| 其他摘要 | Soil erosion in the Loess Plateau is an important issue related to the safety in the
middle and lower reaches of the Yellow River and the sustainable development in the
Loess plateau. Uncountable fruitful results have been achieved in terms of the physical
processes of rainfall on the soil erosion and sediment yield, soil and water conservation
effect of the vegetation, soil erosion spatial and temporal distribution, etc. However, there
have been long many controversies about the soil erosion origin, drives and factors of the
geological time in the Loess Plateau. The main limit is the lack of the verification about the
sedimentary geological records in the downstream. Thus, one long drilling core (1000 m)
named Luyang Lake No.1, one short drilling core (50 m) named Luyang Lake No.2, and
one artificial profile (2.8 m), located in the southeastern of Loess Plateau, were obtained as
the research materials to solve these questions by analyzing paleomagnetic dating,
optically stimulated luminescence dating, carbonates, grain size and magnetic
susceptibility, etc. The deposition rate of 400 m in Luyang Lake No. 1 drilling core, No. 2
drilling core (50 m) and the Tianjiao Lake profile (2.8 m) were calculated, respectively.
The main results are as follows:
(1) The results in Luyang Lake No.1 drilling core 400 m (5.23 Ma) showed that the
deposition rate increased obviously since 2.58 Ma, which was consistent with the bottom
age of loess-paleosol profiles. There were seven obviously accelerating periods in
deposition rate including 4.18-3.58 Ma, 2.5-1.7 Ma, 1.07-0.78 Ma, 0.78-0.12 Ma, 0.12-
0.09 Ma, 67-63 ka, 13-9.8 ka. Among them, the later six stages had a clear correlation with
the uplift activities in the Tibetan Plateau since Qingzang movement (2.58 Ma), indicating
the soil erosion in the Loess Plateau in long time scales was controlled by the uplifting
effect of Tibetan Plateau. The deposition rate could be divided into two stages with 0.12
Ma as the border since Quaternary. Between 2.58 and 0.12 Ma, there were only several
weak increases in deposition rate, ranging from 3-10 cm/ka. But after 0.12 Ma, the
deposition rate significantly increased by 10 fold compared to the previous stage,
indicating that 0.12 Ma was an important turning point for the evolution of soil erosion
environment in the Loess Plateau. (2) No.2 drilling core 50 m (150 ka) sedimentary sequence showed that the intense
tectonic uplift around 120 ka corresponded to the Loess-paleosol S 1 stage, indicating the
high deposition rate also happened in the warm and wet climate situation. In this period,
many erosion surfaces distributed in the upper reaches of Luyang Lake, and in the lower
reaches, the Yellow River completely cut through the Sanmen Gorge in this period. These
evidences showed that the high deposition rate around 120 ka was the combined
interactions of tectonic uplifts, humid climate and the decline of local erosion base level.
(3) The artificial profile 2.8 m (33.4 ka) in the Dongtan of Luyang Lake showed that
the lake level experienced the change of low-high- low process, which is consistent with
the climate change series in Guanzhong Basin since the LGM, indicating the lake level
change was primarily controlled by the East Asian summer monsoon. The dry climate after
4.6 ka in late Holocene caused the extensive withering of Luyang Lake, creating the
conditions for the rapid extinction of Luyang Lake. On this basis, the rapid increases in the
reclamations, salt production and other human activities around the lake accelerated the
lake deposition rate, making the lake turned into a salty swamp and went into the
extinction.
Sedimentary sequences in Luyang Lake showed that soil erosion in the south
Loess Plateau should start in the early of Quaternary, and accelerated around 1.0 Ma.
The high deposition rate periods corresponded to the uplift stage of the Tibetan
Plateau and the river terrace ages in the Loess Plateau, indicating that the evolution of
the soil erosion environmental in the loess plateau since Quaternary was mainly drove
by the tectonic uplifts of Tibet Plateau. The high deposition rate around 120 ka
indicated that the intense erosion event was closely related to the humid climate,
tectonic uplifts and the local environmental change. The increase of human activities
around Luyang Lake in recent two thousand years accelerated the deposition rate and
caused the rapid extinction of Luyang Lake.
Key Words: Loess Plateau; Erosion Environment; Luyang Lake; Deposition rate |
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