosynthetic pathways among grasses, whereas phytolith assemblage analysis detects changes in overall vegetation. Carbon isotope ratios of fossil phytoliths in conjunction with phytolith assemblage analysis suggest that the late Neogene expansion of C4 grasses was largely at the expense of C3 grasses rather than C3 shrubs/trees. Stable isotopic analysis of phytoliths can therefore provide unique information about grass community changes during the Neogene, as well as help test how grass phytolith morphology relates to photosynthetic pathway.

Dozens of well-preserved fossil burrow systems in upper Miocene sedimentary rocks of the Ogallala Group at a site in east-central Nebraska record rodent behavior and the subsurface ecology of grasslands just as the modern Great Plains was developing. These burrow systems include one to four entrance or exit tunnels, large underground nesting chambers at depths of several decimeters below ancient land surfaces, and incisor grooves on the walls. Tunnels average 89.8 mm in diameter, a value similar to the burrow diameters of multiple living North American rodents. Chambers vary in shape and typically exceed 500 mm in length; some attain 1000 mm in length. Living marmotine ground squirrels (tribe Marmotini) construct burrow systems of varying degrees of complexity, but they do not engage in shallow subsurface foraging. Extinct members of this group were the most likely excavators of the fossil burrows. In contrast, extant pocket gophers (family Geomyidae) and, presumably their fossil relatives, are obligate subterranean animals that produce linked deep and shallow burrow subsystems, the latter representing their chief foraging strategy. Our results raise issues regarding the relationships between the architecture of fossil rodent burrow systems and aspects of rodent behavior and life history, such as litter size, developmental rates, seasonal torpor, hibernation, and sociality in grasslands. An improved understanding of the burrowing behaviors of ancient rodents will highly complement the growing body of knowledge about the development of grasslands on Earth over time, but truly ichnological analyses of the burrows and burrowing behaviors of extant rodents are much needed.

The Ogallala Formation of Miocene/Pliocene age is present over a large area of the Great Plains in the central US. The formation is composed of mostly clastic material eroded and transported eastward from the Rocky Mountains by eastward flowing rivers and streams. It contains the life-giving water that supports the population and farming activities of this vast area. However, this precious water resource is fast being depleted. The geology and geologic history, groundwater recharge, irrigation development, and water law and management of the region are outlined so as to highlight the nature and extent of the water problems facing that region and point to a more sustainable path. Further strengthening local water management and coordination with state government as well as better interstate coordination of the High Plains states will help sustain this critical water resource for generations to come.

Calcretes (petrocalcic horizons) are widespread indurated accumulations of carbonate in arid and semiarid regions of the world. Many studies have detailed the formation of these calcretes; however, little direct attention has been given to their degradation. The primary objective of this study was to evaluate the continuity of calcrete on the Llano Estacado as a step toward understanding processes of calcrete degradation across the region. The Llano Estacado is a tableland spanning eastern New Mexico and West Texas with a well-developed calcrete, known as the \"caprock,\" exposed as an escarpment around most of its perimeter. We excavated several deep pits along a transect between the caprock on the eastern margin of the Llano Estacado and a similar outcrop on a playa slope. We documented degradation features in the calcrete and compared the extent of degradation with a calcrete in a more arid region (Diablo Plateau). Our findings show a lack of continuity of the calcrete on the Llano Estacado even in a relatively short transect (220 m) between the two outcrops. Individual calcrete fragments showed evidence of partial dissolution. These observations were confirmed in road cuts and pits excavated for road material across the area. Geomorphic position and surface slope control water movement across the landscape and apparently explain both the variation in the presence of degradational features and the distribution of calcrete fragments in the transect. Observations from the more arid Diablo Plateau suggest that calcrete degradation is widespread. The degradation of calcrete on the Llano Estacado makes the caprock unreliable in some areas as a stratigraphic marker in distinguishing the Ogallala and Blackwater Draw Formations.

Size distributions of distal ashfall particles from correlated 10-Ma layers in Nebraska, measured using laser diffraction methods, are lognormal with mode diameters of ∼90 µm. This ashfall is ∼100% bubble-wall shards of rhyolite glass and apparently represents a distal ashfall from an eruption 1400 km away. Measured terminal velocities of these ash particles are 0.2-18 cm/s, consistent with Stokes Law settling of spherical particles with diameters of 9-50 µm. Surface area of the ash particles, measured with gas adsorption, is 20-30 times the surface area of equivalent Stokes spheres. These results highlight the effects of shape and atmospheric drag in distal ashfalls. They also highlight atmospheric transport and fallout of distal ashfall particles, because these deposits resemble many other ashfalls preserved in the Great Plains of North America throughout the Tertiary and Quaternary. Because the ashfalls preserve major mammalian death assemblages, they demonstrate that deposits with modes of optical diameters >100 µm are still hazardous by aerodynamic definitions of lung disease risk and include particles substantially within hazardous PM10 ranges. The aerodynamically fine particle size may lead to substantial aeolian redistribution, causing local thicknesses of >2 m. Overall, the ashfall thicknesses observed are at least several times larger than would be expected based on exponential thinning from the volcano. Shape measurements of distal ash particles may be necessary to assess risk. The possible health risks in the central United States from a future rhyolitic eruption in the western United States may be significant.

Lithic analysis on the southern High Plains traditionally has focused on behavioral inferences based in part on the geographic relationship between disparately distributed high quality toolstone sources. These sources often are used as a proxy for behavior by which hunter-gatherer mobility within the wider region can be modeled based on the presence or absence of specific material types within anyone assemblage. In contrast, utilization of secondary lithic material sources, such as the alluvial gravels, has received relatively little attention. The corollary of this research tradition is that hunter-gatherer resource scheduling strategies appear to be highly structured and necessarily dependant on logistical trips to the high quality toolstone locations. Two sites used for extensive procurement of Ogallala Formation gravels provide an opportunity to examine the range of material available to hunter-gatherer groups at locations between the high quality sources. Characterization and technological analysis of these secondary sources has been achieved through systematic sampling to record the variability in lithic source material, technology, and geomorphic processes. The results are significant for the interpretation of lithic technology at this location and for the wider understanding of hunter-gatherer populations moving through the rich resource areas at the edge of the southern High Plains.