Soluble salts in a Quaternary loess-soil sequence near Xining and the environmental implications

Dust composition is reflective to environmental conditions of the source areas, the transportation dynamics, and the post-depositional environments. In this study, soil chemical analysis and micromorphological approaches were conducted on a loess-soil sequence of the last 900ka near Xining at the western most part of the Loess Plateau. Thirty representative samples were selected to determine the pH, exchangeable sodium percentage (ESP), cation exchangeable capacity (CEC), and the exchangeable cations. Soil solutions of sixty representative samples were analyzed to determine the composition of soluble salts.All the analyzed samples show alkaline properties with the pH values varing from 8.5 to 10.0. pH values in loess layers (generally >9.0) are higher than in soil layers (generally <9.0). Except the Holocene soil S0 with weak alkaline property, ESP in loess varies from 15 ％ ~25 ％ while it ranges from 20 ％ ~40 ％ in soils. Both loess and soil layers contain a significant amount of soluble salts, including abundant sulfates that are thought to have significant climate impacts. Na+ represents a proportion of more than 70 ％ among cations. Among anions, CO2-3 and HCO-3 are more abundant in loess layers than in soil layers while SO2-4 and Cl- are more abundant in soils. These indicate that loess layers contain more Na2 CO3 and NaHCO3, while soil layers mainly contain NaCl and Na2SO4. Overall, soluble salts are much more abundant in soils than in loess layers.Micromorphological investigations show a weak weathering intensity of the studied soils, characterized by relatively fresh feldspars and biotite grains. All soil samples still contain detrital carbonates, suggesting their weakly developed steppe condition. These do not support a chemical weathering origin of the soluble salts. Because the studied section is located on high-terraces and no hydromorphic features were observed in either loess or soil layers, these soluble salts were unlikely to be originated from groundwater. Consequently, we interpret that these soluble salts were derived from eolian dust during the loess deposition. Salt-bearing dust should have been originated from the surrounding alkaline and saline lakes through eolian erosion.Our results therefore revealed a detrital origin for the abundant soluble salts. These indicate that alkaline/saline lakes have been constantly developed in the surrounding areas over the last 900ka. These lakes were probably seasonal and have experienced wind erosion during dry seasons. A most startling feature is the much higher content of soluble salts in loess layers than in soil layers, indicating that the interglacial dusts contain more soluble salts than glacial dusts. These might be attributable to the more abundant rainfalls associated with the higher temperatures and stronger evaporation during the interglacial times

Diverse Regional Sensitivity of Summer Precipitation in East Asia to Ice Volume, CO2 and Astronomical Forcing

The East Asian summer monsoon (EASM) is an important component of the climate system and it influences about one-third of the world’s population. Numerous paleoclimate records and climate simulations have been used to study its long-term evolution and response to different forcings. The strong regional dependence of the EASM variation questions the relative role of ice sheets and insolation on the EASM precipitation in different sub-regions in East Asia. A Gaussian emulator, which was generated and calibrated by interpolating the outputs of 61 snapshot simulations performed with the model HadCM3, is used to quantitatively assess how astronomical forcing, CO2 and northern hemisphere ice sheets affect the variation of the summer precipitation over the last 800 ky. Our results show that in the north of 25°N of the EASM domain, the variation of the summer precipitation is dominated by precession and insolation. This leads to strong 23-ky cycles in the summer precipitation. However, in the southern part (south of 25°N), the impact of ice volume becomes more important, leading to strong 100-ky cycles. Ice sheets influence the summer precipitation in the south mainly through its control on the location of the Intertropical Convergence Zone (ITCZ) which is very sensitive to ice volume. ITCZ is shifted significantly to the south under large ice sheets conditions. Therefore, the region under control of the ITCZ is more sensitive to the influence of ice volume than other regions. Our results also show that obliquity and CO2 have relatively small effect on the summer precipitation as compared to precession and ice sheets

Different Regional Sensitivity of Summer Precipitation in East Asia to Astronomical Forcing, CO2 and Ice Volume

The relative influence of insolation, CO2, and ice sheets on the East Asian summer monsoon (EASM) is not well understood especially at regional scale. We use a Gaussian emulator based on simulations with HadCM3 to quantitatively assess how astronomical forcing, CO2, and northern hemisphere ice sheets affect the variation of the summer precipitation over the last 800 thousand years. Our results show that in the north of 25° N of the EASM domain, the variation of the summer precipitation is dominated by precession, and ice volume only modulates the effect of insolation through influencing the land-sea pressure contrast. This leads to strong 23-ka cycles in the summer precipitation. In the southern part (south of 25° N),the impact of ice volume becomes more important, leading to strong 100-ka cycles. Ice volume controls the precipitation in the southern part via its dominant control on the location of the Intertropical Convergence Zone and the Hadley cell. The effect of ice volume on summer precipitation depends on background astronomical configurations and vice versa. The relationship between summer precipitation and glaciation level varies among latitudes and for different astronomical configurations. Obliquity and CO2 have little effect on the summer precipitation as compared to precession and ice sheets

Regional sensitivity of East Asian summer monsoon to ice sheet and orbital forcing

The East Asian summer monsoon (EASM) is an important component of the climate system, and it influences the economy and life of a large population. Numerous paleoclimate records have been used to reconstruct the long-term evolution of the EASM. The strong regional dependence of the EASM variation as recorded in various proxy records questions the relative role of ice sheets and insolation on the EASM precipitation in different subregions in East Asia. In this study, we used a Gaussian emulator based on simulations with HadCM3 to investigate the relative importance of the orbital forcing and ice sheets on the summer precipitation in different latitudes of the EASM domain over the last 800 ky. Sensitivity analyses are performed to quantitatively assess the role of different factors. Our results show that a strong precessional signal exists in the long-term variation of the summer precipitation in all latitudes, while precipitation shows a different degree of response to ice volume between the northern and southern part of the EASM domain. In the north of 25°N, the ice sheets only modulate the effect of insolation by influencing the land-sea pressure gradient. Reduced land-sea pressure contrasts lead to a weakening of the EASM. Accordingly, the water vapor flux from the Northwest Pacific, one of the major moisture sources for the EASM precipitation, is also reduced. In the southern part, EASM is more sensitive to the glaciation level. A southward shift of the Intertropical Convergence Zone and the Hadley cell in response to the ice sheet forcing explains the stronger drought in southern China than in northern China. The relationship between precipitation and glaciation level varies for different astronomical configurations, showing the necessity of considering the background astronomical forcing when discussing the effect of ice sheets on the EASM

Asynchronous Holocene Optimum in East Asia monsoon region recorded by stalagmites and its underlying climate dynamics

Reconstructions of Holocene Optimum (HO) in East Asian summer monsoon (EASM) regime from speleothem versus other proxy records have yielded divergent phase relationships with the EASM and local precipitation. This apparent discrepancy has been partly attributed to the uncertainties in the climatic representation of Chinese speleothem oxygen isotope records. Here we conducted a data-model comparison along with a water moisture budget analysis to assess the role of thermodynamic and dynamic components in controlling mid-summer and spring rainfall during early and mid-Holocene, and to compare with the precipitation changes referred by the stalagmite oxygen isotope records. Our results show that 1) a marked southward shift of the HO period from 10500~6500 yr BP in North China (NC) to 9000~5000 yr BP in Yangtze river valley (YRV). During the Holocene, the variation of the summer precipitation is dominated by precession in NC, ice sheet in YRV. 2) An incoherent orbital-scale speleothem oxygen isotope variability in EASM regime indicate that speleothem oxygen isotope is largely controlled by the large-scale circulation and concomitant latitude shifts of monsoon rain belt.3) The intensified hydroclimate in YRV in mid-Holocene was contributed to excessive rainfall in spring, especially for increasing the large-scale/total precipitation ratio, which leads to the lightest speleothem oxygen isotope during the mid-Holocene. The excessive rainfall in spring is mainly from the enhancement of horizontal monsoonal moisture transport that is caused by the anticyclone over Western North Pacific

Roman Warm Period and Late Antique Little Ice Age in an Earth System Model Large Ensemble

International audienceChanges in climate during the Roman Warm Period (RWP, 1-250 CE) and Late Antique Little Ice Age (LALIA, 536-660 CE) play a critical role in early societal evolution, but the climate differences between these two periods and the possible causes of the changes remain poorly explored. Here we use the LOch-Vecode-Ecbilt-CLio-agIsM model Large Common Era Ensemble with 70 members to examine the climate change over these two intervals and compare the results of this ensemble with the latest temperature reconstructions from the Past Global Changes 2k network and the transient simulation for the past 2,000 years from the Community Earth System Model. Results from both proxy reconstructions and climate model simulations show warming in mid-to-high latitudes of the Northern Hemisphere (NH) during the RWP compared with the LALIA. This is likely linked with the increased radiative forcing associated with weaker volcanic eruptions in the RWP, which results in reduced sea ice area and pronounced high-latitude warming through surface albedo and lapse-rate feedbacks. This increases the upper ocean heat content over centennial time scales to maintain warming over the NH high-latitude regions. Moreover, the RWP has drier (wetter) conditions in the eastern (western) equatorial Pacific than the LALIA, and this is related to the zonal sea surface temperature gradient in the equatorial Pacific through modification of the zonal circulation

Modeling the Relationship of ≥2 MeV Electron Fluxes at Different Longitudes in Geostationary Orbit by the Machine Learning Method

The energetic electrons in the Earth’s radiation belt, known as “killer electrons”, are one of the crucial factors for the safety of geostationary satellites. Geostationary satellites at different longitudes encounter different energetic electron environments. However, organizations of space weather prediction usually only display the real-time ≥2 MeV electron fluxes and the predictions of ≥2 MeV electron fluxes or daily fluences within the next 1–3 days by models at one location in GEO orbit. In this study, the relationship of ≥2 MeV electron fluxes at different longitudes is investigated based on observations from GOES satellites, and the relevant models are developed. Based on the observations from GOES-10 and GOES-12 after calibration verification, the ratios of the ≥2 MeV electron daily fluences at 135° W to those at 75° W are mainly in the range from 1.0 to 4.0, with an average of 1.92. The models with various combinations of two or three input parameters are developed by the fully connected neural network for the relationship between ≥2 MeV electron fluxes at 135° W and 75° W in GEO orbit. According to the prediction efficiency (PE), the model only using log10 (fluxes) and MLT from GOES-10 (135° W), whose PE can reach 0.920, has the best performance to predict ≥2 MeV electron fluxes at the locations of GOES-12 (75° W). Its PE is larger than that (0.882) of the linear model using log10 (fluxes four hours ahead) from GOES-10 (135° W). We also develop models for the relationship between ≥2 MeV electron fluxes at 75° W and at variable longitudes between 95.8° W and 114.9° W in GEO orbit by the fully connected neural network. The PE values of these models are larger than 0.90. These models realize the predictions of ≥2 MeV electron fluxes at arbitrary longitude between 95.8° W and 114.9° W in GEO orbit

East Asian climate changes during the Quaternary as viewed from multiple proxies and modeling results

Quaternary monsoon changes in East Asia have been extensively investigated by proxy records from continental and marine archives. However, these proxy indicators often show controversial characteristics in terms of trends and rhythms, perplexing understanding of orbital-scale monsoon dynamics. Here we review the orbital-scale monsoon variability and dynamics in East Asia by comparing multiple proxies from loess, lake, speleothem, and marine records with the HadCM3 modeling result. Evolutionary power spectra of loess grain size and sea surface temperature exhibit a remarkable shift from 41- to 100-kyr cycles across the mid-Pleistocene transition (MPT), whereas other proxy records (e.g. 13C of loess carbonate, pollen concentration in lake sediments, and magnetic mineral compositions in marine sediments) display strong and persistent precession cycles through the Quaternary, along with distinct 100-kyr cycles after the MPT. Simulations with the HadCM3 climate model reveal that the effects of orbital parameters, ice volume, and CO2 concentration on the temperature, precipitation, and southerly winds are seasonally and spatially different in East Asia. In the summer season, orbitally induced insolation plays a dominant role in driving changes in these three climate variables except for summer precipitation in south China (20-30ºN), whilst annual changes in precipitation and temperature are jointly affected by insolation, ice volume, and CO2. Proxy-model comparison suggests that several land-based proxies are sensitive to changes in summer precipitation, annual precipitation, and annual temperature, though their responses to astronomical, ice, and CO2 forcing being quite different between north and south China. Our proxy-model comparison reveals that diverse expression of Quaternary climate periodicities was provoked by different sensitivities of marine and terrestrial proxies to seasonal and/or annual changes in precipitation and temperature, and by different responses of temperature and precipitation to insolation and ice/CO2 forcing. We suggest that understanding of Quaternary climate change can be deepened by further comparison of quantitatively reconstructed paleotemperature and precipitation data with results of high-resolution, regional climate models