пятница, 26 апреля 2019 г.

Holocene changes of landforms and environments in the eastern portion of Asian...

Deserts belong to the most beautiful landforms on Earth although their potential encroachments may threaten daily lives of many people, in particular those in developing countries. Studies of past environmental changes in deserts are of great significance for understanding the nature of arid ecosystems and for establishing effective strategies to overcome the threat of desertification to the global communities in the 21st Century.

Holocene changes of landforms and environments in the eastern portion of Asian mid-latitude deserts
Location of study area. (A) Overview of the drylands in north China. The Gonghe Basin is located in the
 marginal zone influenced by the Asian summer monsoon. The modern Asian summer monsoon limit is
indicated by a dark dashed line redrawn from Gao et al. (1962). (B) The Gonghe Basin (Google Earth™),
with locations of the studied aeolian sections and the Lagan (LG) section (Liu et al., 2013)
indicated by solid blue squares [Mingrui Qiang et al. 2016]

The remains of paleo-landforms and sedimentary sections in deserts allow scientists to reconstruct phases of aeolian activity or occurrence of sand dunes, and the state and timing of environmental stability due to the increase in vegetation coverage. Because of extreme sensitivities of desert ecosystem, the study of deserts can help scientists better identify not only the nature and history of deserts but also the variability of climate system on Earth.

Sand seas dominated by active sand dunes, the most common landforms in deserts, are widely distributed in the arid regions of northern China with a mean annual precipitation less than 200 mm, while the sandy lands characterized with semi-fixed and vegetated (at least partly) dunes mainly occur in the semi-arid regions with a mean annual precipitation of 200-400 mm in the mid-latitudes of Asia. The spatial and temporal variations of these vegetated and active dunes provide clues to decipher changes of global climate systems, human activities and the interactions between natural and human factors.

Based on careful field investigation over decades and geochronological and paleoenvironmental data, Professor Xiaoping Yang of Zhejiang University in Hangzhou and his collaborators from several institutions jointly studied the Holocene histories of the sand seas and sandy lands in the eastern portion of the desert belt in northern China.

Their results show that spatial and temporal heterogeneity has been common in the dune fields of northern China. They demonstrated that it is a must to obtain more than a few sedimentary records in order to correctly reconstruct palaeoclimatic histories in the arid and semi-arid areas with diverse landforms like those in northern China.

Holocene changes of landforms and environments in the eastern portion of Asian mid-latitude deserts
Photographs of the studied aeolian sections [Mingrui Qiang et al. 2016]

One of the difficulties in interpreting paleoenvironmental histories in deserts is that new phase of aeolian activities might be just a reworking of previously deposited sediments from an existing dune system, resulting in a lower probability of preserving older deposits, a potentially large gap in the palaeoenvironmental records. Even though, the team is able to have recognized periods of environmentally stable conditions in the eastern portion of the desert belt in northern China on the basis of large number of new data.

On the contrary to the earlier doctrine of millions of years in age, the new study shows that the sand seas and sandy lands in the eastern portion of the desert belt in northern China is very vulnerable to climate changes and to human activities. The current Kubuqi Sand Sea, the only sand sea in the semi-arid regions of China, has been formed during the Holocene, partly due to agricultural cultivation in historical times.

A long-lasting, although not equally long, stabilization of aeolian landforms had occurred almost in every sandy land in northern China during the middle Holocene according to their studies. The team emphasized that dune activities cannot simply be considered as an indicator of climate aridity as done in some early projects. Due to strong interactions between aeolian, fluvial and lacustrine processes in these sandy lands, the study areas provide an ideal laboratory for understanding Earth surface processes and Earth system.

Such kind of studies may help decision-makers establish ecologically sustainable land use strategies. These research results have just been published online in the journal Science China Earth Sciences under the title «Holocene aeolian stratigraphic sequences in the eastern portion of the desert belt (sand seas and sandy lands) in northern China and their palaeoenvironmental implications».

Source: Science China Press [April 23, 2019]



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