东北印度洋东经90°海岭末次冰消期孢粉源区的转换及机制

CONVERSION AND MECHANISM OF ORIGINAL REGION OF POLLEN ON THE 90°E SEA RIDGE OF THE NORTHEAST INDIAN OCEAN SINCETHE LAST DEGLACIAL PERIOD

海洋沉积物中的孢粉是反映大气环流变化与洋流输运的重要手段。通过对东北印度洋90°E海岭18I105A孔(孢粉分析长度为168 cm,即45.1 cal.ka B.P.以来)的孢粉群落分析,发现在12.6 cal.ka B.P.孢粉组合存在一个明显的转变,孢粉的组成由单缝孢子、松属花粉和栲属、青冈属等壳斗科花粉转变为三缝孢子、桃金娘科花粉和榆科花粉。通过与现代孢粉主要源区对比以及与海岭北部的YDY05孔进行对比,认为18I105A孔的孢粉源区从苏门答腊岛到斯里兰卡、印度半岛等地,发生了重要转变。在12.6 cal.ka B.P.之前,孢粉源区主要来源于苏门答腊岛等近源区域,主要输运方式为安达曼表层洋流,受海平面升降控制;而到了全新世,加强的印度夏季风形成较强的西南季风流、海平面上升使得源区距离增加,阻碍了源区距离较近的东部苏门答腊岛等地的孢粉运输过程,导致孢粉源区变为印度半岛及斯里兰卡,孢粉浓度骤降,该阶段海平面和夏季风起主要控制作用;在12.6 cal.ka B.P.之前,YDY05孔孢粉总浓度远低于18I105A孔,反映当时YDY05孔孢粉源区距离更远,二者具有相似的孢粉组合类型及孢粉变化趋势,表明90°E海岭沉积物中孢粉具有相似的源区及影响因素。孢粉记录的区域气候/降水变化为:18I105A孔孢粉记录了45.1 cal.ka B.P.以来的陆源地的古气候演化历史,苏门答腊岛在海洋同位素阶段(MIS)3时期以干冷气候为主,MIS 2时期为湿冷气候,全新世时期记录了印度和斯里兰卡温暖潮湿的气候变化。

Pollen in marine sediments is an important indicator to reflect the changes of atmospheric circulation and ocean current transport.Based on the results of AMS 14C dating of planktonic foraminifera,the sporopollen analysis of the columnar sediment(core 18I105A)in the Ninetyeast Ridge of the Northeast Indian Ocean was conducted.18I105A is located at 5.1230°N,90.8407°E,with a water depth of 3094 m and a columnar length of 180 cm.A total of 10 samples of mixed planktonic foraminifera were selected for dating.The dating model showed that the top age of the core was 1.5 cal.ka B.P.and the bottom age was 47 cal.ka B.P.The whole core was sampled at 1 cm intervals,eventually obtaining 180 samples.The palynological analysis of 40 samples was carried out at an average interval of 4 cm,and the depth of the oldest identified samples is 168 cm,and its age is estimated to be 45.1 cal.ka B.P.Through the analysis of the pollen community of those 40 samples in core 18I105A of the 90°E Sea Ridge in the Indian Ocean,it was found that there was an obvious change in the pollen assemblage at 12.6 cal.ka B.P.The pollen assemblage changed from monolete-spore,Pinus,Cyclobalanopsis and Castanopsis pollen to trilete-spore,Myrtaceae and Ulmaceae pollen.By comparing with the main source area of modern pollen in the Northeast Indian Ocean and with the YDY05 core collected in the north part of 90°E Sea Ridge,it is suggested that the source area of pollen in core 18I105A has changed from Sumatra Island to Sri Lanka and the Indian Peninsula.Before 12.6 cal.ka B.P.,the source area of pollen mainly came from the near source area such as Sumatra Island,and the main transport factor was the Andaman surface current,which was controlled by the rise and fall of sea level.However,in the Holocene,the strengthening of Indian summer monsoon formed strong southwest monsoon flow and sea level rise increased the distance from the source area,hindered the process of pollen transport in the eastern Sumatra Island,which was close to the source area.This process results in the change of the source area to the Indian Peninsula and Sri Lanka,and transporting distance increased and pollen concentration plummeted.At this stage,sea level and summer monsoon played a major controlling role.Before 12.6 cal.ka B.P.,the total concentration of pollen and spores in YDY05 core was much lower than that in 18I105A core,indicating that the source area of spores in YDY05 core was further away.They have similar pollen assemblage types and concentration variation trends,indicating that pollen sediments in 90°E Sea Ridge have similar source areas and influencing factors.The regional climate/precipitation changes recorded by palynology are as follows:The palynology of the 18I105A core recorded the paleoclimatic evolution history of the terrigenous land since 45.1 cal.ka B.P.The climate of Sumatra Island was mainly dry and cold in the marine isotope stages(MIS)3 period,and wet and cold in the MIS 2 period,and the Holocene period recorded a warm and humid climate change in India and Sri Lanka.