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Sökning: hsv:(NATURVETENSKAP) hsv:(Geovetenskap och miljövetenskap) hsv:(Geologi) > Stevens Thomas 1979

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1.
  • Hällberg, Lars Petter, et al. (författare)
  • Magnetic susceptibility parameters as proxies for desert sediment provenance
  • 2020
  • Ingår i: Aeolian Research. - : Elsevier BV. - 1875-9637 .- 2212-1684. ; 46
  • Tidskriftsartikel (refereegranskat)abstract
    • Magnetic susceptibility in sediments has been thoroughly studied as a paleoenvironmental proxy over the last decades. However, it is unknown to what extent magnetic susceptibility variation is also a diagnostic of different sediment sources. Here we investigate if multiple magnetic susceptibility-based parameters can effectively be used as sediment source indicators. New magnetic property data from the Mu Us and Tengger Deserts in China are compared to previously known sediment provenance based on other well-established proxies. To assess the magnetic properties of these deserts, magnetic susceptibility and its out-of-phase component, its dependence on frequency, temperature and low-field amplitude are analyzed. Our results indicate similar sources for the western Mu Us Desert and the Tengger Desert and a distinct source for the eastern Mu Us, in-line with previously hypothesized provenance patterns. However, magnetic properties within the Tengger Desert sediments are homogenous, which may suggest a uniform provenance for the entire Tengger Desert, that the sediments are greatly reworked, or similar magnetic properties in potential multiple source regions. Magnetite is the major magnetic mineral in the study area and the dominant causes for divergence in magnetic properties are the magnetic mineral concentration and domain state. The results here, in particular from the Mu Us, suggest considerable promise for using magnetic susceptibility parameters in desert sediment provenance research.
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2.
  • Baykal, Yunus, et al. (författare)
  • Detrital zircon U–Pb age analysis of last glacial loess sources and proglacial sediment dynamics in the Northern European Plain
  • 2021
  • Ingår i: Quaternary Science Reviews. - : Elsevier. - 0277-3791 .- 1873-457X. ; 274
  • Tidskriftsartikel (refereegranskat)abstract
    • Loess deposits along the northern fringe of the European loess belt potentially record past changes in dust emission from areas proximal to former ice sheets. Recent chronologies from loess deposits across this region generally agree on greatly enhanced dust deposition rates when the Fennoscandian Ice Sheet reached its maximum extent during the late last glacial. However, uncertainties over the material's source and origin limit understanding of the causes of this enhanced dust activity. In particular, loess in southwestern Poland has been attributed to multiple origins, mainly involving glaciofluvial outwash plains along the Fennoscandian Ice Sheet margin and/or local sources in the mountainous areas of the Sudetes and Western Carpathians. Here we apply detrital zircon U–Pb age analyses for a large number of grains recovered from four loess samples taken from different stratigraphic units exposed at Biały Kościół in southwestern Poland, previously luminescence dated to MIS 4–2, to assess loess provenance as well as its temporal evolution during last glacial Fennoscandian Ice Sheet fluctuations. Furthermore, we analysed the detrital zircon U–Pb age spectra of five samples from potential source sediments to constrain the history of sediment recycling and mixing within the Northern European Plain prior to deflation and loess deposition. The broad range of zircon age components detected in the four loess samples suggests both Fennoscandian and closer Peri-Gondwanan proto-sources while a narrow, dominant Carboniferous age peak is consistent with sourcing from the local Strzelin Hills in the Sudetic foreland. However, the presence of both Fennoscandian and Peri-Gondwanan derived grains in samples from potential source sediments reveals that this mixture of sediment sources is widespread across the Northern European Plain, as a result of long-term glacial and fluvial reworking of cover sediments in the proglacial area throughout the Quaternary. Local rivers draining the Sudetic foreland transported this Fennoscandian-Peri-Gondwanan sediment mixture along with particles denuded from the Strzelin Hills, resulting in a nearby, temporally stable dust source for the Biały Kościół loess during MIS 4–2, while dust emission rates were substantially increased during the last glacial maximum. Given that our model for loess formation at Biały Kościół essentially involves sediment distribution via rivers prior to short distance aeolian transport, we infer that the proportion of northern ice sheet derived particles in European loess deposits is mainly controlled by the drainage pattern of major rivers in relation to Pleistocene ice margins where glaciofluvial sediment is abundant. Based on the presence of Fennoscandian derived zircon grains in European loess deposits, we constrain a southern limit of the influence of northern ice sheet dust sources along the central European highlands that currently divide drainage between the Northern European Plain and the Danube Basins.
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3.
  • Giosan, Liviu, et al. (författare)
  • Fluvial landscapes of the Harappan civilization
  • 2012
  • Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 109:26, s. E1688-E1694
  • Tidskriftsartikel (refereegranskat)abstract
    • 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.
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4.
  • Giosan, Liviu, et al. (författare)
  • On the Holocene evolution of the Ayeyawady megadelta
  • 2018
  • Ingår i: Earth Surface Dynamics. - : Copernicus GmbH. - 2196-6311 .- 2196-632X. ; 6:2, s. 451-466
  • Tidskriftsartikel (refereegranskat)abstract
    • The Ayeyawady delta is the last Asian megadelta whose evolution has remained essentially unexplored so far. Unlike most other deltas across the world, the Ayeyawady has not yet been affected by dam construction, providing a unique view on largely natural deltaic processes benefiting from abundant sediment loads affected by tectonics and monsoon hydroclimate. To alleviate the information gap and provide a baseline for future work, here we provide a first model for the Holocene development of this megadelta based on drill core sediments collected in 2016 and 2017, dated with radiocarbon and optically stimulated luminescence, together with a reevaluation of published maps, charts and scientific literature. Altogether, these data indicate that Ayeyawady is a mud-dominated delta with tidal and wave influences. The sediment-rich Ayeyawady River built meander belt alluvial ridges with avulsive characters. A more advanced coast in the western half of the delta (i.e., the Pathein lobe) was probably favored by the more western location of the early course of the river. Radiogenic isotopic fingerprinting of the sediment suggests that the Pathein lobe coast does not receive significant sediment from neighboring rivers. However, the eastern region of the delta (i.e., Yangon lobe) is offset inland and extends east into the mudflats of the Sittaung estuary. Wave-built beach ridge construction during the late Holocene, similar to several other deltas across the Indian monsoon domain, suggests a common climatic control on monsoonal delta morphodynamics through variability in discharge, changes in wave climate or both. Correlation of the delta morphological and stratigraphic architecture information on land with the shelf bathymetry, as well as its tectonic, sedimentary and hydrodynamic characteristics, provides insight on the peculiar growth style of the Ayeyawady delta. The offset between the western Pathein lobe and the eastern deltaic coast appears to be driven by tectonic-hydrodynamic feedbacks as the extensionally lowered shelf block of the Gulf of Mottama amplifies tidal currents relative to the western part of the shelf. This situation probably activates a perennial shear front between the two regions that acts as a leaky energy fence. Just as importantly, the strong currents in the Gulf of Mottama act as an offshore-directed tidal pump that helps build the deep mid-shelf Mottama clinoform with mixed sediments from the Ayeyawady, Sittaung and Thanlwin rivers. The highly energetic tidal, wind and wave regime of the northern Andaman Sea thus exports most sediment offshore despite the large load of the Ayeyawady River.
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7.
  • Bohm, Katja, et al. (författare)
  • Neogene global climate change and East Asian dust sources : Combined rutile geochemistry and zircon U Pb analysis from the northern Chinese Loess Plateau
  • 2023
  • Ingår i: Global and Planetary Change. - : Elsevier. - 0921-8181 .- 1872-6364. ; 221
  • Tidskriftsartikel (refereegranskat)abstract
    • During the late Neogene, global climate underwent a long-term transition to cooler climates that culminated in the Quaternary icehouse conditions. In the East Asian terrestrial realm, atmospheric dustiness increased, and the volume of aeolian dust deposits expanded significantly. The Neogene Red Clay deposits, located on the Chinese Loess Plateau (CLP), provide an exceptional geologic archive to investigate pre-Quaternary climates and environments. Constraining the provenance of the Red Clay is crucial for unravelling the links between late Neogene Central-East Asian climate, tectonics and desertification. However, Red Clay provenance is highly debated and data are scarce. In this study, we have used a multi-proxy approach at a high sampling resolution to study the provenance of the c. 7–2.6 Ma Baode Red Clay. Our data consist of joint detrital zircon U-Pb ages and detrital rutile trace element geochemistry of the Baode Red Clay, and of a massive rutile geochemistry dataset from 14 potential dust source areas. The data indicate that the Mio-Pliocene and Plio-Pleistocene global climate transitions were coupled with gradual dust provenance shifts. We propose these shifts indicate the intensification of East Asian winter monsoon and/or enhanced Central-East Asian drying as a response to global cooling. We also identify temporary Pliocene provenance changes that interrupt the long-term winter monsoon -controlled dust transport at c. 4 Ma and c. 3.5 Ma. Several indicators point to dust transport by the westerly jet at c. 4 Ma, possibly caused by a shift in the jet position as a response to global cooling, and/or enhanced wet deposition caused by intensified summer monsoon. Alternatively, the role of the paleo-Yellow River should be investigated further. We infer the 3.5 Ma provenance signal was caused by increased Yellow River -transported material from eroding Northeastern Tibetan Plateau and/or western CLP. Implications of our results not only shed light on the links between late Cenozoic global climate change and Central-East Asian dust cycle, but also provide detailed information to further investigate the late Neogene regional geomorphology and its effects on dust emission, transport, and deposition. Our combined rutile-zircon analysis also verifies that a multi-proxy single-grain approach that targets different types of primary source rocks is needed for reliable provenance analysis of the CLP dust.
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8.
  • Bohm, Katja, et al. (författare)
  • The provenance of late Cenozoic East Asian Red Clay : Tectonic-metamorphic history of potential source regions and a novel combined zircon-rutile approach
  • 2022
  • Ingår i: Earth-Science Reviews. - : Elsevier. - 0012-8252 .- 1872-6828. ; 225
  • Tidskriftsartikel (refereegranskat)abstract
    • Constraining the provenance of aeolian mineral dust is critical in understanding past climate changes, atmospheric dust activity, circulation, and sediment generation. On the Chinese Loess Plateau (CLP), use of detrital zircon U-Pb age data as source tracers for the dust has seen a huge growth and lead to breakthroughs in understanding dust provenance. However, significant ambiguities remain especially regarding the provenance of the aeolian Neogene Red Clay (RC). To address this, here we review the state of the art of understanding of Neogene RC provenance, with a focus on single-grain analyses, and introduce detrital rutile geochemistry as a tool to complement zircon U-Pb dating. Furthermore, to better utilise the link between the detrital minerals and their primary origin, we compile primary source region geologic background and single-grain data relevant for use of geochronological and metamorphic provenance proxy minerals. We discuss four major tectonic divisions in northern China and southern Mongolia: North China Craton (NCC), Tarim Craton (TC), Central China Orogen (CCO), parts of the Central Asian Orogenic Belt (CAOB), and briefly summarize the Tibetan-Himalayan orogen. Many of these regions have been tectonically active during the same time periods in the Earth's history, and our analysis demonstrates how use of zircon age data alone has limitations in differentiating between a number of key potential dust sources to the CLP. Addition of a metamorphic source tracer such as rutile allows some of these possible source areas to be distinguished. For example, the proximal northern NCC regions that show high−/ultrahigh-temperature metamorphic conditions can potentially be diagnostic of a northerly source component to CLP dust. Our combined zircon-rutile data analysis of ca. 4 Ma Nihewan RC in northern CLP verifies the utility of the novel rutile provenance proxy in sourcing CLP sediments. The zircon and rutile data suggest similar dust provenance: the dominant sources are proximal areas on the NCC, while contributions from the dry areas in parts of the CAOB, central deserts, and the Yellow River are also likely. Our results also hint at a minor source component deriving from distal western source regions in the TC, and/or in the central parts of the CCO, but rutile data from potential secondary source areas are needed to verify this possibility. We also conclude that multi-proxy single-grain provenance analyses are needed for more reliable provenance analyses.
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  • Resultat 1-10 av 40

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