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1,778 result(s) for "Stevens, Thomas"
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The Avengers. Kree/Skrull War
When Ronan the Accuser seizes control of the Kree Empire and launches an attack against Earth, the Avengers must face the threat, while the shape-shifting Skrulls, hiding among the human population, reveal themselves to wage war.
Loess Plateau storage of Northeastern Tibetan Plateau-derived Yellow River sediment
Marine accumulations of terrigenous sediment are widely assumed to accurately record climatic- and tectonic-controlled mountain denudation and play an important role in understanding late Cenozoic mountain uplift and global cooling. Underpinning this is the assumption that the majority of sediment eroded from hinterland orogenic belts is transported to and ultimately stored in marine basins with little lag between erosion and deposition. Here we use a detailed and multi-technique sedimentary provenance dataset from the Yellow River to show that substantial amounts of sediment eroded from Northeast Tibet and carried by the river’s upper reach are stored in the Chinese Loess Plateau and the western Mu Us desert. This finding revises our understanding of the origin of the Chinese Loess Plateau and provides a potential solution for mismatches between late Cenozoic terrestrial sedimentation and marine geochemistry records, as well as between global CO 2 and erosion records. Theories of Cenozoic mountain uplift and associated global cooling assume that eroded mountain sediments are stored in marine basins. Here, based on detailed provenance data, Nie et al . show that Northeast Tibetan sediments are in fact stored inland, in the Chinese Loess Plateau and Mu Us desert.
A Meta-analysis of Hypothetical Bias in Stated Preference Valuation
Individuals are widely believed to overstate their economic valuation of a good by a factor of two or three. This paper reports the results of a meta-analysis of hypothetical bias in 28 stated preference valuation studies that report monetary willingness-to-pay and used the same mechanism for eliciting both hypothetical and actual values. The papers generated 83 observations with a median ratio of hypothetical to actual value of only 1.35, and the distribution has severe positive skewness. We find that a choice-based elicitation mechanism is important in reducing bias. We provide some evidence that the use of student subjects may be a source of bias, but since this variable is highly correlated with group experimental settings, firm conclusions cannot be drawn. There is some weak evidence that bias increases when public goods are being valued, and that some calibration methods may be effective at reducing bias. However, results are quite sensitive to model specification, which will remain a problem until a comprehensive theory of hypothetical bias is developed. Copyright Springer 2005
Continental‐Hemispheric Scale Dust Events Driven by Last Glacial Alpine Ice Sheet Dynamics
Greenland ice cores demonstrate that transient last glacial cold climate events (stadials) were associated with greatly enhanced atmospheric dust loading. Detailed radiocarbon dating of loess in central Europe reveals concurrent increases in dust activity in dust‐emitting regions. However, the causes of these changes in dust emission and the role of dust in rapid climate change remain unclear. We address this uncertainty through multi‐proxy analysis of loess sources in Hungary. Our results demonstrate that loess particles were dominantly produced by subglacial grinding processes in the eastern Alps. These particles were released along with stadial Alpine Ice Sheet‐driven meltwater pulses, turning major river systems into efficient dust sources for loess deposition in Europe. Concurrent strengthened anticyclonic circulation over northern Europe would have caused wider dispersal of Alpine Ice Sheet‐derived dust as far as Greenland. Resultant continental‐hemispheric scale changes in atmospheric dust loading likely amplified concomitant North Atlantic climate cooling and ice sheet decay during Greenland stadials. Plain Language Summary Atmospheric dust is a fundamental component of Earth's climate system. Quaternary archives from dust‐emitting and source distal regions reveal a close relationship between abrupt changes in dust and climate. However, the causes for this dust‐climate coupling and the role of dust in rapid climate change events remain unclear. Here, we propose a mechanism that explains this coupling. Specifically, we show that last glacial dust in Central Europe was produced by subglacial grinding processes in the Alps. Alpine Ice Sheet recession events greatly enhanced proglacial sediment supply and turned major river systems into efficient but intermittent dust sources. Wider dispersal of Alpine Ice Sheet dust across the North Atlantic region likely amplified abrupt climate fluctuations during the last glacial. Key Points Multi‐proxy source study of detailed 14C dated loess in Europe reveals Alpine Ice Sheet sediment supply drives last glacial dust events Wider dispersal of Alpine Ice Sheet dust potentially affected the North Atlantic region and amplified last glacial climate fluctuations Coherent explanation of millennial‐scale dust‐climate coupling during last glacial period
Shift of large-scale atmospheric systems over Europe during late MIS 3 and implications for Modern Human dispersal
Understanding the past dynamics of large-scale atmospheric systems is crucial for our knowledge of the palaeoclimate conditions in Europe. Southeastern Europe currently lies at the border between Atlantic, Mediterranean, and continental climate zones. Past changes in the relative influence of associated atmospheric systems must have been recorded in the region’s palaeoarchives. By comparing high-resolution grain-size, environmental magnetic and geochemical data from two loess-palaeosol sequences in the Lower Danube Basin with other Eurasian palaeorecords, we reconstructed past climatic patterns over Southeastern Europe and the related interaction of the prevailing large-scale circulation modes over Europe, especially during late Marine Isotope Stage 3 (40,000–27,000 years ago). We demonstrate that during this time interval, the intensification of the Siberian High had a crucial influence on European climate causing the more continental conditions over major parts of Europe, and a southwards shift of the Westerlies. Such a climatic and environmental change, combined with the Campanian Ignimbrite/Y-5 volcanic eruption, may have driven the Anatomically Modern Human dispersal towards Central and Western Europe, pointing to a corridor over the Eastern European Plain as an important pathway in their dispersal.
Pacific freshening drives Pliocene cooling and Asian monsoon intensification
The monsoon is a fundamental component of Earth's climate. The Pliocene warm period is characterized by long-term global cooling yet concurrent monsoon dynamics are poorly known. Here we present the first fully quantified and calibrated reconstructions of separate Pliocene air temperature and East Asian summer monsoon precipitation histories on the Chinese Loess Plateau through joint analysis of loess/red clay magnetic parameters with different sensitivities to air temperature and precipitation. East Asian summer monsoon precipitation shows an intensified trend, paradoxically at the same time that climate cooled. We propose a hitherto unrecognized feedback where persistently intensified East Asian summer monsoon during the late Pliocene, triggered by the gradual closure of the Panama Seaway, reinforced late Pliocene Pacific freshening, sea-ice development and ice volume increase, culminating in initiation of the extensive Northern Hemisphere glaciations of the Quaternary Ice Age. This feedback mechanism represents a fundamental reinterpretation of the origin of the Quaternary glaciations and the impact of the monsoon.
Gene flow of small mammals is inhibited by highways and the urbanized habitat matrix in a large urban forest fragment
Urbanization dramatically modifies landscapes, fragments habitats, and is spreading rapidly. Urbanization can inhibit gene flow in a variety of species, but identifying the impact of specific landscape features on gene flow is difficult without prior knowledge of which landscape features may represent barriers to dispersal. We employed a landscape genetics approach to investigate the effects of urbanization on dispersal and gene flow in the swamp rabbit (Sylvilagus aquaticus), a forest‐obligate species, within the largest urban forest fragment in the United States, the Great Trinity Forest (GTF) in Dallas, TX. We employed a range of spatially explicit genetic analyses to investigate how landscape features, such as roads and the urban matrix, influence dispersal. We used noninvasive techniques to obtain 367 swamp rabbit fecal DNA samples representing 80 individuals from 2019 to 2021. We used traditional population genetics approaches, including STRUCTURE, FST, and genetic spatial autocorrelation, to quantify genetic differentiation within our study population. We then employed a resistance to dispersal mapping landscape genetics approach to link observed genetic differentiation in swamp rabbits to specific landscape features. Global F'ST (0.182) showed significant subdivision within the population, and spatial autocorrelation analysis showed that individuals were genetically more similar than expected at distances under 3000 m and less similar than expected at distances over 7000 m. An examination of the spatial relationships between isolation by distance (IBD) residuals and urban landscape features revealed that the urban matrix inhibited gene flow, and groups of individuals separated by large highways were less related than expected based on the population's IBD trend. Swamp rabbits in the GTF, a single large and continuous forest fragment, showed equivalent genetic differentiation to other small mammals in widely disparate habitat fragments, revealing that even the largest urban habitat fragments may be unable to conserve urban‐avoiding species when they are bisected by highways and surrounded by a dense urban matrix.
Tracing sources of inorganic suspended particulate matter in the Great Barrier Reef lagoon, Australia
Water clarity on the inshore Great Barrier Reef (GBR) is greatly influenced by terrestrial runoff of suspended particulate matter (SPM). Catchment sediment tracing studies often do not extend into the marine environment, preventing the analysis of preferential marine transport. This study employs novel collection and sediment tracing techniques to examine the transport of the terrigenous ‘mineral’ component of plume SPM within the GBR lagoon for two flood events. Utilising geochemical, radionuclide and clay mineral analysis, we trace terrigenous mineral sediments > 100 km from the river mouth. We show that the SPM geochemistry is highly influenced by particle-size fractionation, desorption, and dilution within the plume, rendering traditional tracing methods unviable. However, the ratios of rare earth elements (REE) to thorium (Th) provide stable tracers of mineral SPM transported across the catchment to marine continuum and allow the identification of discrete catchment sources for each flood event. Plume sediment radionuclides are also stable and consistent with sub-surface erosion sources.
Defining paleoclimatic routes and opportunities for hominin dispersals across Iran
Fossil and archaeological evidence indicates that hominin dispersals into Southwest Asia occurred throughout the Pleistocene, including the expansion of Homo sapiens populations out of Africa. While there is evidence for hominin occupations in the Pleistocene in Iran, as evidenced by the presence of Lower to Upper Paleolithic archaeological sites, the extent to which humid periods facilitated population expansions into western Asia has remained unclear. To test the role of humid periods on hominin dispersals here we assess Paleolithic site distributions and paleoenvironmental records across Iran. We developed the first spatially comprehensive, high-resolution paleohydrological model for Iran in order to assess water availability and its influence on hominin dispersals. We highlight environmentally mediated routes which likely played a key role in Late Pleistocene hominin dispersals, including the expansion of H . sapiens and Neanderthals eastwards into Asia. Our combined analyses indicate that, during MIS 5, there were opportunities for hominins to traverse a northern route through the Alborz and Kopet Dagh Mountains and the Dasht-I Kavir desert owing to the presence of activated fresh water sources. We recognize a new southern route along the Zagros Mountains and extending eastwards towards Pakistan and Afghanistan. We find evidence for a potential northern route during MIS 3, which would have permitted hominin movements and species interactions in Southwest Asia. Between humid periods, these interconnections would have waned, isolating populations in the Zagros and Alborz Mountains, where hominins may have continued to have had access to water.
Fluvial landscapes of the Harappan civilization
The collapse of the Bronze Age Harappan, one of the earliest urban civilizations, remains an enigma. Urbanism flourished in the western region of the Indo-Gangetic Plain for approximately 600 y, but since approximately 3,900 y ago, the total settled area and settlement sizes declined, many sites were abandoned, and a significant shift in site numbers and density towards the east is recorded. We report morphologic and chronologic evidence indicating that fluvial landscapes in Harappan territory became remarkably stable during the late Holocene as aridification intensified in the region after approximately 5,000 BP. Upstream on the alluvial plain, the large Himalayan rivers in Punjab stopped incising, while downstream, sedimentation slowed on the distinctive mega-fluvial ridge, which the Indus built in Sindh. This fluvial quiescence suggests a gradual decrease in flood intensity that probably stimulated intensive agriculture initially and encouraged urbanization around 4,500 BP. However, further decline in monsoon precipitation led to conditions adverse to both inundation- and rain-based farming. Contrary to earlier assumptions that a large glacier-fed Himalayan river, identified by some with the mythical Sarasvati, watered the Harappan heartland on the interfluve between the Indus and Ganges basins, we show that only monsoonal-fed rivers were active there during the Holocene. As the monsoon weakened, monsoonal rivers gradually dried or became seasonal, affecting habitability along their courses. Hydroclimatic stress increased the vulnerability of agricultural production supporting Harappan urbanism, leading to settlement downsizing, diversification of crops, and a drastic increase in settlements in the moister monsoon regions of the upper Punjab, Haryana, and Uttar Pradesh.