Neogene Climate Change Research Articles

Upper Neogene climate change in northern Africa based on chemical weathering indices and clay mineralogy: a case study of southeastern Tunisia (Gulf of Gabès)

Upper Neogene climate change in northern Africa based on chemical weathering indices and clay mineralogy: a case study of southeastern Tunisia (Gulf of Gabès)

Ecological polarities of African Miocene apes

Humans have been considered ecologically unspecialized, and our evolution a compromise path through a maze of conflicting influences. Generalist ecological roles contrast with ecological polarities such as competitor, breeder or tolerator. Ecological polarities can be approximated in fossil mammals by relative size of canines for competitors, incisors for breeders, and molars for tolerators. Considered in this way, early Miocene apes of Kenya were generalists, but show a greater range of ecological polarity than modern apes, including ecological polarities found in living apes, macaques, and vervets. As many as six primates in a single Miocene aleosol show diverse ecological polarities, implying competitive exclusion. Middle Miocene monkeys and apes were more molarized and marginally more tolerant than early Miocene primates, so more like humans in that respect. This adaptive shift of 15 ​Ma was at a time of climatic aridity and open vegetation indicated by associated paleosols. A 20 ​m.yr record of Kenyan paleoprecipitation from paleosols indicates that 15 ​Ma was unusually dry after exceptionally wet paleoclimate of 16 ​Ma. This new Kenyan paleosol record of paleoclimate is from the same localities as the fossil apes, and uninfluenced by whole ocean mixing, salinity and ice volume effects which compromise similar marine isotopic proxies of global change. Molarization of apes at 15 ​Ma ago was only one of a series of adaptations selected by at least 9 dry alternating with wet episodes which shifted forest to shrubland ecotones over the past 20 ​m.yr. Our evolutionary lineage ran a gauntlet of Neogene climatic and vegetation changes in Africa by adopting a generalist ecological role.

Early‐Middle Eocene Hydroclimate Variations Recorded by Environmental Magnetism in the Linxia Basin, NE Tibetan Plateau

AbstractAlthough Neogene climate change reconstruction in central Asia is important for understanding global cooling, Asian monsoon evolution, and the Tibetan Plateau stepwise uplift during the Cenozoic, the paleoenvironmental evolution during the Paleogene in this region is still ambiguous due to a lack of corresponding continuous and complete terrestrial records with precise age constraints. Recently, a new fluvial sedimentary sequence with magnetostratigraphic ages spanning from ∼53 to 40 Ma was found in the Linxia Basin, NE Tibetan Plateau, thereby providing a window to understand Eocene climate change in the Asian interior. From this fluvial sedimentary sequence, we present early‐middle Eocene hydroclimate variations based on a combined method of rock magnetism and diffuse reflectance spectroscopy. Our results show magnetic enhancement of fluvial sediments at 51.7 Ma resulted from abundant single‐domain magnetite inputs from the surrounding mountains through strong erosion and transport in a relatively wet environment. Subsequently, at 47.6 Ma, magnetic weakening of floodplain sediments corresponds to an increase in hematite concentrations produced via low‐temperature oxidation in a prolonged dry environment. Comparisons of paleoclimatic proxies, such as organic geochemistry and rock magnetism from neighboring basins, tectonic deformation of the NE Tibetan Plateau, and sea level change in the Paratethys Sea suggest that the hydroclimate variation from relatively wet to dry climate in the early‐middle Eocene in this area was mainly controlled by global climatic change and probably superimposed by the uplift of the Tibetan Plateau.

Geomorphology of the northwestern Kurdistan Region of Iraq: landscapes of the Zagros Mountains drained by the Tigris and Great Zab Rivers

ABSTRACT We present the geomorphological map of the northwestern part of the Kurdistan Region of Iraq, where the landscape expresses the tectonic activity associated with the Arabia-Eurasia convergence and Neogene climate change. These processes influenced the evolution of landforms and fluvial pathways, where major rivers Tigris, Khabur, and Great Zab incise the landscape of Northeastern Mesopotamia Anticlinal ridges and syncline trough compose the Zagros orogen. The development of water and wind gaps, slope, and karsts processes in the highlands and the tilting of fluvial terraces in the flat areas are the main evidence of the relationship between tectonics, climate variations and geomorphological processes. During the Quaternary, especially after the Last Glacial Maximum, fluctuating arid and wet periods also influenced local landforms and fluvial patterns of the area. Finally, the intensified Holocene human occupation and agricultural activities during the passage to more complex societies over time impacted the evolution of the landscape in this part of Mesopotamia.

Marine plankton show threshold extinction response to Neogene climate change

Ongoing climate change is predicted to trigger major shifts in the geographic distribution of marine plankton species. However, it remains unclear whether species will successfully track optimal habitats to new regions, or face extinction. Here we show that one significant zooplankton group, the radiolaria, underwent a severe decline in high latitude species richness presaged by ecologic reorganization during the late Neogene, a time of amplified polar cooling. We find that the majority (71%) of affected species did not relocate to the warmer low latitudes, but went extinct. This indicates that some plankton species cannot track optimal temperatures on a global scale as assumed by ecologic models; instead, assemblages undergo restructuring and extinction once local environmental thresholds are exceeded. This pattern forewarns profound diversity loss of high latitude radiolaria in the near future, which may have cascading effects on the ocean food web and carbon cycle.

Global Diversification of Anelosimus Spiders Driven by Long-Distance Overwater Dispersal and Neogene Climate Oscillations.

Vicariance and dispersal events, combined with intricate global climatic history, have left an imprint on the spatiotemporal distribution and diversity of many organisms. Anelosimus cobweb spiders (Theridiidae), are organisms ranging in behavior from solitary to highly social, with a cosmopolitan distribution in temperate to tropical areas. Their evolutionary history and the discontinuous distribution of species richness suggest that 1) long-distance overwater dispersal and 2) climate change during the Neogene (23-2.6 Ma), may be major factors in explaining their distribution and diversification. Here, we test these hypotheses, and explicitly test if global Miocene/Pliocene climatic cooling in the last 8 Ma affected Anelosimus radiation in parallel in South America and Madagascar. To do so, we investigate the phylogeny and spatiotemporal biogeography of Anelosimus through a culmination of a 20-year comprehensive global sampling at the species level (69 species, including 84% of the known 75 species worldwide, represented by 268 individuals) using nucleotide data from seven loci (5.5 kb). Our results strongly support the monophyly of Anelosimus with an Oligocene ($\sim $30 Ma) South American origin. Major clades on other continents originate via multiple, long-distance dispersal events, of solitary or subsocial-but not social-lineages, from the Americas. These intercontinental dispersals were to Africa, Madagascar (twice), and SE Asia/Australasia. The early diversification of Anelosimus spiders coincides with a sudden thermal increase in the late Oligocene ($\sim $27-25 Ma), though no causal connection can be made. Our results, however, strongly support the hypothesis that global Neogene climatic cooling in the last 8 Ma drove Anelosimus radiation in parallel in South America and Madagascar, offering a rare empirical evidence for diversification of a socially diverse group driven by an interplay between long-distance dispersal and global Neogene climatic changes. [Cobweb spiders; diversification; global biogeography; long-distance dispersal; molecular phylogenetics; neogene climate changes; sociality; vicariance.].

Refinement of Miocene sea level and monsoon events from the sedimentary archive of the Maldives (Indian Ocean)

International Ocean Discovery Program (IODP) Expedition 359 cored sediments from eight borehole locations in the carbonate platform of the Maldives in the Indian Ocean. The expedition set out to unravel the timing of Neogene climate changes, in particular the evolution of the South Asian monsoon and fluctuations of the sea level. The timing of these changes are assessed by dating resultant sedimentary alterations that mark stratigraphic turning points in the Neogene Maldives platform system. The first four turning points during the early and middle Miocene are related to sea-level changes. These are reliably recorded in the stratigraphy of the carbonate sequences in which sequence boundaries provide the ages of the sea-level lowstand. Phases of aggradational platform growth give precise age brackets of long-term sea-level high stands during the early Miocene and the early to middle Miocene Climate Optimum that is dated here between 17 to 15.1 Ma. The subsequent middle Miocene cooling coincident with the eastern Antarctic ice sheet expansion resulted in a long-term lowering of sea level that is reflected by a progradational platform growth. The change in platform architecture from aggradation to progradation marks this turning point at 15.1 Ma.An abrupt change in sedimentation pattern is recognized across the entire archipelago at a sequence boundary dated as 12.9–13 Ma. At this turning point, the platform sedimentation switched to a current-controlled mode when the monsoon-wind-driven circulation started in the Indian Ocean. The similar age of the onset of drift deposition from monsoon-wind-driven circulation across the entire archipelago indicates an abrupt onset of monsoon winds in the Indian Ocean. Ten unconformities dissect the drift sequences, attesting changes in current strength or direction that are likely caused by the combined product of changes in the monsoon-wind intensity and sea level fluctuations in the last 13 Ma. A major shift in the drift packages is dated with 3.8 Ma that coincides with the end of stepwise platform drowning and a reduction of the oxygen minimum zone in the Inner Sea.The strata of the Maldives platform provides a detailed record of the extrinsic controlling factors on carbonate platform growth through time. This potential of carbonate platforms for dating the Neogene climate and current changes has been exploited in other platforms drilled by the Ocean Drilling Program. For example, Great Bahama Bank, the Queensland Plateau, and the platforms on the Marion Plateau show similar histories with sediment architectures driven by sea level in their early history (early to middle Miocene) replaced by current-driven drowning or partial drowning during their later history (Late Miocene). In all three platform systems, the influence of currents on sedimentations is reported between 11 and 13 Ma.

Reconstruction of Mediterranean flora, vegetation and climate for the last 23 million years based on an extensive pollen dataset

In the last 46 years extensive effort has been devoted to the botanical identification of pollen grains from Neogene deposits. This robust approach is the only reliable way to reconstruct past flora and vegetation from pollen archives, additionally allowing fruitful comparisons with plant macroremains. Rich peri-Mediterranean fossil pollen data provide a comprehensive history of plant ecosystems during the last 23 million years. Successive disappearance of thermophilous-hygrophilous taxa has occurred. Severe changes affected the North Mediterranean vegetation which varied from prevalent forests to dominant open environments. Avicennia mangrove progressively perished from North to South and was replaced by Glyptostrobus swamps which in turn disappeared too in a diachronous way. Subtropical evergreen forests left room for Mediterranean sclerophyllous communities, and finally to Artemisia steppes which alternated with mesophilous forests during glacial-interglacial cycles. The South Mediterranean plant ecosystems contrasted with open subdesertic associations where Avicennia persisted up to the early Pliocene before the steppes invaded the lowlands. Neogene climate changes benefited the Mediterranean sclerophyllous plants that were already present in the early Miocene. Their subsequent fluctuations relate not only to temperature variations but maybe also to phases with high instability in seasonality and low variability in warmth. When present, Microtropis fallax is a serious candidate for identifying the primary Mediterranean sclerophyllous assemblages. Interpretation and climate quantification of the pollen data show a well-marked latitudinal contrast in the Mediterranean area both in temperature and humidity/ dryness. The thermic latitudinal gradient is characterized by a significant increase in the late Miocene towards its modern value. In the late Pliocene, climate evolved from overall warm temperatures and dry seasons to cooler winters and dry summers. General dryness and colder conditions developed during glacial periods, alternating with moister and warmer conditions during interglacials. Present refuges of Zelkova and Pterocarya are discussed. Desiccation of the Mediterranean led subdesertic plants and cedar to migrate in opposite ways. The present distribution of Cedrus illustrates its weakness against coolings although it seems to have been saved by geodynamic events.

Diversification in tropics and subtropics following the mid-Miocene climate change: A case study of the spider genus Nesticella.

Caves may offer suitable refugia for troglophilic invertebrates during periods of unfavourable climatic conditions because of their stable microclimates. As a consequence, allopatric divergence from their epigean counterparts may occur, leading to formation of truly hypogean communities (the Climatic Relict Hypothesis). Unlike the well-studied effects of Pleistocene glaciations, we know little about how ancient climate changes drove the development of cave-dwelling organisms living at both middle and lower latitudes. We investigate the evolutionary history of the troglophilic spider genus Nesticella (Araneae, Nesticidae) in relation to Asian Neogene (23-2.6Ma) climatic changes. Our analyses discern clear differences in the evolution of the two main clades of Nesticella, which occur in temperate/subtropical and tropical latitudes. Eastern Asian Nesticella gradually evolved greater sedentariness and a strict subterranean lifestyle starting from the middle Miocene Epoch (~15-14Ma) in conjunction with the progressive deterioration of the climate and vegetational shifts. Caves appear to have acted as refugia because of their internally uniform temperature and humidity, which allowed these spiders to survive increasing external seasonality and habitat loss. In contrast, a uniform accumulation of lineages, long-lasting times for dispersals and the lack of a comparable habitat shifting characterized the tropical lineage. This difference in pattern likely owes to the mild effects of climate change at low latitudes and the consequent lack of strong climatic drivers in tropical environments. Thus, the mid-Miocene climatic shift appears to be the major evolutionary force shaping the ecological differences between Asian troglophilic invertebrates and the driver of the permanent hypogean communities in middle latitudes.

Miocene–Pliocene vegetation change in south-western Africa (ODP Site 1081, offshore Namibia)

Abstract Aridification is an important component of Late Neogene climate change in south-western Africa probably caused by modifications in the atmospheric circulation in relation to the initiation and intensification of the Benguela Upwelling System due to globally steepening of the meridional pressure gradient. Intensification of the meridional pressure gradient influenced the climate intensively which had then an impact on the vegetation. However, vegetation changes of south-western Africa from the Miocene to Pliocene have not yet been reported and only indirectly investigated by sedimentological data. Here, we present a pollen record of marine ODP Site 1081 retrieved 160 km offshore Namibia covering the time between 9 and 2.7 Ma. Using an endmember unmixing model we distinguished three vegetation phases as follows: a relative wet phase, during the Tortonian, showing higher representations of Cyperaceae, a transition phase during the Messinian, when especially grasses expanded, and a dry one covering the Pliocene with a strong representation of desert and semi-desert plants. The three phases indicate ongoing aridification probably caused by intensified meridional pressure gradients. Additionally, aquatic vegetation indicators appear in our pollen record from around 5 Ma on, which we attribute to a relocation of the lower course of the Cunene River to its modern outlet in the Atlantic Ocean. Redirection of the Cunene River toward the Atlantic would have deprived the palaeolake Cunene of an important source of fresh-water ultimately resulting in desiccation of the lake and the formation of the Etosha Pan.

Molecular data and ecological niche modelling reveal a highly dynamic evolutionary history of the East Asian Tertiary relict Cercidiphyllum (Cercidiphyllaceae)

East Asia's temperate deciduous forests served as sanctuary for Tertiary relict trees, but their ages and response to past climate change remain largely unknown. To address this issue, we elucidated the evolutionary and population demographic history of Cercdiphyllum, comprising species in China/Japan (Cercdiphyllum japonicum) and central Japan (Cercdiphyllum magnificum). Fifty-three populations were genotyped using chloroplast and ribosomal DNA sequences and microsatellite loci to assess molecular structure and diversity in relation to past (Last Glacial Maximum) and present distributions based on ecological niche modelling. Late Tertiary climate cooling was reflected in a relatively recent speciation event, dated at the Mio-/Pliocene boundary. During glacials, the warm-temperate C. japonicum experienced massive habitat losses in some areas (north-central China/north Japan) but increases in others (southwest/-east China, East China Sea landbridge, south Japan). In China, the Sichuan Basin and/or the middle-Yangtze were source areas of postglacial northward recolonization; in Japan, this may have been facilitated through introgressive hybridization with the cool-temperate C. magnificum. Our findings challenge the notion of relative evolutionary and demographic stability of Tertiary relict trees, and may serve as a guideline for assessing the impact of Neogene climate change on the evolution and distribution of East Asian temperate plants.

500 Paleovegetation in response to Neogene climate change in the Hengduan Mountains, SW China

500 Paleovegetation in response to Neogene climate change in the Hengduan Mountains, SW China

Late Pliocene vegetation and climate of Zhangcun region, Shanxi, North China

To understand the Neogene climatic changes in eastern Asia and evaluate the intercontinental climatic differences, we have quantitatively reconstructed the vegetation successions and climatic changes in the late Pliocene Zhangcun area based on the palynological data and explored the regional climatic differences between central Europe and eastern Asia. The late Pliocene palynological assemblage of Zhangcun, Shanxi was composed of 63 palynomorphs, belonging to 50 families, covering angiosperms (90.2%), gymnosperms (9.7%), ferns (0.09%), and other elements (0.02%). Four periods of vegetation succession over time were recognized. In period 1, a needle- and broad-leaved mixed forest prevailed with a cool and dry climate. Period 2 was characterized by an expansion of forest with a warmer and wetter climate. The number of conifers increased and that of herbs decreased in period 3, and the climate became cool and dry. In period 4, the forest was dominated by conifers and reflecting a cooler climate. The data of seven climatic parameters in general and four periods estimated by the Coexistence Approach suggested that (1) The late Pliocene temperatures and precipitations were higher than today. (2) The Neogene climate of both Central Europe and North China exhibited a general cooling and drying trend although the mean annual temperature dropped by ca. 1 °C in North China, vs. ca. 7 °C in Central Europe from the middle Miocene to the late Pliocene. (3) The decline of the mean maximum monthly precipitation might signal a weakening of the summer monsoon. (4) The decline of both the mean coldest monthly temperature and the mean minimum monthly precipitation might be linked to the strengthening of the winter monsoon in eastern Asia. (5) The rapid uplift of the Tibetan Plateau strengthened the climatic cooling and drying during the late Pliocene of the Zhangcun region.

Paleosol record of Neogene climate change in the Australian outback

Effects in Australia of a global spike of warm-wet climate during the middle Miocene (ca16 Ma) have been controversial, with one faction arguing for inland rain forest and the other faction for dry woodland. This question is here addressed using the Oligocene–Miocene sequence of fossil mammal localities at Lake Palankarinna, South Australia, which includes numerous paleosols that have been dated by paleomagnetism, palynology, biostratigraphy, and radiometric methods to straddle this paleoclimatic event. Paleosols of the Oligocene–Miocene Etadunna and Pliocene Tirari Formations formed in arid paleoclimates and include pedogenic gypsum. The Maralji paleosol, developed on early Miocene Etadunna Formation and overlain by late Miocene–Pliocene Mampuwordu Sands, is middle Miocene in age based on mammal fauna at correlative horizons in nearby Lake Ngapakaldi. The Maralji paleosol has shallow calcareous nodules and stout root traces suggesting vegetation like dry woodland (mallee). Mallee vegetation now grows no closer than 1200 km to the southwest, so middle Miocene warm-wet climate enabled range extension of mallee and woody thickening of plants in the Australia outback. There is no evidence in the outback of middle Miocene rain forest, which may have expanded its range to form kaolinitic Ultisols near Sydney, Mudgee and Gulgong, all in New South Wales. Nor is there evidence so far inland of swamp woodlands and heaths like those producing brown coals in the Latrobe Valley, Victoria.

Neogene climate change and uplift in the Atacama Desert, Chile: COMMENT: COMMENT

[Rech et al. (2006)][1] describe a variety of palaeosol characteristics from Miocene sections present along the southwest margin of the Calama Basin in the Atacama Desert of northern Chile. They identified a change in soil character from a calcic vertisol to an extremely mature gypsisol with

Neogene climate change and uplift in the Atacama Desert, Chile: COMMENT: REPLY

We welcome comments by Hartley regarding our Neogene climate reconstruction of the Atacama Desert ([Rech et al., 2006][1]) and the opportunity to further discuss the resolution and sensitivity of soils and sedimentological deposits for inferring hyperaridity in the geologic record. In regard to our

Neogene climate change and emergence of C 4 grasses in the Namib, southwestern Africa, as reflected in ratite 13C and 18O

Stable light isotopes in ratite eggshells have been shown to be reliable indicators of shifts in climate and environmental conditions in the past. Here, we show that δ 18O and δ 13C values in fossil and modern ratite eggshells collected in the aeolianite deposits of the southern and central Namib Desert track regional distinctions and global climate shifts throughout the Neogene. δ 18O values, although variable, are consistently higher in the central compared to the southern Namib throughout the record. δ 18O trends during the Miocene differ for the two regions, but track each other post-Miocene. Throughout the Miocene, δ 13C values for ratite eggshells from both the central and southern Namib regions are indistinguishable showing that the flora remained C 3 throughout. The overall negative (− 3‰) shift in mean values for Miocene biostratigraphic zones is consistent with the response of C 3 photosynthesis to pCO 2 shifts from 180 to 320 ppmv as estimated from marine alkenone studies and/or evolution of the δ 13C of the atmospheric CO 2. Evidence for C 4 plants occurs post-Miocene, with the development of the southern, winter rainfall and central/northern, summer rainfall zonation apparent today. These data provide independent corroboration that the expansion of C 4-dominated ecosystems after ∼ 7 Ma cannot be attributed to a reduction of pCO 2 below a 500 ppmv threshold, as earlier proposed. Proliferation of C 4 plants in the Namib after ∼ 5 Ma and, elsewhere, may be related rather to energy budgets and rainfall seasonality shifts resulting from large-scale atmospheric and oceanic circulation reorganisation.

Neogene climate change and uplift in the Atacama Desert, Chile

The relationship between Andean uplift and extreme desiccation of the west coast of South America is important for understanding the interplay between climate and tectonics in the Central Andes, yet it is poorly understood. Here we use soil morphological char- acteristics, salt chemistry, and mass independent fractionation anomalies ( 17 O values) in dated paleosols to reconstruct a middle Miocene climatic transition from semiaridity to extreme hyperaridity in the Atacama Desert. Paleosols along the southeastern margin of the Calama Basin change from calcic Vertisols with root traces, slickensides, and gleyed horizons to an extremely mature salic Gypsisol with pedogenic nitrate. We interpret this transition, which occurred between 19 and 13 Ma, to represent a change in precipitation from 200 mm/yr to 20 mm/yr. This drastic reduction in precipitation likely resulted from uplift of the Central Andes to elevations 2 km; the uplift blocked moisture from the South American summer monsoon from entering the Atacama. The mid-Miocene Gyp- sisol with pedogenic nitrate is located at elevations between 2900 and 3400 m in the Cal- ama Basin, significantly higher than modern nitrate soils, which occur below 2500 m. Modern and Quaternary soils in this elevation zone contain soil carbonate and lack ped- ogenic gypsum and nitrate. We infer that 900 m of local surface uplift over the past 10 m.y. displaced these nitrate paleosols relative to modern nitrate soils and caused a return to wetter conditions in the Calama Basin by decreasing local air temperatures and creating an orographic barrier to Pacific air masses.

Carbon and oxygen isotopic variability in Neogene paleosol carbonates: constraints on the evolution of the C4-grasslands of the Great Plains, USA

Understanding the development of grassland ecosystems is critical for understanding the evolution of their associated vertebrate faunas. We focus on the central and southern portions of the Great Plains, which is the largest continuous grassland in North America and has a well-documented mammalian fossil record. Modern biomass in this region is dominated by C4 grasses, but paleontological evidence indicates the region was forested prior to the middle Miocene, implying the region was formerly dominated by C3 plants. We examine long- and short-term variations in the abundance of C4 grasses and climatic conditions in the central and southern Great Plains using the stable carbon and oxygen isotope composition of paleosol carbonates from 28 sections of 11 lithostratigraphic units in the region. The carbon isotope composition (δ13C) of paleosol carbonate reflects the proportion of C3 and C4 plants that grew in an ancient soil. The long-term pattern of δ13C values indicates that the percentage of C4 grasses in the Great Plains was moderate (ca. 20%) throughout most of the Miocene, increased from 6.4 to 4.0 Ma, and reached modern levels by 2.5 Ma. Based on the range in δ13C values within sections and the mean δ13C values for different carbonate morphologies that reflect different intervals of carbonate accumulation, the abundance of C4 biomass did not vary substantially on short time scales. The variability in C4 biomass on short time scales does not correlate with age of section, latitude, longitude, or the mean percentage of C4 biomass. The oxygen isotope composition (δ18O) of soil carbonate is controlled by soil temperature and the δ18O value of soil water. Great Plains paleosol δ18O values vary considerably in relation to time and latitude. Removing the variation due to latitude with least squares linear regression reveals a temporal pattern that is consistent with the global pattern of Neogene climate change. Quantitative interpretation of the δ18O record is difficult without independent control on temperature or the oxygen isotope composition of soil water, both of which have probably varied with long-term climate change. Comparisons of measured δ18O values for each section with predicted values based on modern temperature data and estimated modern meteoric water δ18O values suggest that (1) most Miocene sections have δ18O values consistent with warmer temperatures and more positively shifted soil water δ18O values than today, (2) three Miocene sections in western Nebraska probably had soil water compositions that were more negative than similar-aged localities in the region, and (3) the Plio-Pleistocene sections have δ18O values consistent with cooler temperatures and more negative soil water δ18O values than today. The paleosol δ13C values do not correlate strongly with the pattern of climate change recorded by the δ18O values, suggesting that long-term change in mean temperature is not the main control on the abundance of δ13C biomass in the Great Plains. We propose two testable hypotheses to account for our data and other published data that bear on the evolution of the Great Plains ecosystem.

Carbon and oxygen isotopic variability in Neogene paleosol carbonates: constraints on the evolution of the C 4-grasslands of the Great Plains, USA

Understanding the development of grassland ecosystems is critical for understanding the evolution of their associated vertebrate faunas. We focus on the central and southern portions of the Great Plains, which is the largest continuous grassland in North America and has a well-documented mammalian fossil record. Modern biomass in this region is dominated by C 4 grasses, but paleontological evidence indicates the region was forested prior to the middle Miocene, implying the region was formerly dominated by C 3 plants. We examine long- and short-term variations in the abundance of C 4 grasses and climatic conditions in the central and southern Great Plains using the stable carbon and oxygen isotope composition of paleosol carbonates from 28 sections of 11 lithostratigraphic units in the region. The carbon isotope composition ( δ 13C) of paleosol carbonate reflects the proportion of C 3 and C 4 plants that grew in an ancient soil. The long-term pattern of δ 13C values indicates that the percentage of C 4 grasses in the Great Plains was moderate (ca. 20%) throughout most of the Miocene, increased from 6.4 to 4.0 Ma, and reached modern levels by 2.5 Ma. Based on the range in δ 13C values within sections and the mean δ 13C values for different carbonate morphologies that reflect different intervals of carbonate accumulation, the abundance of C 4 biomass did not vary substantially on short time scales. The variability in C 4 biomass on short time scales does not correlate with age of section, latitude, longitude, or the mean percentage of C 4 biomass. The oxygen isotope composition ( δ 18O) of soil carbonate is controlled by soil temperature and the δ 18O value of soil water. Great Plains paleosol δ 18O values vary considerably in relation to time and latitude. Removing the variation due to latitude with least squares linear regression reveals a temporal pattern that is consistent with the global pattern of Neogene climate change. Quantitative interpretation of the δ 18O record is difficult without independent control on temperature or the oxygen isotope composition of soil water, both of which have probably varied with long-term climate change. Comparisons of measured δ 18O values for each section with predicted values based on modern temperature data and estimated modern meteoric water δ 18O values suggest that (1) most Miocene sections have δ 18O values consistent with warmer temperatures and more positively shifted soil water δ 18O values than today, (2) three Miocene sections in western Nebraska probably had soil water compositions that were more negative than similar-aged localities in the region, and (3) the Plio-Pleistocene sections have δ 18O values consistent with cooler temperatures and more negative soil water δ 18O values than today. The paleosol δ 13C values do not correlate strongly with the pattern of climate change recorded by the δ 18O values, suggesting that long-term change in mean temperature is not the main control on the abundance of δ 13C biomass in the Great Plains. We propose two testable hypotheses to account for our data and other published data that bear on the evolution of the Great Plains ecosystem.