Article

Sedimentology, stratigraphy, and paleoclimate at the late Miocene Coffee Ranch fossil site in the Texas Panhandle

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Abstract

The late Miocene Coffee Ranch fossil assemblage contains some of the earliest evidence of C4 herbivory in North America. However, little is known regarding the environmental setting associated with the fauna, and a general lack of detailed paleoclimate data exists for the late Miocene from the North American continental interior. In this study, the sedimentary environments, stratigraphy, geochronology, and paleoclimate are interpreted for a series of outcrops of the late Miocene Ogallala Formation in the Texas Panhandle that includes the Coffee Ranch locality. Updated magnetostratigraphy coupled with previously published geochronology indicates that all exposed strata were deposited over a ~ 277 kyr interval within chron C3An.2n from ~ 6.42–6.70 Ma. Depositional environments include fluvial channels, floodplain ponds, floodplain paleosols, eolian paleosols, riverine tufa, and reworked volcanic ash. Trunk and tributary fluvial channels are differentiated using channel dimensions, sedimentary structures, and bounding surface architecture, and indicate seasonally-variable discharge. Paleosols from nine pedofacies are described and preserve a spectrum of weakly developed Entisols and weakly to moderately mature Inceptisols and Vertisols. Constitutive mass-balance calculations reveal that mature paleosols formed along distinct pedogenic pathways. Calcic Vertisols accumulated pedogenic carbonate and exhibited either net volumetric dilation or collapse as a result of mineral weathering. In contrast, non-calcic Vertisols show patterns of decalcification and variable degrees of volumetric dilation and collapse. The presence of the paleosols described in this study requires paleoprecipitation between of 900–1150 mm/yr—approximately double modern values—and a westward shift of the continental udic-ustic soil moisture boundary by ~ 400 km during the late Miocene. This finding suggests that climates in the southern midcontinent were not sufficiently arid to select for C4 photosynthetic pathways over C3 photosynthesis in the late Miocene.

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... Coffee Ranch LF (and thus the shared taxa of Optima) has produced evidence of early C4 herbivory in North American horses (Wang et al. 1994;Sharp and Cerling 1998;Passey et al. 2002) and is dated to approximately 6.62 Ma (Passey et al. 2002;Lukens et al. 2017). Pedogenic analysis sug gests an understory of ≤25% C4 biomass Koch 2003, 2004) in a climate that was considerably wetter than today (900-1150 mm/yr) and at least moderately warmer (Lukens et al. 2017). ...
... Coffee Ranch LF (and thus the shared taxa of Optima) has produced evidence of early C4 herbivory in North American horses (Wang et al. 1994;Sharp and Cerling 1998;Passey et al. 2002) and is dated to approximately 6.62 Ma (Passey et al. 2002;Lukens et al. 2017). Pedogenic analysis sug gests an understory of ≤25% C4 biomass Koch 2003, 2004) in a climate that was considerably wetter than today (900-1150 mm/yr) and at least moderately warmer (Lukens et al. 2017). These results are consistent with eco system reconstructions by Fraser and Theodor (2013), who estimated a mean annual precipitation (MAP) of 992 mm/ yr based on the proportion of hypsodont ungulates in the assemblage. ...
... These results are consistent with eco system reconstructions by Fraser and Theodor (2013), who estimated a mean annual precipitation (MAP) of 992 mm/ yr based on the proportion of hypsodont ungulates in the assemblage. Though grasses were undoubtedly present, the high degree of hypsodonty in the Coffee Ranch and Optima faunas is thought to be a result of increased eolian sand and silt upon brushland and woody savannah vegetation (Fox and Koch 2004;Fraser and Theodor 2013;Lukens et al. 2017). Elsewhere, hypsodonty has also been consid ered more closely related to open habitat foraging than to grass consumption, at least in eastern hemisphere ungulates (Mendoza and Palmqvist 2007). ...
Article
The Optima Local Fauna represents an important glimpse into the ecological transition between savannah and grassland during the late Miocene (Hemphillian) of what is now the southcentral Great Plains of North America. Though dominated by horses, herbivores from the Optima are morphologically diverse, bearing adaptations for both browsing and grazing lifestyles. Likewise, the carnivorans show similar ranges of size and presumed dietary behavior. In this study, we used carbonate isotope, mesowear, and tooth breakage and wear analyses to investigate the dietary complexity of mammals from a single site collected by the Oklahoma Museum of Natural History. Seventeen taxa were analyzed, including five perissodactyls (Teleoceras hicksi, Dinohippus interpolatus, Neohipparion eurystyle, Nannippus ingenuus, and Astro�hippus ansae), four artiodactyls (Texoceros guymonensis, Pediomeryx hemphillensis, Megatylopus matthewi, and Platy�gonus sp.), a single proboscidean (Mammut sp.), two rodents (Dipoides indet. and Umbogaulus monodon), and five carni�vorans (Agriotherium schneideri, Amphimachairodus coloradensis, Borophagus secundus, Eucyon davisi, Pliotaxidea cf. nevadensis). Both stable isotope analysis and dental mesowear indicate a broad dietary partitioning occurred among the Optima herbivores, where the artiodactyls were identified as mixed feeders and the perissodactyls were recovered as grazers. In the carnivorans, the large felid Amphimachairodus coloradensis was a hypercarnivore with limited tooth breakage and an enriched δ13C signature, indicating low carcass utilization and a prey preference for horses. The canids had a more generalized diet, with B. secundus showing a greater proportional consumption of carcasses through a higher tooth breakage rate. The large ursid Agriotherium schneideri is here interpreted as an omnivore based on depleted δ13C values. Overall, we found evidence for a diversity of dietary niches in both carnivores and herbivores during the late Hemphillian in Oklahoma, likely driven by the expansion of grasslands in the region.
... These processes are important to consider in the geologic record of the southwestern portion of the North American Great Plains, as dissolution of subsurface evaporites has occurred throughout the Neogene and Quaternary (Gustavson et al., 1982;Goldstein and Collins, 1984;Gustavson and Budnik, 1985;Gustavson, 1986). The presence of tufas and groundwater-mediated carbonates at Neogene localities in the Texas Panhandle (Lukens et al., 2017) and the Meade Basin of southwest Kansas (Lukens et al., 2019) suggests that groundwater discharge could have contributed to the development of localized, landscapescale saline and/or sodic soils at critical points in the C 3 -C 4 transition (Fox and Koch, 2003;Fox et al., 2012). ...
... In this paper, we focus on the late Miocene (6.4-6.7 Ma) Coffee Ranch locality in the Texas Panhandle ( Fig. 1), which has previously been interpreted to have mostly (~75%) C 3 landcover with a laterally homogenous understory of C 4 grass (Fox and Koch, 2004). We note that the study by Fox and Koch (2004) predates a more detailed analysis of the stratigraphy and paleopedology at Coffee Ranch (Lukens et al., 2017); therefore, the interpretations of low abundance and low lateral variability in C 4 vegetation can now be more rigorously tested. We reconstruct plantsensitive colloidal properties of paleosols using pedotransfer functions that relate whole-soil elemental composition to chemical and physical pedogenic variables. ...
... Paleomagnetic analyses indicate that all exposed strata were deposited over a ~ 277 kyr interval within chron C3An.2n from ~6.42-6.70 Ma (Lindsay et al., 1975;Lukens et al., 2017). ...
Article
Modern physical and chemical soil properties can favor or exclude C3 and C4 plants, yet little is known regarding these relationships from deep-time records that track the evolution and expansion of C4 vegetation. In this study, we used a multi-proxy approach to reconstruct vegetation (C3 vs. C4 biomass) and pedogenic properties (soil alkalinity, salinity, sodicity, and texture) from paleolandscapes at Coffee Ranch, Texas, a site from which fossil horses provide the earliest evidence for C4 herbivory in the Great Plains of North America. Local proportion of C4 biomass was assessed using stable carbon isotope ratios of calcium carbonates (δ¹³Ccc) and organic matter (δ¹³Com) analyzed on four different paleosol types, freshwater tufa, and reworked carbonate nodules in fluvial channel lags. Using a Monte Carlo uncertainty analysis, we interpret δ¹³Ccc (range = −8.5 to −5.2‰ VPDB) and δ¹³Com values (range = −25.9 to −24.2‰ VPDB) to be consistent with C4 biomass low in abundance and variability at the study site, but with large uncertainties that would be overlooked using simple linear mixing model approaches. Paleo-pedogenic properties were reconstructed using pedotransfer functions and provide evidence of possible salinity and sodicity in two of five paleosol profiles. However, saline-sodic conditions and soil texture were not correlated with δ¹³C values, contrary to some modern mixed C3-C4 biomes. Using late Miocene CO2 and paleoclimate model reconstructions, we argue that conditions were at or near crossover thresholds favoring C4 over C3 photosynthesis in the Great Plains despite the low abundance of C4 vegetation across the paleolandscapes. This study presents evidence that abiotic factors that select for C4 plants in modern systems—high growing season temperature, low atmospheric CO2, salinity-sodicity, and soil texture—were less influential in the late Miocene than biotic factors (i.e., ecological feedbacks) that suppressed C4 plants prior to their increase in abundance in the Great Plains in the Pliocene.
... Paleosols were formed on landscapes in the geological past, and as such recorded copious environmental information (Retallack, 2007). Paleopedology often involves comparisons with modern analogues, which are then used to reconstruct the paleoenvironment and paleoclimate qualitatively and quantitatively (Maher, 2011;Lukens et al., 2017;Driese et al., 2018). The research on the pedogenic features of paleosols is an important approach in paleopedology, for soil development is related to environmental conditions such as precipitation and temperature. ...
... Generally, the more degree of pedogenesis for the paleosols, the wetter and warmer for the represented paleoclimate. In the past decade, quantitative reconstruction of paleoclimate has gradually become the focus of paleosol studies (Sheldon and Tabor, 2009;Beverly et al., 2017;Lukens et al., 2017;Li et al., 2016Li et al., , 2018. The mean annual precipitation (MAP) and the mean annual temperature (MAT) during the period of paleosol formation can be calculated using some empirical relationships established by the pedogenic indices of modern soil and the characteristics values of modern climate, e.g., bulk geochemical data in soil can be used to estimate paleoclimate using the proxies of (K 2 O + Na 2 O)/Al 2 O 3 , (K 2 O + Na 2 O + CaO + MgO)/Al 2 O 3 , and chemical index of alteration (CIA) (Sheldon and Retallack, 2002;Retallack 2013). ...
... To compare pedogenic intensities a normalized pedogenic index (NPI) was calculated. Multi proxies for pedogenic properties were used to calculate the NPI, and they were classified as two categories, i.e., ones that usually increase with pedogenic process such as clay, MS, CIA, Rb/ Sr, Ba/Sr, and total rare earth element concentration (ΣREE); and ones that usually decrease with pedogenic process such as CaCO 3 , Sr, (K 2 O + Na 2 O)/Al 2 O 3 , CaO/Al 2 O 3 , and (K 2 O + Na 2 O + CaO + MgO)/Al 2 O 3 (Liu, 1985;Lü et al., 1994;Retallack, 2001;Zhang et al., 2007;Sheldon and Tabor, 2009). The B horizon, which reflects relatively stable and longterm pedogenesis is usually used to estimate the degree of soil development and its correlation with climate (Sheldon and Tabor, 2009). ...
Article
The pedogenic features of Holocene paleosols and their paleoclimatic significance in the arid upper Minjiang River valley of the eastern Tibetan Plateau, southwest China, were investigated using elemental geochemistry, particle size distribution, and magnetic susceptibility indices in combination with the min–max normalization and paleoclimate reconstruction methods. Paleosols developed in a semi-humid environment and were classified as Inceptisols and Entisols. Pedogenic intensity of these paleosols was higher in the Middle Holocene and lower in the Early and Late Holocene. Mean annual precipitation (MAP) and mean annual temperature (MAT) inferred from the paleosols formed in the Middle Holocene are ~ 700 mm and ~ 13 °C, respectively. Modern climate is cooler and drier than that of the Middle Holocene and resembles that of the Early Holocene and the period after the last glacial maximum. Climatic changes in the study region were sensitive to the Asian monsoon evolution, which was controlled by orbital forcing. Relative warmer and wetter conditions perhaps triggered the ancient cultural immigration and prosperity at ~ 6000 cal. yr BP in the arid valley region.
... For most samples of the LFF, PPM 1.0 yields higher MAP values compared to the estimations from the CIA-K; however, in the KKF MAP (CIA-K) are greater than those for the MAP (PPM1.0) . Such differences have been reported by other authors (Lukens et al., 2017(Lukens et al., , 2019, but are yet poorly understood. Although there is variability between each proxy, they are within standard error margins, with the exception of one Btv horizon of the P2a in LFF that indicates the wettest (perhumid, 2338 mm/yr − 1 ) conditions. ...
Article
The Eocene Las Flores and Koluel-Kaike formations in southern Patagonia (~48° S, Golfo San Jorge Basin, Argentina) are pedogenically modified fluvial and fluvio-eolian successions, respectively, which document early-middle Eocene environments at mid-paleolatitudes in the Southern Hemisphere. In order to reconstruct the paleoenvironment for the Las Flores and Koluel-Kaike formations, we performed a multiproxy and coordinated study of sediments and paleosols of both units, considering abiotic components. Using detailed sedimentology and paleopedology (macro- and micromorphology), bulk paleosol geochemistry and clay mineralogy, and organic carbon concentrations and stable isotope (δ¹³Corg) compositions, we show that the Las Flores and Koluel-Kaike paleosols are overall Ultisol-like paleosols, mineralogically and chemically consistent with a high to high-moderate degree of weathering, and developed on different parent materials (sedimentary with sandy and silty texture vs. silty volcaniclastic). Climate proxies and a comparison with modern Ultisols with similar features suggest that these paleosols formed under a broadly tropical-temperate and humid-subhumid climate with distinct seasonality. Overall, these combined data record long-term environmental conditions during the Paleogene (early-middle Eocene), and preserve a record of Eocene terrestrial climate in the Southern Hemisphere. This research is relevant for understanding latitudinal climatic gradients during warm periods like the Eocene, a key knowledge gap for future predictions, and these sites are particularly important because mid-latitude reconstructions in the Southern Hemisphere are the poorest resolved.
... In North China, fossil mammal tooth and soil carbonate carbon-isotope data record the appearance of C 4 grasses at ∼8 Ma (Passey et al., 2009;). On the North American Great Plains, paleosol records indicate a multi-stage increase in C 4 biomass from the early Late Miocene into the Early Pleistocene (Fox et al., 2011;Lukens et al., 2017). Thus, C 4 grassland expansion since ∼11 Ma has been characterized by regionally different patterns . ...
Article
C4-grasslands are known to have first expanded globally in the Late Miocene, and recent research has hinted at a second expansion phase during the Early Pliocene; however, the geographic extent of this second event and its driving force are debated. In this study, we present organic carbon-isotopic evidence from a high-resolution drillcore in North China spanning the interval from ∼8.0 Ma to the present. A prominent positive δ¹³Corg shift at ∼4.1 Ma marks C4-grassland expansion on the North China Plain, and rapid oscillations of >6‰ from ∼3.2 to 2.2 Ma imply large shifts between C3- and C4-dominated floras. Our δ¹³Corg profile mirrors carbon isotope shifts in loess-red clay deposits of the Chinese Loess Plateau, confirming the regional nature of this event. Furthermore, coeval carbon isotope records from Africa, North America, and South America provide evidence that the Early Pliocene C4-grassland expansion was global in scale and temporally distinct from the Late Miocene event. Whereas the Late Miocene event may have been related to aridification and altered fire regimes, this mechanism cannot account for the Early Pliocene expansion owing to contemporaneous increases in humidity at many Northern Hemisphere sites. We hypothesize that C4-grassland expansion in the Early Pliocene was triggered by falling atmospheric CO2 levels, as evidenced by multiple pCO2 proxy records and supported by quantum photosynthetic efficiency modeling.
... Lithofacies were assigned using standard nomenclature (Miall 1978(Miall , 2013 and methods similar to recent sedimentology studies at other Neogene localities in the Great Plains (Joeckel et al. 2014;Lukens et al. 2017a). Sedimentary deposits exhibiting soil features (paleosols) were described using a modification of U.S. Department of Agriculture-Natural Resources Conservation Service (USDA-NRCS) field methods (Schoeneberger 2002), and include observations of horizonation, ped structure, color, authigenic and translocated minerals, grain size, bounding contacts, root traces, and invertebrate trace fossils, where present. ...
Article
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Terrestrial paleoenvironmental reconstructions from the Pliocene Epoch (5.3–2.6 Ma) of the Neogene Period are rare from the North American continental interior, but are important because they provide insight into the evolutionary context of modern landscapes and ecological systems. Pliocene marine records indicate that global climate was warmer and atmospheric pCO2 was higher than pre industrial conditions, spurring efforts to understand regional climate and environmental variability under conditions potentially analogous to future warming scenarios. In this study, we investigate sedimentary environments and paleoclimate conditions from the Meade Basin of southwest Kansas, a moderately sized basin formed from dissolution and withdrawal of deep evaporites. Pliocene intervals of the Meade Basin have yielded classic faunal assemblages representing the early to middle Blancan North American Land Mammal Age (~ 4.5–3.2Ma).We reconstruct the paleoenvironments using a multidisciplinary approach of lithofacies analysis, paleopedology, and ichnology. The stratigraphic interval we examined is bounded by large scale, fluvial trunk channels that show paleocurrent trends to the south-southwest—tangential to modern drainages—likely due to local halotectonic subsidence during the Neogene. The stratigraphic interval between these fluvially dominated phases consists of palustrine landscapes with temporally and laterally variable subaqueous and subaerial facies. Paleosols are abundant; however, most pedotypes are poorly to variably drained, and so their elemental compositions do not reflect local climate state. The few mature, oxidized, and relatively well-drained paleosols observed contain elemental signatures consistent with subhumid climate conditions. Frequent and recursive ponding events are discerned through the tiering of burrows (Camborygma isp.) similar to those produced by modern freshwater decapod crustaceans (i.e., crayfish). The drivers of these flooding events are most likely episodic halotectonic subsidence and groundwater discharge, though influence from intervals of relatively wetter climate cannot be ruled out. By the late middle Pliocene, landscapes returned to fluvially dominated environments as sedimentation began to outpace accommodation. Our results collectively indicate that climate was likely wetter than modern conditions in the early to middle Pliocene in the western Great Plains, contrary to forecasts for the region under current pCO2-driven warming.
... Bulk-measured CaO from B horizons is already used in a number of widely utilized paleosol proxies for climate, including the chemical index of altera- tion minus potassium (CIA-K; Sheldon et al. 2002), CALMAG (Nordt and Driese 2010b), the paleosol weathering index (PWI; Gallagher and Sheldon 2013), and the paleosol-paleoclimate model (PPM 1.0 ; Stinchcomb et al. 2016). These proxies are now routinely applied throughout the geologic record (Nesbitt and Young 1982;Maynard 1992;Kahmann and Driese 2008;Beverly et al. 2015;Nordt et al. 2015;Driese and Ashley 2016;Sheldon et al. 2016;Lukens et al. 2017aLukens et al. , 2017bDriese et al. 2018). The extension of bulk geochemical indices to the reconstruction of edaphic variables is necessary in order to advance the growing discipline of deep-time Critical Zone science. ...
Article
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Soil pH is essential for understanding weathering, nutrient availability, and biological-edaphic relationships. However, standard pH measurement on soils requires friable material, thereby excluding most paleosols. In this article, bulk geochemical proxies for pH are developed for soil B horizons using indices that track pH-dependent mineralogical transformations. Geochemical relationships within a continental-scale modern soil data set (n=619) reveal a close association between pH and log-transformed CaO and little influence of refractory oxides on pH. These results guided the formulation of three geochemical indices that consist of ratios of Fe2O3, TiO2, and Al2O3 to CaO, herein referred to as FeCa, TiCa, and AlCa. After careful screening for anthropogenic influence, pedotransfer functions relating each index to pH were derived using sigmoidal regressions on a calibration data set (n=305). Each index has similar predictive capacity for pH (r2=0.70–0.74, root mean square error=0.83–0.88). The models were cross-validated on an external testing set (n=130), which returned root mean square prediction errors (RMSPEs) similar to regression results (RMSPE=0.81–0.86). While soil pH shows a significant correlation with mean annual precipitation, partial correlation analysis of FeCa, TiCa, AlCa, and a number of widely used paleosol weathering indices revealed that the relationship between B horizon composition and pH is significant, even when climate is held constant. This finding implies that bulk geochemical indices used in pedotransfer functions for climate primarily track pH, which in turn responds to climate state. A case study is examined, where the pH transfer functions were applied on a succession of Triassic alluvial paleosols that experienced a large range of soil-forming conditions. Reconstructed pH values closely track interpreted vegetation, climate, and pedogenesis. These pedotransfer functions offer a new pathway to estimate an ecologically significant parameter in deep-time Critical Zones.
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A new paleovegetation reconstruction based on both phytolith and paleosol carbon isotopic data is presented for the middle–late Miocene (11.2–9.5Ma) of southwestern Montana. Both sources of data indicate an open-habitat, C3 grassland ecosystem with a small C4 component (b10%). However, while a penecontemporaneous record from further south within the same region indicated significant spatial and temporal variability in C4 abundance, the new record indicates relatively little change either in space or in time. Because paleoclimatic conditions reconstructed at these two sites are the same,we hypothesize that other ecological factors controlled C4 abundance across the region. The relative abundance of C3 and C4 plants in the Miocene is the same as in modern environments at both sites, suggesting that their distribution was likely established by the middle–late Miocene. This is consistent with the idea that paleoelevation was also similar, which agrees with independent constraints on the regional tectonic history. The Montana records indicate a more limited distribution of C4 vegetation than is observed in the Great Plains and elsewhere globally, as well as variable timing in the shift to C4 dominance in ecosystems. Taken together, this local, regional, and global variability during a time of little or no climate change suggests that C4 distribution and expansion was driven by local, rather than global, environmental factors.
Chapter
The patterns of marine magnetic anomalies for the Late Cretaceous through Neogene (C-sequence) and Late Jurassic through Early Cretaceous (M-sequence) have been calibrated by magnetostratigraphic studies to biostratigraphy, cyclostratigraphy, and a few radiometrically dated levels. The geomagnetic polarity time scale for the past 160 myr has been constructed by fitting these constraints and a selected model for spreading rates. The status of the geomagnetic polarity time scale for each geological period is summarized in Chapters 11–22 as appropriate. PRINCIPLES Magnetic field reversals and magnetostratigraphy The principal goal of magnetostratigraphy is to document and calibrate the global geomagnetic polarity sequence in stratified rocks and to apply this geomagnetic polarity time scale for high-resolution correlation of marine magnetic anomalies and of polarity zones in other sections. The basis of magnetostratigraphy is the retention by rocks of a magnetic imprint acquired in the geomagnetic field that existed when the sedimentary rock was deposited or the igneous rock underwent cooling. The imprint most useful for paleomagnetic directions and magnetostratigraphy is recorded by particles of iron oxide minerals. Most of the material in this chapter is updated from summaries in Harland et al. (1990) and Ogg (1995). Excellent reviews are given in Opdyke and Channell (1996) for magnetostratigraphy and McElhinny and McFadden (2000) for general paleomagnetism.
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The late Miocene palaeorecord provides evidence for a warmer and wetter climate than that of today, and there is uncertainty in the palaeo-CO 2 record of at least 200 ppm. We present results from fully coupled atmosphere-ocean-vegetation simulations for the late Miocene that examine the relative roles of palaeogeography (topography and ice sheet geometry) and CO2 concentration in the determination of late Miocene climate through comprehensive terrestrial model-data comparisons. Assuming that these data accurately reflect the late Miocene climate, and that the late Miocene palaeogeographic reconstruction used in the model is robust, then results indicate that: 1. Both palaeogeography and atmospheric CO2 contribute to the proxy-derived precipitation differences between the late Miocene and modern reference climates. However these contributions exibit synergy and so do not add linearly. 2. The vast majority of the proxy-derived temperature differences between the late Miocene and modern reference climates can only be accounted for if we assume a palaeo-CO2 concentration towards the higher end of the range of estimates.
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Based on slacks-based measure (SBM) mechanism, this paper builds a dynamic indicator to evaluate the ecological economic transition in Chinese provinces since the reform. The measurement indicates that Chinese ecological economic development could be divided into five stages since the reform: the poor development during 1985-1992 and the beginning of this century (2003-2009), and the good performance in early reform period (1981-1984), the period of 1993-2002 and in present years (2010-2012). The evaluation indicator of ecological transition, produced from the economic theory, takes both the growth quality and speed into account and thus could be employed as a more appropriate alternative of conventional GDP evaluation criterion. The heterogeneity and inconsistency of evaluation for each province revealed that Chinese ecological economic transition is still at the unstable early stage and more appropriate environmental regulations should be implemented to support the long-run process of ecological economic development in China.
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The Ash Hollow Formation (AHF) of the Ogallala Group is an important sedimentary archive of the emergence of the Great Plains and it contains major groundwater resources. Stratal patterns of constituent alluvial lithofacies demonstrate that the AHF much more heterogeneous than is commonly assumed. Very fine- to fine-grained sandstone dominate overall, chiefly lithofacies Sm (massive to locally stratified sandstone). Stacked, thin sheets of Sm with accretionary macroform surfaces are common, indicating that many sandstone architectural elements originated as compound-bar deposits in dominantly sand-bed streams. Channel forms are difficult to identify and steep cutbanks are absent. Multiple units of lithofacies Sm show dense, and sometimes deep, burrowing by insects well above water tables under ancient floodplains. Massive, pedogenically modified siltstones (Fm), which compose floodplain fines architectural elements, are subsidiary in volumetric abundance to sandstones. Paleosols in these siltstones lack evidence for well-developed B horizons and advanced stages of maturity. Thin lenses of impure carbonate and laminated mud (lithofacies association Fl + C), which appear in most exposures, are deposits of ponded water in abandoned channels. Paleosols, ponded-water elements, and large vertebrate burrows in both Sm and Fm indicate that episodes of floodplain deposition, bar accretion, and channel filling were regularly followed by intervals of nondeposition on floodplains and by channel migration and abandonment. This study documents a major downdip change in the Ogallala Group overall, from source-proximal gravelly successions in the Wyoming Gangplank and deep, narrow paleovalley fills extending eastward into the Nebraska Panhandle. The lithofacies composition, stratigraphic architecture, and stratal dimensions of the AHF in the present study area are compatible with the planform geometries and floodplain soils of modestly-sized, sandy, low-sinuosity braided streams in Nebraska today, namely the modern North, South, and Middle Loup Rivers, rather than being the signatures of “big rivers.”
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Equids from the late Miocene (Hemphillian North American Land Mammal Age) of Texas evolved C4 dominated diets earlier than their Nebraska counterparts, leading researchers to hypothesize a northward expansion of C4 grasses between 8 and 6 Ma. However, competing hypotheses from multiple paleoecological proxies support one of three hypotheses: that northward C4 expansion occurred at the expense of C3 woodlands, C3 grasslands, or was not occurring during the Hemphillian. We test these alternative hypotheses by comparing the ecosystems of Hemphillian Texas and Nebraska (6.3–7.5 Ma) using hoofed mammal dietary reconstruction (hypsodonty and digestive strategy) and by incorporating published tooth enamel isotopes, paleosol isotopes, and phytolith assemblage data. We also use hypsodonty, mesowear, and microwear in a detailed comparison of two localities that are roughly contemporary (Cambridge, Nebraska at 7 Ma and Coffee Ranch, Texas at 6.6 Ma) to rule out time averaging as an explanatory variable. Through analogy with modern African communities, we reconstruct Texas and Nebraska ecosystems as bushland and woodland, respectively. Using the proportions of hypsodont taxa we estimate mean annual precipitation values of 1217 mm/year for Nebraska and 1368 mm/year for Texas. Using rarefaction we also do not find differences in the richness of brachydont, hypsodont, and hindgut fermenting taxa. We therefore conclude that both ecosystems were largely similar in hoofed mammal faunal structure during the late Miocene. When compared to published paleosol and phytolith data, which show no differences in C3/C4 biomass among latitudes, our results allow us to reject hypotheses of northward C4 expansion during the late Miocene. We therefore suggest that the equid enamel isotopes may have been biased by two potential factors: the primary reliance on a single locality of appropriate age and possible behavioural lag in C4 feeding among Nebraska equids. These results suggest that the formation of a modern C3/C4 latitudinal gradient occurred later than is previously suggested by equid enamel carbon isotopes.
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The Early Eocene Climatic Optimum (EECO) has been characterized as a prolonged warm event indicated by increased atmospheric pCO2, temperatures, precipitation, and biological turnover. A new paleoenvironmental reconstruction using integrated pedological, geochemical, and isotopic data from the Green River Basin (Green River/Great Divide region) provides a high-resolution record of environmental and climatic change throughout the EECO. Our reconstruction indicates that this region, and likely much of the margin of paleolake Gosiute, was a stable, fluvially-controlled floodplain environment with evidence of large scale continuous soil development and features comparable with modern Alfisols (temperate forest soils). Regional climate data from multiple proxies indicates that the period was warm-temperate and semi-arid to sub-humid, with a peak interval from about 51.5–50.9 Ma that exhibits significantly warmer (~ 7 °C) and wetter (~ 750 mm yr− 1) conditions, resulting in major changes to the local weathering regime. Isotopic analyses also indicate a rapid increase to high atmospheric pCO2 values (~ 1700 ppmV) and a shift in the δ13C composition of pedogenic carbonates during this peak interval that appear to define and provide a cause for this significant regional response to global climatic change. The new data, when combined with foraminiferal δ13C records, are consistent with CO2 ventilation from a deep marine reservoir source. This multi-proxy reconstruction suggests that the EECO may have had a superimposed “peak” of climatic and ecological change on land.
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The origin and spread of grasslands is one of the key evolutionary events in the Cenozoic, which is characterized by long-term cooling and drying climatic conditions. One way to examine the relationship between vegetation and climate is to study the isotopic composition of organic carbon preserved in paleosols. Paleosols that preserve organic matter in the form of carbonaceous root traces provide direct evidence of the proportion of C3 to C4 biomass that grew in the soil, in contrast with pedogenic carbonate δ13C values, which may also reflect complicating factors including mixing from atmospheric δ13C in low productivity ecosystems. A high temporal and spatial resolution reconstruction of past ecosystems was derived from thirty-five paleosols in a thirty-four meter section of the Sixmile Creek Formation at Timber Hills, Montana (USA) that was deposited during the Miocene (10.2–8.9 Ma ago). Phytoliths were extracted from paleosol samples to compare vegetation assemblages to inferences based on isotopic compositions, with both proxies giving similar results. Isotopic results from organic matter indicate both a small component of C4 photosynthetic plants locally prior to their regional expansion to dominance in the late Miocene through the early Pleistocene, and large variation in the abundance of C4 plants (0–25%) in this ecosystem both laterally and on a 100 Kyr timescale. In contrast, pedogenic carbonate δ13C values from this site indicate a high proportion of C4 photosynthesis that is at odds with both phytolith and δ13Corg results, suggesting that the carbonate values are biased by diagenesis or diffusion of atmospheric CO2, and that a similar issue may impact previous paleovegetation reconstructions based on pedogenic carbonates. Quantitative reconstructions of mean annual temperature and mean annual precipitation indicate little local variability through time and that the fluctuations in C4 proportion were not climatically driven. Instead, the variable proportion of C4 photosynthesis is best explained by ecosystem-scale variables such as succession, fluvial avulsion, and fire regime.
Article
A 617 site palaeobotanical dataset for the Mid to Late Miocene is presented. This dataset is internally consistent and provides a comprehensive overview of vegetational change from 15.97 to 5.33 Ma. The palaeobotanical dataset has been translated into the BIOME4 classification scheme to enable direct comparison with climate model outputs. The vegetation change throughout the Langhian, Seravallian, Tortonian and Messinian is discussed. The data shows that the Langhian, which includes the end of the Mid-Miocene Climatic Optimum, represents a world significantly warmer than today. The high northern latitudes were characterised by cool-temperate forests, the mid-latitudes by warm-temperate mixed forests, the tropics by tropical evergreen broadleaf forests and Antarctica by tundra shrub vegetation. Cool-temperate mixed forest existed during the Seravallian in the high northern latitudes, a reduction in warm-temperate mixed forests in the mid latitudes and a loss of tundra on Antarctica was noted. Tortonian vegetation distribution indicates that further cooling had occurred since the Seravallian. The major changes in vegetation include the first evidence for cold taiga forest in the high northern latitudes and a further reduction of warm-temperate mixed forests. By the Messinian, this cooling trend had eliminated warm-temperate mixed forests from the western USA and Australia and had formed mid-latitude deserts. Despite the cooling trend throughout the Mid to Late Miocene, the vegetation distribution of all four reconstructed stages reflect the vegetation of a world warmer than the pre-industrial conditions. The latitudinal distribution of bioclimatic zones suggests that the latitudinal temperature gradient for the Langhian would have been significantly shallower than at present and has gradually, but asymmetrically, become more modern towards the end of the Miocene. First the southern hemisphere distribution of bioclimatic zones became more modern, probably due to the climatic effects of a fully glaciated Antarctica. The northern hemisphere bioclimatic zone gradient continued to be shallower than modern throughout the Miocene and slowly became more modern by the Messinian.
Article
The Hemphillian Land Mammal Age was defined by Wood et al. (1941) on local faunas from the Coffee Ranch of Hemphill County and from near Higgins, in Lipscomb County, Texas. Both large and small mammals of the Coffee Ranch are known but only a tentative list of the megafauna of Higgins has been published (Schultz, 1977), and no small mammals have been reported. Small mammal fossils collected at the Higgins B Quarry in 1984 include eight taxa, two of which are new (the chiropteran Pizonyx wheeleri, sp. nov., and the geomyid rodent Pliosaccomys higginsensis, sp. nov.). Comparisons of both large and small mammals of Coffee Ranch and Higgins local faunas show few genera and few species in common. The Higgins local fauna is early Hemphillian, with numerous Clarendonian elements. The Coffee Ranch local fauna is early late Hemphillian. It is urged that the type locality and type local fauna of the Hemphillian Land Mammal Age be restricted to the Coffee Ranch alone.New material from the Coffee Ranch includes specimens of Cupidinimus sp. and permits better characterization of the eomyid, Comancheomys rogersi.
Article
Many interpretations of oxygen levels in the Precambrian atmosphere use interpretations of Fe distributions measured in paleosols. This article addresses the current lack of knowledge concerning "baseline" Fe translocations in modern soil analogs for the 2.25-Ga Hekpoort paleosol at Waterval Onder, South Africa, as well as the lack of understanding of diagnostic petrographic features related to Fe translocation preserved in the Precambrian paleosols. Petrographic features related to Fe translocations include redox-related Fe depletions and enrichments, and possibly Fe-Mn nodules, which compare favorably with similar pedogenic features in Paleozoic paleo-Vertisols and modern analog Vertisols. Additionally, preservation of sepic-plasmic microfabrics in the Hekpoort paleosol, which are characteristic of clay soils experiencing shrink-swell, supports previous paleo-Vertisol interpretations. Total Fe losses in Vertisols are 10%-50%; losses increase with increasing soil age and with increasing mean annual precipitation but appear to be independent of parent material differences (unconsolidated sediments, sedimentary rocks, mafic rocks). Total Fe losses in Paleozoic paleo-Vertisols are comparable to those of Vertisols. Nearly complete (75%-99%) loss of total Fe characterizes the upper 220 cm of the Hekpoort paleosol, if the underlying basalt is assumed to be the parent material. The apparent greater mobility of Fe in the Hekpoort paleosol most likely reflects the lower (but not anoxic) PO2 conditions postulated for this time by some researchers but could also indicate oxic conditions and the presence of a significant terrestrial biomass as a source of organic ligands that enhanced Fe mobility, which has also been recently proposed.
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The objectives of this synthesis are (1) to review the factors that influence the ecological, geographical, and palaeoecological distributions of plants possessing C4 photosynthesis and (2) to propose a hypothesis/model to explain both the distribution of C4 plants with respect to temperature and CO2 and why C4 photosynthesis is relatively uncommon in dicotyledonous plants (hereafter dicots), especially in comparison with its widespread distribution in monocotyledonous species (hereafter monocots). Our goal is to stimulate discussion of the factors controlling distributions of C4 plants today, historically, and under future elevated CO2 environments. Understanding the distributions of C3/C4 plants impacts not only primary productivity, but also the distribution, evolution, and migration of both invertebrates and vertebrates that graze on these plants. Sixteen separate studies all indicate that the current distributions of C4 monocots are tightly correlated with temperature: elevated temperatures during the growing season favor C4 monocots. In contrast, the seven studies on C4 dicot distributions suggest that a different environmental parameter, such as aridity (combination of temperature and evaporative potential), more closely describes their distributions. Differences in the temperature dependence of the quantum yield for CO2 uptake (light-use efficiency) of C3 and C4 species relate well to observed plant distributions and light-use efficiency is the only mechanism that has been proposed to explain distributional differences in C3/C4 monocots. Modeling of C3 and C4 light-use efficiencies under different combinations of atmospheric CO2 and temperature predicts that C4-dominated ecosystems should not have expanded until atmospheric CO2 concentrations reached the lower levels that are thought to have existed beginning near the end of the Miocene. At that time, palaeocarbonate and fossil data indicate a simultaneous, global expansion of C4-dominated grasslands. The C4 monocots generally have a higher quantum yield than C4 dicots and it is proposed that leaf venation patterns play a role in increasing the light-use efficiency of most C4 monocots. The reduced quantum yield of most C4 dicots is consistent with their rarity, and it is suggested that C4 dicots may not have been selected until CO2 concentrations reached their lowest levels during glacial maxima in the Quaternary. Given the intrinsic light-use efficiency advantage of C4 monocots, C4 dicots may have been limited in their distributions to the warmest ecosystems, saline ecosystems, and/or to highly disturbed ecosystems. All C4 plants have a significant advantage over C3 plants under low atmospheric CO2 conditions and are predicted to have expanded significantly on a global scale during full-glacial periods, especially in tropical regions. Bog and lake sediment cores as well as pedogenic carbonates support the hypothesis that C4 ecosystems were more extensive during the last glacial maximum and then decreased in abundance following deglaciation as atmospheric CO2 levels increased.
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The latest Cenozoic (<6 Ma) ash beds in the western United States have been intensively studied for several decades. The more wide-spread of these ash beds are well-documented event horizons that are of great value in studies of the timing and pace of geological, climatological, and biological events throughout the region. Because explosive volcanism was not restricted to latest Neogene time in this region, many older ash beds are likely to prove as useful as younger beds as event horizons, once they are located, characterized, and dated. As a first step in developing a useful chronology of older Cenozoic ash beds in the western United States, we have sampled and analyzed silicic fallout tuffs in middle to late Miocene sedimentary basins across the northern Basin and Range province. The northern Basin and Range basins, ideally situated in the vicinity of major coeval silicic volcanic centers, contain numerous relatively unaltered, silicic fallout tuffs. We have correlated tuffs between all sampled sections on the basis of glass shard composition. The composite stratigraphie sequence established by the correlations contains more than 200 individual tuffs, including 59 widely distributed tuffs termed correlative tuffs. The tuffs vary widely in composition, but most are in one of two compositional groups: gray metaluminous vitric tuffs (Gm tuffs) or white metaluminous vitric tuffs (Wm tuffs). Distribution patterns, compositional characteristics, and correlation with ash-flow tuffs show that the source for most Gm tuffs was the Snake River Plain volcanic province along the northern edge of the northern Basin and Range, and the source for most Wm tuffs was the southwestern Nevada volcanic field in the southern part of the northern Basin and Range. The northern Basin and Range tuffs range in age from ca. 16-6 Ma. The ages of individual tuffs are determined variously by direct isotopic dating, by correlation to previously dated fallout and ash-flow tuffs, or by interpolation age estimation. Ages for most tuffs are known to within 0.25 m.y. (1σ) or less and for many tuffs to within 0.1 m.y. or less. The sequence and ages of tuffs established in this study provide insights into the evolution of the northern Basin and Range basins and patterns of explosive volcanism in coeval volcanic centers, and contribute to the development of a high-resolution stratigraphy and chronology of coeval sedimentary deposits throughout the western United States.
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Some 51 separate units of vitric fallout tuff are recognized in the Trapper Creek section. Petrographic and chemical characteristics indicate that most are from SRPVP sources. New 40Ar/39Ar laser-fusion dating, along with prior isotopic age determinations, show that the Trapper Creek tuffs span the period ca. 13.9-8.6 Ma. The Trapper Creek tuffs provide insight into the characteristics of explosive silicic volcanism within the SRPVP during middle-late Miocene time. Variations in the character of SRPVP explosive silicic eruptions may reflect changes in the structure, composition, or state of stress in the crust beneath the eastward propagating SRPVP, or, perhaps, changes in the Yellowstone hot-spot plume that may drive the SRPVP volcanism. -from Authors
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Evidence that these processes have been active in the region since the middle Tertiary includes Pliocene lake sediments and Quaternary terrace alluvium that have been deformed by dissolution-induced subsidence as well as former sinkholes filled with lacustrine sediments of the lower Ogallala Formation (Miocene).-from Author
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Mass-balance relationships of 10 major elements in a Vertisol climosequence and chronosequence formed on alluvial terraces on the Texas Gulf Coastal Plain indicate that the soils develop characteristic element translocation patterns in response to climatic forcing and reach mass-flux equilibrium within a relatively short period of time. Vertisols within the climosequence (age 35 ka, 800 1500 mm mean annual precipitation [MAP]) approach a net mass flux of ˜-16% (±3%) of parent element contents, corresponding to a weathering flux of -1.0 × 10-3 to -1.3 × 10-3 g cm-2 yr-1, when MAP exceeds 900 mm. Net mass-flux assessments in a Vertisol chronosequence (0.4 35 ka, 1000 mm MAP) show that this equilibrium is achieved within 5 6 k.y. Below 900 mm MAP, positive net mass fluxes indicate that Vertisol profiles are gaining material (at a rate of 0.2 × 10-3 g cm-2 yr-1). Vertisols forming in drier MAP areas approach equilibrium with climate by different mechanisms and at different rates, compared to their wetter counterparts, and may not be as sensitive to millennial-scale climate shifts.