TY - JOUR A1 - Barbolini, Natasha A1 - Woutersen, Amber A1 - Dupont-Nivet, Guillaume A1 - Silvestro, Daniele A1 - Tardif-Becquet, Delphine A1 - Coster, Pauline M. C. A1 - Meijer, Niels A1 - Chang, Cun A1 - Zhang, Hou-Xi A1 - Licht, Alexis A1 - Rydin, Catarina A1 - Koutsodendris, Andreas A1 - Han, Fang A1 - Rohrmann, Alexander A1 - Liu, Xiang-Jun A1 - Zhang, Y. A1 - Donnadieu, Yannick A1 - Fluteau, Frederic A1 - Ladant, Jean-Baptiste A1 - Le Hir, Guillaume A1 - Hoorn, M. Carina T1 - Cenozoic evolution of the steppe-desert biome in Central Asia JF - Science Advances N2 - The origins and development of the arid and highly seasonal steppe-desert biome in Central Asia, the largest of its kind in the world, remain largely unconstrained by existing records. It is unclear how Cenozoic climatic, geological, and biological forces, acting at diverse spatial and temporal scales, shaped Central Asian ecosystems through time. Our synthesis shows that the Central Asian steppe-desert has existed since at least Eocene times but experienced no less than two regime shifts, one at the Eocene-Oligocene Transition and one in the mid-Miocene. These shifts separated three successive "stable states," each characterized by unique floral and faunal structures. Past responses to disturbance in the Asian steppe-desert imply that modern ecosystems are unlikely to recover their present structures and diversity if forced into a new regime. This is of concern for Asian steppes today, which are being modified for human use and lost to desertification at unprecedented rates. Y1 - 2020 U6 - https://doi.org/10.1126/sciadv.abb8227 SN - 2375-2548 VL - 6 IS - 41 PB - American Association for the Advancement of Science CY - Washington ER - TY - JOUR A1 - Govin, Gwladys A1 - van der Beek, Pieter A. A1 - Najman, Yani A1 - Millar, Ian A1 - Gemignani, Lorenzo A1 - Huyghe, Pascale A1 - Dupont-Nivet, Guillaume A1 - Bernet, Matthias A1 - Mark, Chris A1 - Wijbrans, Jan T1 - Early onset and late acceleration of rapid exhumation in the Namche Barwa syntaxis, eastern Himalaya JF - Geology N2 - The Himalayan syntaxes, characterized by extreme rates of rock exhumation co-located with major trans-orogenic rivers, figure prominently in the debate on tectonic versus erosional forcing of exhumation. Both the mechanism and timing of rapid exhumation of the Namche Barwa massif in the eastern syntaxis remain controversial. It has been argued that coupling between crustal rock advection and surface erosion initiated in the late Miocene (8-10 Ma). Recent studies, in contrast, suggest a Quaternary onset of rapid exhumation linked to a purely tectonic mechanism. We report new multisystem detrital thermochronology data from the most proximal Neogene clastic sediments downstream of Namche Barwa and use a thermo-kinematic model constrained by new and published data to explore its exhumation history. Modeling results show that exhumation accelerated to similar to 4 km/m.y. at ca. 8 Ma and to similar to 9 km/m.y. after ca. 2 Ma. This three-stage history reconciles apparently contradictory evidence for early and late onset of rapid exhumation and suggests efficient coupling between tectonics and erosion since the late Miocene. Quaternary acceleration of exhumation is consistent with river-profile evolution and may be linked to a Quaternary river-capture event. Y1 - 2020 U6 - https://doi.org/10.1130/G47720.1 SN - 0091-7613 SN - 1943-2682 VL - 48 IS - 12 SP - 1139 EP - 1143 PB - American Institute of Physics CY - Boulder ER - TY - JOUR A1 - Kaya, Mustafa Yuecel A1 - Dupont-Nivet, Guillaume A1 - Proust, Jean-Noël A1 - Roperch, Pierrick A1 - Meijer, Niels A1 - Frieling, Joost A1 - Fioroni, Chiara A1 - Altiner, Sevinç Özkan A1 - Stoica, Marius A1 - Aminov, Jovid A1 - Mamtimin, Mehmut A1 - Guo, Zhaojie T1 - Cretaceous evolution of the Central Asian Proto-Paratethys Sea BT - tectonic, eustatic, and climatic controls JF - Tectonics N2 - The timing and mechanisms of the Cretaceous sea incursions into Central Asia are still poorly constrained. We provide a new chronostratigraphic framework based on biostratigraphy and magnetostratigraphy together with detailed paleoenvironmental analyses of Cretaceous records of the proto-Paratethys Sea fluctuations in the Tajik and Tarim basins. The Early Cretaceous marine incursion in the western Tajik Basin was followed by major marine incursions during the Cenomanian (ca. 100 Ma) and Santonian (ca. 86 Ma) that reached far into the eastern Tajik and Tarim basins. These marine incursions were separated by a Turonian-Coniacian (ca. 92-86 Ma) regression. Basin-wide tectonic subsidence analyses imply that the Early Cretaceous sea incursion into the Tajik Basin was related to increased Pamir tectonism. We find that thrusting along the northern edge of the Pamir at ca. 130-90 Ma resulted in increased subsidence in a retro-arc basin setting. This tectonic event and coeval eustatic highstand resulted in the maximum observed geographic extent of the sea during the Cenomanian (ca. 100 Ma). The following Turonian-Coniacian (ca. 92-86 Ma) major regression, driven by eustasy, coincides with a sharp slowdown in tectonic subsidence during the late orogenic unloading period with limited thrusting. The Santonian (ca. 86 Ma) major sea incursion was likely controlled by eustasy as evidenced by the coeval fluctuations in the west Siberian Basin. An early Maastrichtian cooling (ca. 71-70 Ma), potentially connected to global Late Cretaceous trends, is inferred from the replacement of mollusk-rich limestones by bryozoan- and echinoderm-rich limestones. Y1 - 2020 U6 - https://doi.org/10.1029/2019TC005983 SN - 0278-7407 SN - 1944-9194 VL - 39 IS - 9 PB - American Geophysical Union CY - Washington ER - TY - GEN A1 - Tardif-Becquet, Delphine A1 - Fluteau, Frédéric A1 - Donnadieu, Yannick A1 - Le Hir, Guillaume A1 - Ladant, Jean-Baptiste A1 - Sepulchre, Pierre A1 - Licht, Alexis A1 - Poblete, Fernando A1 - Dupont-Nivet, Guillaume T1 - The origin of Asian monsoons BT - a modelling perspective T2 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe N2 - The Cenozoic inception and development of the Asian monsoons remain unclear and have generated much debate, as several hypotheses regarding circulation patterns at work in Asia during the Eocene have been proposed in the few last decades. These include (a) the existence of modern-like monsoons since the early Eocene; (b) that of a weak South Asian monsoon (SAM) and little to no East Asian monsoon (EAM); or (c) a prevalence of the Intertropical Convergence Zone (ITCZ) migrations, also referred to as Indonesian-Australian monsoon (I-AM). As SAM and EAM are supposed to have been triggered or enhanced primarily by Asian palaeogeographic changes, their possible inception in the very dynamic Eocene palaeogeographic context remains an open question, both in the modelling and field-based communities. We investigate here Eocene Asian climate conditions using the IPSL-CM5A2 (Sepulchre et al., 2019) earth system model and revised palaeogeographies. Our Eocene climate simulation yields atmospheric circulation patterns in Asia substantially different from modern conditions. A large high-pressure area is simulated over the Tethys ocean, which generates intense low tropospheric winds blowing southward along the western flank of the proto-Himalayan-Tibetan plateau (HTP) system. This low-level wind system blocks, to latitudes lower than 10 degrees N, the migration of humid and warm air masses coming from the Indian Ocean. This strongly contrasts with the modern SAM, during which equatorial air masses reach a latitude of 20-25 degrees N over India and southeastern China. Another specific feature of our Eocene simulation is the widespread subsidence taking place over northern India in the midtroposphere (around 5000 m), preventing deep convective updraught that would transport water vapour up to the condensation level. Both processes lead to the onset of a broad arid region located over northern India and over the HTP. More humid regions of high seasonality in precipitation encircle this arid area, due to the prevalence of the Intertropical Convergence Zone (ITCZ) migrations (or Indonesian-Australian monsoon, I-AM) rather than monsoons. Although the existence of this central arid region may partly result from the specifics of our simulation (model dependence and palaeogeographic uncertainties) and has yet to be confirmed by proxy records, most of the observational evidence for Eocene monsoons are located in the highly seasonal transition zone between the arid area and the more humid surroundings. We thus suggest that a zonal arid climate prevailed over Asia before the initiation of monsoons that most likely occurred following Eocene palaeogeographic changes. Our results also show that precipitation seasonality should be used with caution to infer the presence of a monsoonal circulation and that the collection of new data in this arid area is of paramount importance to allow the debate to move forward. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 1436 KW - earth system model KW - early eocene KW - tibetan plateau KW - climate-change KW - oligocene climate KW - summer monsoon KW - global monsoon KW - ice sheet KW - part 1 KW - China Y1 - 2020 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-516770 SN - 1866-8372 IS - 1436 ER - TY - THES A1 - Meijer, Niels T1 - Asian dust, monsoons and westerlies during the Eocene N2 - The East Asian monsoons characterize the modern-day Asian climate, yet their geological history and driving mechanisms remain controversial. The southeasterly summer monsoon provides moisture, whereas the northwesterly winter monsoon sweeps up dust from the arid Asian interior to form the Chinese Loess Plateau. The onset of this loess accumulation, and therefore of the monsoons, was thought to be 8 million years ago (Ma). However, in recent years these loess records have been extended further back in time to the Eocene (56-34 Ma), a period characterized by significant changes in both the regional geography and global climate. Yet the extent to which these reconfigurations drive atmospheric circulation and whether the loess-like deposits are monsoonal remains debated. In this thesis, I study the terrestrial deposits of the Xining Basin previously identified as Eocene loess, to derive the paleoenvironmental evolution of the region and identify the geological processes that have shaped the Asian climate. I review dust deposits in the geological record and conclude that these are commonly represented by a mix of both windblown and water-laid sediments, in contrast to the pure windblown material known as loess. Yet by using a combination of quartz surface morphologies, provenance characteristics and distinguishing grain-size distributions, windblown dust can be identified and quantified in a variety of settings. This has important implications for tracking aridification and dust-fluxes throughout the geological record. Past reversals of Earth’s magnetic field are recorded in the deposits of the Xining Basin and I use these together with a dated volcanic ash layer to accurately constrain the age to the Eocene period. A combination of pollen assemblages, low dust abundances and other geochemical data indicates that the early Eocene was relatively humid suggesting an intensified summer monsoon due to the warmer greenhouse climate at this time. A subsequent shift from predominantly freshwater to salt lakes reflects a long-term aridification trend possibly driven by global cooling and the continuous uplift of the Tibetan Plateau. Superimposed on this aridification are wetter intervals reflected in more abundant lake deposits which correlate with highstands of the inland proto-Paratethys Sea. This sea covered the Eurasian continent and thereby provided additional moisture to the winter-time westerlies during the middle to late Eocene. The long-term aridification culminated in an abrupt shift at 40 Ma reflected by the onset of windblown dust, an increase in steppe-desert pollen, the occurrence of high-latitude orbital cycles and northwesterly winds identified in deflated salt deposits. Together, these indicate the onset of a Siberian high atmospheric pressure system driving the East Asian winter monsoon as well as dust storms and was triggered by a major sea retreat from the Asian interior. These results therefore show that the proto-Paratethys Sea, though less well recognized than the Tibetan Plateau and global climate, has been a major driver in setting up the modern-day climate in Asia. N2 - Die ostasiatischen Monsune prägen das heutige asiatische Klima, doch ihr geologischer Ursprung und ihre Antriebsmechanismen sind nach wie vor umstritten. Der südöstliche Sommermonsun bringt Feuchtigkeit, während der nordwestliche Wintermonsun Staub aus dem trockenen asiatischen Inland aufwirbelt und das chinesische Lössplateau bildet. Der Ursprung dieses Lösses und damit des Monsuns wurde vor 8 Millionen Jahren vermutet (Ma). In den letzten Jahren sind diese Lößablagerungen jedoch weiter in das Eozän (56-34 Ma) zurückverlegt worden, einer Periode, die durch bedeutende Änderungen sowohl in der regionalen Geographie als auch im globalen Klima gekennzeichnet ist. Inwieweit diese Rekonfigurationen die atmosphärische Zirkulation antrieben und ob es sich bei den lößartigen Sedimenten um monsunartige Ablagerungen handelt, bleibt jedoch umstritten. In dieser Dissertation untersuche ich die terrestrischen Ablagerungen des Xining-Beckens, die zuvor als Löss aus dem Eozän identifiziert wurden, um die paläo-umweltbedingte Entwicklung der Region abzuleiten und die geologischen Prozesse zu identifizieren, die das asiatische Klima geprägt haben. Ich überprüfe die Staubablagerungen im geologischen Archiv und komme zu dem Schluss, dass diese durch eine Mischung aus windgetriebenen und wassergelagerten Sedimenten dargestellt werden, im Gegensatz zu dem rein windgetriebenen Material, das als Löß bekannt ist. Doch durch die Verwendung einer Kombination der oberflächenmorphologien von Quartz, Herkunftsmerkmalen und unterscheidenden Korngrößenverteilungen kann windgetriebener Staub in einer Vielzahl von Umgebungen identifiziert und quantifiziert werden. Dies hat wichtige Auswirkungen auf die Nachverfolgung der Aridifizierung und der Staubflüsse in dem gesamten geologischen Archiv. Frühere Umkehrungen des Erdmagnetfeldes werden in den Ablagerungen des Xining-Beckens aufgezeichnet und ich verwende diese zusammen mit einer datierten vulkanischen Ascheschicht, um das Alter genau auf die Eozän-Periode einzugrenzen. Eine Kombination aus Pollenansammlungen, geringen Staubhäufigkeiten und anderen geochemischen Daten deutet darauf hin, dass das frühe Eozän relativ feucht war, was auf einen verstärkten Sommermonsun aufgrund des wärmeren Treibhausklimas zu dieser Zeit hinweist. Eine anschließende Verschiebung von überwiegend Süßwasser zu Salzseen spiegelt einen langfristigen Aridifizierungstrend wider, der möglicherweise durch die globale Abkühlung und die kontinuierliche Hebung des Tibetischen Plateaus angetrieben wurde. Überlagert wird diese Aridifizierung von feuchteren Intervallen, die durch eine Zunahme in Seeablagerungen gekennzeichnet werden und mit den Hochständen des inländischen proto-Paratethys-Meeres korrelieren. Dieses Meer bedeckte den eurasischen Kontinent und versorgte dadurch die winterlichen Westwinde mit zusätzlicher Feuchtigkeit im mittleren bis späten Eozän. T2 - Asiatischer Staub, Monsune und Westwind während des Eozäns KW - Paleoclimatology KW - Asia KW - Eocene KW - Stratigraphy KW - Asien KW - Stratigrafie KW - Monsoon KW - Monsun KW - Paläoklimatologie KW - Eozän Y1 - 2020 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-488687 ER - TY - JOUR A1 - Tardif-Becquet, Delphine A1 - Fluteau, Frédéric A1 - Donnadieu, Yannick A1 - Le Hir, Guillaume A1 - Ladant, Jean-Baptiste A1 - Sepulchre, Pierre A1 - Licht, Alexis A1 - Poblete, Fernando A1 - Dupont-Nivet, Guillaume T1 - The origin of Asian monsoons BT - a modelling perspective JF - Climate of the Past N2 - The Cenozoic inception and development of the Asian monsoons remain unclear and have generated much debate, as several hypotheses regarding circulation patterns at work in Asia during the Eocene have been proposed in the few last decades. These include (a) the existence of modern-like monsoons since the early Eocene; (b) that of a weak South Asian monsoon (SAM) and little to no East Asian monsoon (EAM); or (c) a prevalence of the Intertropical Convergence Zone (ITCZ) migrations, also referred to as Indonesian-Australian monsoon (I-AM). As SAM and EAM are supposed to have been triggered or enhanced primarily by Asian palaeogeographic changes, their possible inception in the very dynamic Eocene palaeogeographic context remains an open question, both in the modelling and field-based communities. We investigate here Eocene Asian climate conditions using the IPSL-CM5A2 (Sepulchre et al., 2019) earth system model and revised palaeogeographies. Our Eocene climate simulation yields atmospheric circulation patterns in Asia substantially different from modern conditions. A large high-pressure area is simulated over the Tethys ocean, which generates intense low tropospheric winds blowing southward along the western flank of the proto-Himalayan-Tibetan plateau (HTP) system. This low-level wind system blocks, to latitudes lower than 10 degrees N, the migration of humid and warm air masses coming from the Indian Ocean. This strongly contrasts with the modern SAM, during which equatorial air masses reach a latitude of 20-25 degrees N over India and southeastern China. Another specific feature of our Eocene simulation is the widespread subsidence taking place over northern India in the midtroposphere (around 5000 m), preventing deep convective updraught that would transport water vapour up to the condensation level. Both processes lead to the onset of a broad arid region located over northern India and over the HTP. More humid regions of high seasonality in precipitation encircle this arid area, due to the prevalence of the Intertropical Convergence Zone (ITCZ) migrations (or Indonesian-Australian monsoon, I-AM) rather than monsoons. Although the existence of this central arid region may partly result from the specifics of our simulation (model dependence and palaeogeographic uncertainties) and has yet to be confirmed by proxy records, most of the observational evidence for Eocene monsoons are located in the highly seasonal transition zone between the arid area and the more humid surroundings. We thus suggest that a zonal arid climate prevailed over Asia before the initiation of monsoons that most likely occurred following Eocene palaeogeographic changes. Our results also show that precipitation seasonality should be used with caution to infer the presence of a monsoonal circulation and that the collection of new data in this arid area is of paramount importance to allow the debate to move forward. KW - earth system model KW - early eocene KW - tibetan plateau KW - climate-change KW - oligocene climate KW - summer monsoon KW - global monsoon KW - ice sheet KW - part 1 KW - China Y1 - 2020 U6 - https://doi.org/10.5194/cp-16-847-2020 SN - 1814-9332 SN - 1814-9324 VL - 16 IS - 3 SP - 847 EP - 865 PB - Copernicus Publications CY - Göttingen ER -