Environmental Information From Aeolian Loess Deposition At Langgangshan Islands Of East China Sea

Loess deposition is a special kind of dust-sediment that came into being under particular environments in quaternary period. Loess deposition in China is widely distributed, continuously deposited and sensitively responded to paleo-environment and paleo-climate changes. So it is of great theoretic significance to study loess in Quaternary research. Xiashu loess that is distributed over the west hilly lands of Yangtze Delta and the aeolian loess and the secondary loess over plain of East China, islands and submarine paleo-deltas of East China Sea, are all important components of loess in China as a special type. Based on a great deal of field investigations of the islands of East China Sea, an aeolian loess profile at Langgangshan islands was chosen as the study object. After many systematic sampling in the profile, the grain size, the environmental magnetic characteristics, the organic matter, the carbonate, the clay minerals, active iron and optically stimulated luminescence (OSL) dating were systematically analyzed. The main results can be concluded as follows:1. The grain size composition of the aeolian loess on Langgangshan Islands is basically consistent with that of other inland loess in China, which accords with the character of grain size composition of Chinese loess deposit. The loess at Langgangshan belongs to clay-clayey soil deposit by grain size composition, which coincides with that e sandy-loess zone, loess zone and clayey-loess zone distribute from west to east in China. Compared with that of other inland loess, the content of >50μm grains of the loess is relatively higher. As a matter of fact, it indicates that the study profile received not only far aeolian deposit from northwest by strong winter monsoon as other inland loess but also proximal one from inshore shelf area during the last glacial age. And the content of >50μm grains of the loess can be regarded as an index that shows the proportion of proximal aeolian deposit.2. The content of magnetic grains in the study profile is high and the average magnetic susceptibility varies from 69.3 X 10~-8m~3kg~-1 to 93.8 X 10~-8m~-3kg~-1. And the magnetic characters are attributed to moist climate and strong pedogenesis. The type and grain size of magnetic grains in the study profile are similar to those in other inland loess profiles, both mainly consisting of pseudo-single domain (PSD) and multi-domain (MD) ferrimagnetic minerals, which indicates that the island loess have something in common with other inland loess, such as material source anddynamical cause of deposit. Not being high correlation with the content of <1μm grains, magnetic susceptibility cannot be an index that denotes the degree of regional summer monsoon and pedogenesis. But the lower segment of magnetic susceptibility can be considered as the degree of regional winter monsoon.3. The content of organism is lower in Langgangshan loess, which has something to do with little organism being born in dry-cold climate during loess deposit and some being lost in moist climate subsequently. Series of oxidation-reduction layers are parted by the ratio of Fe~3+/ Fe~2+in the study profile. Langgangshan loess is distinct in low of carbonate from other inland loess. High strength long-term eluviation results in that almost all of carbonate lost in the loess profile. The microelement content of Langgangshan loess is close to that of other loess, which denotes that all loess in China is identical in material component and material source. But the loess also appears lower in microelement contrasting to other loess because of moist climate since Holocene. Primary component of clay minerals in the loess is illite as well as other loess. While the distinctly higher content of vermiculite is the significant character of clay minerals in Langgangshan loess, which is attributed to excessive rainfall around Langgangshan islands.4. The dating data of the three samples from 10cm, 115cmand 225cm of the study profile determined by infra-red stimulated luminescence (IRSL) are separately 40.2 ± 2.7kaB.