dc.description.abstract | The goal of this research is to investigate the characteristics of the paleo-climatic
and paleo-environmental changes in Ilan, NE Taiwan by applying environmental
magnetic analyses to a lacustrine core MHL5-A, drilled from Mei-Hwa lake in Ilan,
and expect further to understand the evolution of the East Asia monsoon system.
This study is a part of an integrated project, named as 「Asia Paleo-environmental
Changes」, held by Institute of Earth Sciences, Academia Sinica. The purpose of
this project is to systematically study the paleo-climatic and paleo-environmental
changes since late Pleistocene in East Asia.
Taiwan is located at the intersection of the East Asia. Winter monsoon generated
from Siberia high atmospheric pressure and the summer monsoon originated from
the Indian Ocean. Nonetheless, the heavy rainfalls were generally brought mainly
by the NE winter monsoon and the typhoons during summer and autumn, but not
the summer monsoon. This is due to the high Central Range stems the SW summer
monsoon and horn shape topography faces the NE winter monsoon in Ilan. By
measuring magnetic proxies, such as magnetic susceptibility (χ), natural remanent
magnetization (NRM), anhysteretic remanent magnetization (ARM), and
isothermal remanent magnetization (IRM) etc., we hope to probe into the patterns
of the paleo-climatic and paleo-environmental changes in the study area, Ilan.
The age model of the core was established by 7 C-14 dating data. The results
lead to totally of about 5200 yrs covered by this 28.63 meters core. And the
climatic condition could be concluded into 4 types by the variation trends of the
used magnetic proxies, major by S-ratio, ARM/χ, ARM/SIRM, etc. Twenty
episodes then could be compartmentalized for the last 5200 yrs in Ilan. The
appearance and characteristics of these 4 climate patterns are summarized as follows: (1) During 5300~5000, 4850~4500, 4300~4100, 3700~2950, 2500~2300,
1950~1850, 1700~1400, 1100~700, and 500~400 yr BP: The values of S-ratio
varied small and almost ranged in 0.9~1. This signifies low-oxidized magnetite is
the major magnetic mineral contained in the sediments. The climate then could be
steady and wet. (2) During 5000~4950, 3850~ 3700, 2950~2500, and 1850~1700
yr BP: The values of S-ratio were almost lower than 0.9, even some were lower
than 0.8. Besides the grain size of magnetite is relatively coarser. This indicates
that high-oxidized hematite is the major magnetic mineral in the sediments. The
climate then could be dry. Surface soils may have highly weathered and oxidized
to form much more high-oxidized hematite. As the torrential rain happened,
hematite with coarser grain-sized magnetite could be transported from the
upstream into the lake. (3) During 4950~4850, 4500~4300, 4100~3850, 2300
~1950, 1400~1100, and 700~500 yr BP: The values of S-ratio changed frequently
between 0.8 and 1. The abundance of magnetic minerals also altered frequently.
The climate in these episodes could be very unstable and rainfall happened
frequently. (4) Till the last 400 yrs, the climate also changed frequently, but not so
fast as the climate condition in (3). And it seems had 100~150 years’ period
variation. Moreover, the most different thing from previous episodes is that
relatively much finer grain-sized magnetite were conducted into the lake and it has
happened four times. The delivered mechanism could be due to the floods of
nearby Lo-tung River, which induced much fine grain- sized magnetite overflow
into the lake. Between the occurrences of each flood, the climate might gradually
become dry. So there were less magnetic minerals inputted into the lake. But still
some paramagnetic minerals were eroded into the lake.
In conclusion, the paleo-climatic and paleo-environmental changes severely in
Ilan since the late middle Holocene. | en_US |