缅甸Twintaung玛珥湖碳酸盐年纹层及100年来微区原位元素分析

Abstract

We report on maar lakes and lacustrine carbonate varves in Myanmar. Carbonate varves are well developed in maar lake Twin Taung, the deepest of five maar lakes in Myanmar. Lamination consists of light and brown-colored laminate couplets in thin sections. A light- colored layer is composed mainly of calcite crystals with thickness 50-400 mm, and a dark-colored layer consists of other organic and siliceous matter. Climatic conditions in the study region are determined by the Indian monsoon and have a pronounced rainfall seasonality. The dry and hot season is between December and April, and warm and humid conditions prevail in summer and autumn. These characteristics are key to varve formation. In the dry and hot season, calcite crystals form with less precipitation and strong evaporation, whereas clastic and organic matter are deposited during the warm and humid season. An independent chronology derived from 137Cs and 210Pb shows good agreement with counted laminations. To compare with instrumental data, synchrotron radiation X-ray fluorescence measurement was used for extracting minor element datasets with high resolution. Based on principal component analysis and comparison between elemental variation and regional rainfall data, we interpret the elemental data in terms of climate variables such as precipitation and temperature. The first component includes lithogenic elements such as K, Ti, Fe, Cu, Zn, Rb and Zr, and might be controlled by various factors such as physical and chemical weathering processes, rainfall, and dust input. The second component, calcium and strontium, might be principally regulated by the balance between evaporation and precipitation. During drought periods, the calcium ion might supersaturate, causing carbonate calcite precipitation from the water column. Our preliminary result suggests that elements Ca and Sr in the annually laminated sediment might be useful indicators for studying paleoclimatic change at high resolution.