中国台湾大鬼湖沉积物所保留之降尘记录

本文以中国台湾大鬼湖沉积物岩芯回推此区过去1600年以来PM10与空飘重金属之变化情形.湖区沉积物主要来自集水区冲刷来的岩屑及植物碎屑,一般为黑色,平均粒径30～50μm,PM10约10%～20%.但在干冷时期,由空飘来的祖国大陆北方沙尘较多,形成相对亮度指数较高的白层,其磁化率较高、平均粒径约10μm,且PM10可达60%;而从1350年以来,无机元素沉积累积速率较前期有所增加,且白层中大部分无机元素皆有较高的累积速率,可能导因于空飘物质输入量增加.较暖湿时期,则以亲有机质元素之沉积累积速率较高.

EOLIAN DUST FLUX RECORDED IN THE SEDIMENTS OF GREAT GHOST LAKE, TAIWAN, CHINA

During winter and spring, cold, dry air originated from the Asian continent carries eolian dust from China's deserts and the Loess Plateau to Taiwan region, and results in an abrupt increase in PM10 （sediments finer than 10um） in the atmosphere. During dry periods, the southward movement of the winter monsoon belt and the declining incidences of typhoon lead to a deficiency in rainfall in Taiwan region. At the same time, there are more eolian dust emitted from China's deserts and the Loess Plateau, but less rain to wash these out. As a result, more dust reach Taiwan region. Because of the remoteness of the lake and the anoxic condition of the lake water, Sediments of the subalpine Great Ghost Lake are well preserved, and serve as archives of paleo-eolian dust flux and climatic events. In this report, we use the relative brightness index （RBI）, grain size, magnetic susceptibility, as well as elemental and organic compositions to unravel the variations in PM10 and eolian trace metal fluxes during the past 1 600 years. These sediments are mostly black in color with many thin white laminae an indication of events happening in the normal sedimentary environment. Black sediments composed of silt to fine sand--size siliciclastics and land plants debris are transported to the lake mainly from the surrounding watershed. During an arid climate, sediments that come from the watershed decrease due to lesser runoff, but more wind--blown dust comes from the northwestern China due to less vegetation cover in the deserts and the Loess Plateau. White laminae thus replace normal, black sedimentation. Well--sorted white laminae have a 60% PM10 and a median grain size of 10um; but the PM10 of black sediments is only 10%--20% and the average grain size is 30-50um. Both black and white sediments are single--mode in their grain size distributions with characteristic sizes, but sediments of the upper part of the core have hi--modal distributions. This also implies that these black and waste laminae have two different sources. Boundaries between the white and black laminae are sometimes sharp sometimes gradual. Further, white laminae have similar grain size as that of the loess, and their occurrences correspond to the frequency of dust events recorded in the northern China as well as in the ice cores of Peru. When the climate was dry and cold （420 520A. D., 620-- 700A. D., 850-- 930A. D., 1320-- 1470A. D., 1570A. D., 1730-- 1800A. D. and 1960A. D.）, concentrations of TOC, TN, ac, TS and organophiles （Br, Ca, Cd, Cu, Fe, Mn and Ph） were low, but RBI, magnetic susceptibility, PM10, and lithophiles （Al, Ba, Cs, K, Mg, Na, Rb and Si） were high. The latter group of elements have positive correlation with Al, but prominently negative correlation with TOC. This may indicate that they tend to be enriched in lithogenic matters. Such kind of elemental distributions may reflect the increase of wind--blown dusts and the decline of land--plants in the watershed during dry, cold Periods. Since 1350 A.D., RBI and MAR of the inorganic particles start to increase. Fluxes of inorganic elements in the white laminae are also higher.This represents increasing long--range transport of crustal materials from arid regions in China. Besides, the Ba / Ti ratio of loess in the Loess Plateau is high, so is the ratio in the major white laminae. This is another indication that the white laminae originated from eolian dust. On the contrary, black sediments which are an indicator of warm and humid climate, have lower RBI and fluxes of lithophiles, but higher magnetic susceptibility, content of organic matters and flux of organophiles. The positive correlation between TOC and the organophiles indicates that these elements may be incorporated into land plants during a wetter climate. The relationship of higher C / P, N/ P and S / P ratios to higher TOC also implies that the black sediments were deposited when land plants were in full flourish during the humid climate. Besides these two groups of elements discussed above, there is still another group of elements. These (As, Ce,