The origin of loess microstructure: 3D insights from initial aeolian dust packing

Major and trace elements as well as strontium isotopic composition have been analyzed on the acid-insoluble (AI) phase of the loess-paleosol sequence from Luochuan, Shaanxi Province, China. Results show that the chemical composition of AI phase of loess and paleosols is distinctive to the average composition of upper continental crust (UCC), characterized by depletion of mobile elements Na, Ca and Sr. The distribution pattern of elements in AI phase reveals that initial dust, derived from a vast area of Asian inland, has suffered from Na- and Ca-removed chemical weathering compared to UCC. Some geochemical parameters (such as CIA values, Na/K, Rb/Sr and 87 Sr/ 86 Sr ratios) display a regular variation and evolution, reflecting that the chemical weathering in the source region of loess deposits has decreased gradually since 2.5 Ma with the general increase of global ice volume. This coincidence reflects that the aridity of Asian inland since the Quaternary is a possible regional response to the global climate change.

Atmospheric mineral dust is a poorly constrained yet extremely important component of the climate system. Provenance studies from geologic dust archives are crucial to understand the drivers of the dust cycle over long time scales. Our multi‐technique provenance analysis of a rare Paleogene (35–27 Ma) eolian dust sequence from Ulantatal, ∼400 km northwest of the Chinese Loess Plateau (CLP), shows that Paleogene dust transporting winds generally varied between northwesterly and westerly, the same as those in the late Neogene‐Quaternary bipolar icehouse. We propose that, as today, westerly wind circulation patterns would have been modulated by an Arctic Oscillation (AO)‐like situation, and that the warm Eocene favored a long‐term negative phase of AO, leading to meridional westerly circulation and the dominance of a northwesterly dust transport pathway. After the Eocene‐Oligocene transition (EOT), long‐term positive phase of AO‐like conditions initiated, leading to stronger and more zonal westerlies. The Siberian High (SH) also formed or strengthened at the EOT and started to control dust storm activity along the northwesterly transport pathway. We argue that increased Paleogene Northern Hemisphere (NH) ice volume was the ultimate driver of this modern‐type dust transport regime in the Ulantatal region, possibly also controlling initial Ulantatal dust sequence formation via the development of the SH and modern‐type eolian regime. The similarity between the Ulantatal and late Neogene northern CLP dust provenance signals suggests that the increased NH ice volume, via its control on the northwesterly dust transport, could have promoted increased loess formation also in the late Miocene.

Aeolian dust from hot spots in eastern China and Mongolia can be carried downwind to provinces in China, neighboring countries, the Pacific islands, and cities far beyond the source region. Although dust sources of huge extent have been identified in several countries, few effective countermeasures are available to combat dust emissions in arid regions. We analyzed Moderate Resolution Imaging Spectroradiometer (MODIS) images (1 km spatial resolution) that captured dust emission and dispersion during 2000–2013 to determine dust sources in eastern China and Mongolia. MODIS level 1B data and the brightness temperature difference (BTD) algorithm provided efficient discrimination of dust in this study. The derived dust information, in conjunction with the MODIS land cover product (1 km spatial resolution) and high-resolution Landsat data (30 m spatial resolution; Landsat 8, Operational Land Imager sensor) were used to identify the locations and specific sources of dust. Dust emissions appear to be sporadic in time and space, controlled by both environmental factors and human activity, although past studies have indicated that many dust emissions are from consistently active hot spots. Analysis of MODIS data indicated that three subregions of the eastern China and Mongolia source region are the dominant sources of dust: Horqin Sandy Land, Otintag Sandy Land, and the southeastern Mongolian Gobi; each of these subregions contains dust emission hot spots. We identified the locations of consistent hot spots and verified that some individual dust emissions originated from those hot spots. Our data also indicated that hot spots in southeastern Mongolia have migrated northward since 2006. Our study showed that hot spots such as dry lakes, river beds, mines, and croplands contribute to dust emissions in the eastern China and Mongolia source region. Dust hot spots coincide with regions of expanding industry in Otintag Sandy Land and in some areas of the Mongolian Gobi and with agricultural areas in Horqin Sandy Land and in some parts of the Mongolian Gobi. In Horqin and Otintag sandy lands, dust sources are associated with ephemeral water bodies. Water conservation can be an important countermeasure for initial dust emissions in the Horqin Sandy Land. In the Otintag Sandy Land, attention should be paid to human activities, for example, minimizing the effects of mining disturbances, improving dust suppression in industrial areas, and controlling water use by industry. In Mongolia, protective farming techniques and water conservation in dust emitting basins, and dust suppression and water resource protection in mining zones, must be considered to combat dust emission. MODIS level 1B data can be used to locate dust hot spots and to identify future sources of dust entrainment. Dust hot spots identified from MODIS level 1B data provide small-scale information about dust emission that can be used to locate pollution hot spots, increase understanding of the global dust cycle, and improve dust modeling.

A series of experiments to determine the direct emission of dust-sized particles from Gobi surfaces by clean wind (wind without sand), and the potential for aeolian abrasion of Gobi surfaces and beds of gravel and mobile sand to produce fine ( 100 μm in diameter and then PM10 by sedimentation was acquired. The Gobi surface provided most of the emitted fine particles during the initial dust emission processes, but subsequently, release of the clay coatings of particles by abrasion becomes the dominant source of fine materials. Under sand-laden winds, PM10 production rates produced by aeolian abrasion of Gobi surfaces ranged between 0.002 and 0.244% of blown materials. After removal of sand, silt, or clay with low resistance to erosion from the Gobi surfaces by the wind, the PM10 production rates caused by aeolian abrasion were similar to those from gravel and sand beds. The results also indicated that after the dust-sized particles with low resistance to erosion were removed, the production of dust-sized particles was unrelated to wind velocity. Under aeolian processes, Gobi deserts in this region therefore play a major role in dust emissions from arid and semiarid China.

Wetting-induced collapse of loess: Tracing microstructural evolution

The transport and fallout of dust during the advance of offshore dust cloud over water surface is studied. Some important issues concerning this problem are re‐examined, particularly the effect of water surface presence on the concentration field and deposition rates of dust. The analysis will show that an inner or surface layer may exist near the water surface, where surface retention or absorption processes are dominating and that large dust concentration gradients will be sustained there. A logarithmic concentration profile is shown to exist, extending from the surface roughness height throughout the surface layer thickness, which will be shown to be dependent upon the wind strength and the dust particles sedimentation velocity. The surface layer structure will not be dependent upon the particular profile assumed by the initial dust concentration at the land‐sea boundary, and the analysis will show that the dust fallout rates due to both sedimentation and surface absorption effects combined can be significantly larger than those commonly obtained by considering sedimentation effects alone. The theory can be used in assessing both the magnitude and the extent of aeolian sediments contribution to marine environment which exist adjacent to large desert area.

Effect of rock fragment eccentricity on eolian deposition of atmospheric dust