南冲绳海槽沉积物中淡水硅藻的发现及1000年以来台风降雨的重建

FRESHWATER DIATOMS IN THE SOUTHERN OKINAWA TROUGH AND ITS IMPLICATION TO TYPHOON PRECIPITATION DURING THE LAST MILLENNIUM

利用主成分分析法对冲绳海槽南部MD05-2908孔近1000年来沉积物硅藻数据进行研究发现,MD05-2908孔各样品在主成分一上的得分情况与该于孔淡水硅藻含量、夏季海水表层盐度变化以及台湾近1000年来的降水异常有良好的相关性,因此,各样品在主成分一上的得分可作为台湾东北部台风降雨指标,用来指示台风强降雨事件对南冲绳海槽海洋环境的影响情况.近1000年来台湾东北部台风强降雨事件波动增加,在1500～1800A.D.期间达到峰值;此外,5次典型的强台风降雨记录,分别发生于1040A.D.、1310A.D.、1530A.D.、1690A.D.和1900A.D.,其平均重现周期为215年（160～270年）.利用MD05-2908孔各样品在主成分一上的得分重建的台风记录与广东、台湾等地已有的台风记录有较好的对应关系,进一步验证了硅藻作为研究工具重建古台风记录的可行性.

The sedimentary record studied for the present paper is from the MD05-2908 gravity core（24°48.04＇N, 122°29.35＇E, 1276m water depth）retrieved on the R/V Marion Dufresne IMAGES XII cruise in 2005 on the continental slope of the SOT.The sediments are mainly composed of nearly homogenous dark grey hemipelagic clay and fine silt.No visible turbidites or tephra layers were found.A total of 86 samples with 9cm sampling iutervals, collected from 2cm to 746cm of the core were analyzed for diatom contents.Six AMS 14C ages were obtained from the planktonic foraminiferal species Globigerinoides ruber and G.sacculifer at the AMS 14C Dating Centre at the Woods Hole Oceanographic Institution, Massachusetts, USA.The calibrated age is 1045a B.P. The dominant taxa in the studied core record of MD05-2908 are Thalassionema nitzschioides, Nitzschia bicapitata, Cyclotella striata, Cyclotella stylorum, Diploneis bombus, Diploneis suborbicularis, Azpeitia nodulifera, Rhizosolenia bergonii, Alveus marina, Fragilaria spp.and Amphora spp. In this paper, a high-resolution diatom record from Core MD05-2908 revealed typhoon precipitation in northeastern Taiwan and paleoceanographic changes in the Southern Okinawa Trough during the last millennium by Principal Component Analysis（PCA）.In a PCA species plot, the species are represented by vectors and positioned based on their scores related to the ordination axes.The first two axes accounted for 61.8 % （λ1=0.433, λ2=0.185）of the variation in diatom components.Results show that axis 1 is positively correlated with the relatively freshwater species Fragilaria spp.and Amphora spp., but negatively correlated to the Kuroshio indicator species A.nodulifera, R.bergonii, A.africana and A.neocrenulata. Thus, the sample scores on axis 1 can represent changes in typhoon precipitation in northeastern Taiwan, and reflect changes in river discharge into the Southern Okinawa Trough.Good coherence between sample scores and changes in abundance of freshwater species, and summer sea-surface salinity in Core MD05-2908 supports that diatom records are potentially a valuable tool for quantitative reconstruction of palaeo-typhoon. Fluctuate increase in typhoon precipitation based on sample scores on axis 1, reflects strengthened river discharge into the study area over the last millennium, with the maximum discharge occuring at 1450A.D.Five high typhoon precipitation events occurred in 1040A.D., 1310A.D., 1530A.D., 1690A.D., and 1900A.D., respectively, with an average 215-year cycle（160~270 years）.High typhoon rainfall during the LIA may be ascribed to strengthened La Nia activity in the Northwest Pacific Ocean.Changes in typhoon rainfall in northeastern Taiwan are consistent with the history of typhoon landfalls in Guangdong and grain size record in northern Taiwan.