LATE TRIASSIC OBLIQUE EXTRUSION OF UHP/HP COMPLEXES IN THE ATBASHI ACCRETIONARY COMPLEX OF SOUTH TIANSHAN, KYRGYZSTAN

The exhumation and tectonic emplacement of eclogites and blueschists take place in forearc accretionary complexes by either forearc- or backarc-directed extrusion, but few examples have been well analysed in detail. Here we present an example of oblique wedge extrusion of UHP/HP rocks in the Atbashi accretionary complex of the Kyrgyz South Tianshan.The exhumation and tectonic emplacement of eclogites and blueschists take place in forearc accretionary complexes by either forearc- or backarc-directed extrusion, but few examples have been well analysed in detail. Here we present an example of oblique wedge extrusion of UHP/HP rocks in the Atbashi accretionary complex of the Kyrgyz South Tianshan

Eolian dust dispersal patterns since the last glacial period in eastern Central Asia: insights from a loess-paleosol sequence in the Ili Basin

The extensive loess deposits of the Eurasian midlatitudes provide important terrestrial archives of Quaternary climatic change. As yet, however, loess records in Central Asia are poorly understood. Here we investigate the grain size and magnetic characteristics of loess from the Nilka (NLK) section in the Ili Basin of eastern Central Asia. Weak pedogenesis suggested by frequency-dependent magnetic susceptibility (chi fd%) and magnetic susceptibility (MS) peaks in primary loess suggest that MS is more strongly influenced by allogenetic magnetic minerals than pedogenesis, and may therefore be used to indicate wind strength. This is supported by the close correlation between variations in MS and proportions of the sand-sized fraction. To further explore the temporal variability in dust transport patterns, we identified three grain size end-members (EM1, mode size 47.5 mu m; EM2, 33.6 mu m; EM3, 18.9 mu m) which represent distinct aerodynamic environments. EM1 and EM2 are inferred to represent grain size fractions transported from proximal sources in short-term, near-surface suspension during dust outbreaks. EM3 appears to represent a continuous background dust fraction under non-dust storm conditions. Of the three end-members, EM1 is most likely the most sensitive recorder of wind strength. We compare our EM1 proportions with mean grain size from the Jingyuan section in the Chinese loess plateau, and assess these in the context of modern and Holocene climate data. Our research suggests that the Siberian High pressure system is the dominant influence on wind dynamics, resulting in loess deposition in the eastern Ili Basin. Six millennial-scale cooling (Heinrich) events can be identified in the NLK loess records. Our grain size data support the hypothesis that the Siberian High acts as teleconnection between the climatic systems of the North Atlantic and East Asia in the high northern latitudes, but not for the mid-latitude westerlies

LATE TRIASSIC OBLIQUE EXTRUSION OF UHP/HP COMPLEXES IN THE ATBASHI ACCRETIONARY COMPLEX OF SOUTH TIANSHAN, KYRGYZSTAN

The exhumation and tectonic emplacement of eclogites and blueschists take place in forearc accretionary complexes by either forearc- or backarc-directed extrusion, but few examples have been well analysed in detail. Here we present an example of oblique wedge extrusion of UHP/HP rocks in the Atbashi accretionary complex of the Kyrgyz South Tianshan

Moisture evolution in Central Asia since 26 ka: Insights from a Kyrgyz loess section, Western Tian Shan

Central Asia (CA) is located in the hinterland of the Eurasian continent, and faces an extreme risk of continued aridification under the current global warming trend. To predict future climate change in CA, paleoclimate studies provide essential references. However, the mechanisms underlying climate changes in CA remain relatively poorly known. Here, we investigate moisture variations in CA, as recorded in a 6-m loess section in the Western Tian Shan, Kyrgyzstan, which has accumulated since-26 ka based on the radiometric AMS C-14 dating. We reconstructed the evolution of the wind regime and moisture conditions in the Western Tian Shan with a modified grain size index (GSI(m)) and a new L* proxy (Delta L*). While the frequency-dependent magnetic susceptibility (cfd), redness (a*) and yellowness (b*) did not provide reliable paleoclimatic indicators, due to weak pedogenesis and other controlling factors. The GSI(m) records demonstrated a precipitation maximum in summer before-15 ka, shifting to a maximum in spring after-15 ka, probably associated with large-scale modulation of the latitudinal climatic zones and atmospheric circulation in response to retreat of the Northern Hemisphere ice sheets. Based on the Delta L* records, we place emphases on the important role of temperature-modulated evaporation in moisture evolution across CA; however, this has only influenced moisture evolution in spring and summer, with little effect on winter moisture conditions. Moreover, the Delta L* records may also indicate that atmospheric humidity can partly affect the long-term effective moisture variations. (C) 2020 Elsevier Ltd. All rights reserved

Heavy mineral assemblages and sedimentation rates of eastern Central Asian loess: Paleoenvironmental implications

Central Asian loess deposits record the evolution of aridification across the Asian interior. However, paleoclimatic proxies are strongly influenced by the variation of the dust source and accumulation process. In order to decipher the paleoenvironmental changes, a sensitive paleoclimatic proxy with clear source and accumulation process will be needed. In this study, heavy mineral assemblages and sedimentation rates were investigated in a 30-kyr loess section in the Ili Basin, Xinjiang, northwestern China. During Marine Isotope Stage (MIS) 2, trends in dust sedimentation rate were similar to those of the Chinese Loess Plateau (CLP), in anti-phase with Northern Hemisphere Summer Insolation (NHSI) at 65 degrees N. Amphibole/epidote ratios (contents and mean grain size) were low during cold phases, such as the late Last Glacial Maximum (LGM) and Heinrich Event (H) 1; their ratios were higher during adjacent warm periods; this was attributed to aeolian sorting during transportation. Namely, greater sorting between amphibole and epidote occurred in warmer phases, and weaker sorting occurred in colder phases. The amphibole/epidote ratio could serve as a proxy for wind intensity, and thus the Siberian High (SH) intensity. Amphibole/epidote ratio was a more appropriate proxy to record wind intensity than dust sedimentation rate which was also influenced by sediment availability. A higher amphibole/epidote ratio corresponded to a more negative East Asian stalagmite delta O-18, indicating an anti-phase relationship between the SH and East Asian Summer Monsoon (EASM) during the MIS 2

Pronounced changes in paleo-wind direction and dust sources during MIS3b recorded in the Tacheng loess, northwest China

Central Asia loess deposits can yield detailed records of aridification across the Asian interior, as well as the atmospheric circulation. In this study, we presented grain size, heavy mineral assemblage, rock magnetism and anisotropy of magnetic susceptibility of a loess section in the Tacheng Basin in Xinjiang region, NW China. The results of anisotropy of magnetic susceptibility indicated that a near-surface SE wind prevailed in the Tacheng area since Marine Isotope Stage (MIS) 3, while a near-surface SW prevailing wind developed during MIS3b. We inferred that the change of wind direction was due to glacial advance during MIS3b. Combining the result of anisotropy of magnetic susceptibility with grain size, heavy mineral assemblage data and previously published low-field mass susceptibility revealed that dust transported by the near-surface SW wind had a greater ability to enhance magnetic susceptibility and higher amphibole/epidote ratio. Additionally, a strong Siberian High during Heinrich Event (H)5 at ca. 48 ka blocked the advection of moisture into the region via mid-latitude westerlies, causing glacial recession and the weakening of the prevailing SW wind. Our study has implications for identifying changes in paleo-wind direction and dust source, which can aid understanding of the influence of westerlies and the Siberian High on dust accumulation, as well as their interactions

Projected Rainfall Erosivity Over Central Asia Based on CMIP5 Climate Models

Climate change-induced precipitation variability is the leading cause of rainfall erosivity that leads to excessive soil losses in most countries of the world. In this paper, four global climate models (GCMs) were used to characterize the spatiotemporal prediction of rainfall erosivity and assess the effect of variations of rainfall erosivity in Central Asia. The GCMs (BCCCSM1-1, IPSLCM5BLR, MIROC5, and MPIESMLR) were statistically downscaled using the delta method under Representative Concentration Pathways (RCPs) 2.6 and 8.5 for two time periods: “Near” and “Far” future (2030s and 2070s). These GCMs data were used to estimate rainfall erosivity and its projected changes over Central Asia. WorldClim data was used as the present baseline precipitation scenario for the study area. The rainfall erosivity (R) factor of the Revised Universal Soil Loss Equation (RUSLE) was used to determine rainfall erosivity. The results show an increase in the future periods of the annual rainfall erosivity compared to the baseline. For all GCMs, with an average change in rainfall erosivity of about 5.6% (424.49 MJ mm ha−1 h−1 year−1) in 2030s and 9.6% (440.57 MJ mm ha−1 h−1 year−1) in 2070s as compared to the baseline of 402 MJ mm ha−1 h−1 year−1. The magnitude of the change varies with the GCMs, with the largest change being 26.6% (508.85 MJ mm ha−1 h−1 year−1), occurring in the MIROC-5 RCP8.5 scenario in the 2070s. Although annual rainfall erosivity shows a steady increase, IPSLCM5ALR (both RCPs and periods) shows a decrease in the average erosivity. Higher rainfall amounts were the prime causes of increasing spatial-temporal rainfall erosivity

Aeolian dust dynamics in the Fergana Valley, Central Asia, since similar to 30 ka inferred from loess deposits

Knowledge of the interactions among atmospheric dynamics, dust emissions and climate system is essential to understand the physical mechanisms for the dust lifecycle, their role in loess formation as well as the predictions of future dust concentration. However, these issues still remain relatively poorly known in Central Asia (CA). The extensive loess deposits on the CA pediments provide a promising archive to explore atmospheric dust dynamics and climatic conditions in the past and their association with loess formation. This study investigates the granulometric and magnetic properties of a loess section (named Osh section) in the Fergana Valley, which provides a sensitive record of atmospheric dust dynamics since 30 ka based on radiometric (AMS C-14) dating. The frequency-dependent magnetic susceptibility (chi(fd)) and the mean grain size are used to reconstruct the broad-scale effective moisture and summer atmospheric dynamics pattern in CA, respectively. The results show that the precession forcing exerts a huge influence on the wind-regime variabilities in CA, but with different physical processes under the impact of the Northern Hemisphere ice sheet (NHIS) before and after 15 ka. The origin of the sedimentation rate variations in the Osh loess is also linked to the NHIS-modulated changes of the atmospheric circulation patterns. Either the strengthened westerlies or the increased surface roughness from higher vegetation cover in loess-deposition areas have significantly accelerated the loess accumulation. As a result, these complicated influence factors of sedimentation rate change in the Osh loess section, especially during the Holocene epoch, may hamper accessibility of the authentic dust emission flux and atmospheric dust concentration in CA. (C) 2021 ChinaUniversity of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V