高雄縣大崗山石灰岩洞穴石筍(DGS-01)沉積環境與特徵

Abstract

In order to study paleoclimate and paleoenvironment in the southwestern Taiwan, we collected a stalagmite (DGS-01) which is 13.7cm in length from Jianfei Cave in Dagangshan Hill in 2009. The Pb-210 dating on the upper 0.6 cm of the stalagmite shows that the stalagmite stopped to grow at least 100 years ago. Three ICP-MS 230Th/U dates have been made on the stalagmite, but with large uncertainties due to high detrital Th contents. The two reasonable 230Th/U dates and Pb-210 dating indicate that the stalagmite had grown from 6000 years ago to about 100 years ago. The growing rates of the stalagmite are 0.040 mm/year in the upper 5.1 cm and 0.012 mm/year between 5.1 and 11.2 cm. The stalagmite DGS-01 exhibits clear growth laminations without apparent growth hiatus. In general, light δ18O and δ13C layers reflecting wetter climate are corresponding to lighter lamination bands, and vice versa. However, other non-climatic factors may affect the growth laminations too. Therefore, we should be cautious when we use the characteristics of stalagmite lamination to interpret climate change. From the trace elemental analyses, we calculate the mean Mg/Ca partitioning coefficient, DMg = 0.0255, and the mean Sr/Ca partitioning coefficient, DSr = 0.1216. Those parameters are the basic data for using the trace elements of the stalagmite DGS-01 to reconstruct paleoclimate. According to the δ18O and δ13C values of 140 samples from the upper 0-14mm of the staglagmite DGS-01, the climatic conditions during 100-450 years in the southwestern Taiwan can be separated into 3 stages. During the I stage (11.7-14mm in depth, from 450~390 years ago), the climatic conditions were dry under weaker summer monsoon, and C4/C3 plant ratio was increased in vegetation. During the II stage (4.8-11.6mm in depth, from 390 to 220 years age), the climatic conditions were relatively wet and stable under stronger summer monsoon. With more rainfall, the vegetation intensity was increased and the C3/C4 plant ratio might be increased. During the III stage (0-4.7mm in depth, 220 to 100 years ago), the averageδ18O and δ13C values were light representing wetter climate and more C3 vegetation. However, the variations of the δ18O and δ13C values were much larger than the previous stages, indicating climate change dramatically. The three stages were all in the Little Ice Age (LIA), reflecting that the climatic modes in the monsoon area had not only cold/dry and warm/wet patterns, but also cold/wet and warm/dry. The δ18O and δ13C records of stalagmite DGS-01 reveal 20-40 years periodical change, similar to the Bruckner cycle (35 years) and the 30-40 years period of the monsoon in the eastern Asia. Our preliminary study on the stalagmite provides the first Holocene record of high-resolution speleothem record in Taiwan. With future work on dating and complete comparative analysis, we are able to obtain climate and environmental changes in Taiwan during the past 6000 years.