Lu Huayu

Evolution of the monsoon and dry climate in East Asia during late Cenozoic: A review

Climate in Eastern Asia is composed of monsoon climate in the east, arid and semi-arid climate in the north and west, and the cold and dry climate of Qinghai-Tibetan Plateau in the southwest. The underlying causes for the evolution of East Asian climate during late Cenozoic have long been investigated and debated, particularly with regards to the role played by the Qinghai-Tibetan Plateau uplift and the global cooling. In this paper, we reviewed major research developments in this area, and summarized the important results. Based on a synthesis of data, we propose that the Qinghai-Tibetan Plateau uplift alone cannot fully explain the formation of monsoon and arid climates in Eastern Asia during the past 22–25 Ma. Other factors such as the global ice volume and high-latitude temperature changes have also played a vital role. Moreover, atmospheric CO2 changes may have modulated the monsoon and dry climate changes by affecting the location of the inter-tropical convergence zone (ITCZ), which controls the monsoon precipitation zone and the track of the East Asian winter monsoon during late Cenozoic. The integration of high-resolution geological record and numerical paleoclimate modeling could make new contributions to understanding the climate evolution and variation in eastern Asia in future studies. It could facilitate the investigation of the regional differences in East Asian environmental changes and the asynchronous nature between the uplift of Qinghai-Tibetan Plateau and their climatic effects. These would be the keys to understanding underlying driving forces for the evolution of the East Asian climate.

Taphonomic and paleoenvironmental issues of the Pleistocene loessic Paleolithic sites in the Qinling Mountains, central China

Hinterland intermountain basins and northern and southern piedmonts of the Qinling Mountains accumulated a large amount of loess during the Pleistocene. The loess strata not only record local paleogeographic and paleoenvironmental changes, but also contain rich hominin fossils and Paleolithic remains. In the northern piedmont of the Qinling Mountains and the lower valley of the South Luohe River, the loess strata have multi loess-paleosol sequences with aeolian loess continuously accumulating during glacial and interglacial cycles. In contrast to the northern piedmont, loess stratigraphy in the hinterland intermountain basins of the Qinling Mountains is relatively thin and contains finer loess particles. In this “mini” type of loess stratum, the density of Paleolithic remain generally is higher than the Loess Plateau in the north of the Qinling Mountains. Based on stratigraphic, chronological, and lithic artifacts analysis in recent years, it appears that the regional lithic assemblage belongs to the Oldowan (Mode I) lithic industry, and it is dominated by choppers, cores, flakes, and simple retouched flake tools from 1.15 to 0.6 Ma. Paleolithic open-air sites such as Gongwangling and Chenjiawo in the Lantian area, Shangbaichuan and Liuwan in the Luonan Basin, Qiaojiayao in the Lushi Basin, Longgangsi and Yaochangwan in the Hanzhong Basin, Guanmiao in the Ankang Basin, and the Yunxian Man Site in the Yunxian Basin are representative sites in the region; from 400 to 250 ka, the Longyadong cave site in the Luonan Basin inherited the characteristics of the local Mode I lithic industry, the stone assemblage is made up of cores, flakes, and small retouched flake tools, such as scrapers, points, and burins; during the period from 250 to 50 ka, bi-facially retouched Acheulean tools (Model II), such as hand-axes, picks, and cleavers, were commonly found in the Qinling Mountains region. The emergence of a large number of Model II artifacts indicates that local lithic industries went through a major transition process. Zhanghuokou, Guoyuan, and Huaishuping sites in the Luonan Basin, Diaozhai, Ganyu, Laochihe, and Xiehu sites in the Lantian area, Hejialiang site in the Hanzhong Basin are representative sites in this period; to the turn of the Late Pleistocene and Early Holocene, it may also exist a small flake-retouched tools lithic industry in the piedmonts of the northern and southern sides of the Qinling Mountains. The lithic assemblages in different stages of the Qinling Mountains region reflect the hominin behavioral changes and the development of lithic technology during the Pleistocene.

Crescentic dune migration and stabilization: Implications for interpreting paleo-dune deposits as paleoenvironmental records

Paleo-dune deposits have been widely used as a proxy indicator of past dune activity, which is further used to reconstruct paleoclimates. However, recent studies have critically examined the reproducibility of dune chronologies and the complexity of paleo-dune deposits as paleoenvironmental records. This paper addresses questions on the paleoenvironmental implications of dune chronostratigraphies that have been raised by those reviews, in the specific case of crescentic dunes, using a case study from the Mu Us dune field, north-central China. The processes of turn-over and stabilization of relatively small crescentic dunes are first investigated by observational evidence. In combination with the analysis of a simplified sand preservation model and stratigraphic records, the effect of dune morphodynamics on sand preservation is demonstrated. It is especially evident that thick, nearly instantaneously deposited sand units record dune stabilization near the very end of a dune activity episode, while thin sand units may signal the preservation of sand deposited earlier in episodes of activity. Interpreting the distribution of luminescence ages that indicate sand deposition over time is not as simple as assumed in some previous work. Low frequency of sand ages could indicate an interval of either dune field stabilization or extensive dune activity but poor sand preservation. A peak of sand age frequency likely represents a shift in dune field activity towards stabilization, not a peak of active dune extent, especially if it partially overlaps with an independently identified interval of stabilization (e.g. one recorded by paleosols). The nature and magnitude of these biases in the distribution of sand ages over time are strongly affected by the magnitude of net sand accumulation, which is in turn related to sand supply, transport capacity and sand availability, and ultimately climate change. Relatively short dune stabilization and turn-over time (101 to 102 yrs) indicate that dune geomorphic processes can quickly respond to short-term disturbance, but the chronology of that response must be interpreted in light of how those processes influence age distributions.

Sand dunes as potential sources of dust in northern China

While saltation bombardment of sand grains on a fine substrate can produce considerable dust, the well-sorted nature of sand dunes tends to preclude them from consideration as major dust sources. Recent research, however, has revealed that sand dunes can, in some cases, be large sources of dust. We used the PI-SWERL (Portable In-Situ Wind Erosion Laboratory) to measure in the field the potential of sand dunes and other desert landforms to emit particulate matter <10 μm (PM-10) dust in the Tengger, Ulan Buh, and Mu Us deserts of northern China. Combined with high resolution particle size measurements of the dune sand, an assessment of sand dunes as a dust source can be made. Large active transverse dunes tend to contain little to no stored PM-10, yet they produce a low dust flux. Coppice dunes stabilized by vegetation contain appreciable PM-10 and have very high dust emission potential. There is a positive correlation between the amount of PM-10 stored in a dune and its potential dust flux. Saltation liberates loose fines stored in dunes, making them very efficient dust emitters compared to landforms such as dry lake beds and washes where dust particles are unavailable for aeolian transport due to protective crusts or sediment cohesion. In cases where large dunes do not store PM-10 yet emit dust when active, two hypotheses can be considered: (1) iron-oxide grain coatings are removed during saltation, creating dust, and (2) sand grains collide during saltation, abrading grains to create dust. Observations reveal that iron oxide coatings are present on some dune sands. PI-SWERL data suggests that low dust fluxes from dunes containing no stored dust may represent an estimate for the amount of PM-10 dust produced by removal of iron oxide coatings. These results are similar to results from dunes in the United States. In addition, PI-SWERL results suggest that dust-bearing coppice dunes, which cover vast areas of China’s sandy deserts, may become major sources of dust in the future if overgrazing, depletion of groundwater, or drought destabilizes the vegetation that now partially covers these dunes.

Climate changes in China since the Last Glacial Maximum and Holocene human adaptation

Based on various geological records of high-resolutions, this project aims at addressing the climate and environmental histories in China since the Last Glacial Maximum (LGM) and human impacts on the Holocene environmental changes. Significant progresses have been obtained with regards to the impacts of past temperature changes to Asian monsoon and to the arid/semi-arid ecosystems in China. The results also suggest that changes in the concentrations of greenhouse gases in the pre-industrial era were mainly attributable to natural driven forces although early agriculture had significantly affected the landscapes.

What causes the ice ages in the late Plioceneand Pleistocene

For more than a century, two families of ice age theories have been proposed: insolation based theories proposed by Adhemar, and atmospheric CO 2 ones proposed by Tyndall. The major technique advance of deep-sea sediment drilling, as well as new interpretations of stable oxygen isotopic composition of the deep-sea fossil foraminifera, which is regarded as a proxy index of continental ice volume and sea water temperature, established the now well recognized glacial-interglacial variations in climate over the late Pliocene and Pleistocene, e.g. ~3 Ma. A landmark progress came from Hays et al. (1976) who unambiguously demonstrated that the change in insolation induced by the Earth’s axial tilt (obliquity), the wobble of this tilt (precession), and the degree of circularity of the Earth’s orbit around the sun (eccentricity) brings on ice ages (glaciations) every hundred thousand years or so during the Pleistocene. Many subsequent observations, from the deep-sea to continental records, confirmed the co-variation between insolation and the alternation of glacial ice age phases, and warmer interglacial phases, at least in terms of frequency domains. However, these findings cannot explain the so called ~100 ka conundrum, where the size of 100 ka insolation forcing is relatively small yet the apparent ice sheet response is large, as well as the bi-polar symmetry in climate changes at precessional frequency of ~23 ka, despite the fact that the precession cycle drives insolation changes in antiphase between the two hemispheres. As such, other factors may be considered in the driving of Northern Hemisphere Glaciation (NHG) global glacial-interglacial variations. Several hypotheses have been put forward, such as: (1) the gradual decreasing CO 2 drove global cooling through to a threshold value where insolation changes may force the ice age cycles; (2) the effect of local insolation that modulated the East Antarctica Ice Sheet after the gradual CO 2 decline that drove its growth to maximal extent; (3) the Southern Ocean “marine biological pump” changes forced by the ocean water stratification and current changes that modulated the atmospheric CO 2 thus triggered ice ages; (4) the clearing of regolith under the North American ice sheets which modulate ice sheet thickness and extent changes which allow the ice sheets to grow further south and be influenced more by precessional cycles, as well as strengthened the fresh bedrock weathering and drawdown more atmospheric CO 2 ; and (5) because ice mass balance depends on whether the temperature is above or below the freezing point, a physically more relevant parameter to measure insolation forcing should be the insolation integrated over a given threshold that allows for ice melting, and thus triggering the glaciations. Unfortunately, we cannot determine which one is right or several of them acting together so far. The most distinct features of the ice age climate are both the onset of the NHG at ~2.7 Ma and the transition of ~41 ka based glacial-interglacial cycles to cycles of a ~100 ka dominated frequency at ~1.0 Ma, namely the Early Middle Pleistocene Transition. We suggest that, both insolation and CO 2 changes together forced the climate of the ice ages: the stepwise cooling that occurred at ~2.7 Ma and ~1.0 Ma may be linked with gradual atmospheric CO 2 reduction, during which the atmospheric CO 2 levels fell to a threshold value that triggered high-latitude (both poles) ice expansion. Under a new cooling state since ~2.7 Ma, the dominant climatic cycle at ~41 ka was forced by obliquity (orbital tilt), while the ~23 ka precession cycle was cancelled by opposing effects at both poles. On the other hand, we propose that the ~100 ka cycles that became dominant following approximately 1 Ma were forced by a CO 2 based “marine biological pump” change in the Southern Ocean, or is related to stochastic behavior of the ice sheets. The ~400 ka eccentricity cycle, which is a stronger absolute influence on insolation is an outstanding rhythm in the late Cenozoic, yet is not as strong in climate records as the precessional and obliquity cycles during the late Pliocene and Pleistocene, and thus needs further examination. In summary, we suggest ice ages are caused by the integrated forcing of insolation, atmospheric CO 2 , ocean conveyer and ice sheet feedback etc., in which the insolation forcing is primary, a threshold value of the other facts was attained, the ice age happened. However, our hypothesis is very tentative, more investigation is still needed on what causes the ice ages.

Inversion of the asymmetry factor for desert areas of China

Deserts, which have high surface albedo and wide area, are important components of the earth system. It is very important for the research of surface radiation and energy balance to understand the anisotropic scattering of desert areas. The emergence and development of multi-angle remote sensing made possi]e the inversion of the anisotropic scattering of desert areas at the regional or global scale. Firstly, this paper explored the accuracy of the inversion of asymmetry factor using the Hapke model and the simulated single- and multi-phase MISR data. Based on the results, the asymmetry factor of representative surface of desert areas in northwestern China was retrieved. The values of the asymmetry factor retrieved from MISR data were compared with the values retrieved from the laboratory data. The results showed that the single-phase MISR data could be used for the inversion of asymmetry parameter of desert areas. The sign of the asymmetry parameter for the laboratory measurements was positive, which suggests that the surface of laboratory samples is forward scattering. The sign of the asymmetry parameter for MISR data was negative; that is, it is backscattering. The values of the asymmetry parameters retrieved from MISR data were related to the character of the land surface. At Loulan, where the surface was smoother than other sites, retrieved values exhibited the largest negative values of asymmetry factor, suggesting the strongest backscattering. The sand dune area of the Kumtag Desert, which has the greatest roughness, had only slightly negative asymmetry factor values. These findings indicated that at the sensor scale, a rough surface (e.g., dunes) does not necessarily mean more backscattering than a smooth surface. This finding has significant implications for empirical methods (e.g., using the normalized index of backward-scattered radiance minus forward-scattered radiance as an indicator of surface roughness), which should be used carefully for analyzing surface roughness from the remote sensing data.