黑潮源区水体结构对早更新世以来西太暖池变动的响应

It has been widely accepted that the low-latitude tropical ocean plays a key role in global climate systems. This is particular so in the western Pacific warm pool (WPWP) due to the largest volume and highest temperature water mass in the world. Changes in the tilt of the low-latitude Pacific thermocline are closely related on interannual timescales to the coupled ocean-atmosphere El Ni?o-Southern Oscillation (ENSO) phenomenon . Kuroshio source area , which linked the low-latitude ocean and high latitude ocean to each other, was sensitive to the changes of WPWP.Then the varition of WPWP directly affected the strength and pathway of Kuroshio current which transported a large of water and heat to high latitude. In view of the lack of long term continuous and relatively high resolution paleoceanography records in tropical western Pacific, we choose the core MD06-3050 located in Kuroshio source area to reconstruct the variation of upper water structure and hydrology condition since early Pleistocene(2Ma). The accurate age model was built based on the oxygen isotope data of benthic foraminifera. Planktonic foraminifera G.ruber and P.obliquiloculata respectively lived in mix layer and thermocline are picked to measure shell Mg/Ca ratio for SST and thermocline water temperature construction. We calculated surface and thermocline seawater δ18O compostion from paired Mg/Ca and δ18O measurements of G.ruber and P.obliquiloculata, respectively. The calcareous nannofossil assemblages are identified under polar microscope. These proxies are combined to achieve the kuroshio source area upper water mass features in early Pleistocene, such as nutricline, thermocline and paleoproducitivity. Through comparing our records with records in eastern and western Pacific, combined with orbital parameters, we can discuss the long term and orbital timescale response of Kuroshio source area to changes of WPWP since early Pleistocene, especially since the finally east-western Pacific asymmetric was built.

西太平洋暖池作为全球气候变化的引擎，通过年际尺度及长期ENSO过程影响着热带及热带以外区域的气候。位于菲律宾以东至巴士海峡的黑潮源区, 是赤道热能向西北太平洋边缘海输送的重要通道, 其水体结构与西太暖池的变动息息相关。鉴于热带西太平洋连续沉积的长时间尺度古环境记录的匮乏，本项目以黑潮源区MD06-3050岩心为研究对象，结合底栖有孔虫与浮游有孔虫氧同位素数据建立准确的年代地层，利用混合层与温跃层种有孔虫壳体Mg/Ca比值结合δ18O 与δ13C，以及钙质超微化石群落组合来重建混合层与温跃层的温度、盐度梯度以及初级生产力变化，从而重建2Ma以来黑潮演化历史，阐明黑潮源区上部水体结构对西太暖池变动响应的细节过程。通过与暖池区太平洋气候指标的对比分析，探讨早更新世，尤其是赤道东西太平洋不对称性特征建立以来，黑潮在长时间尺度上对暖池的响应过程和机制。

本项目以西太平洋暖池边缘黑潮源区的长柱状样岩心MD06-3050为研究对象，综合运用氧同位素年代学、古生物及其相关地球化学等方法，重建了更新世以来黑潮源区温跃层、营养跃层、次表层水体温度、初级生产力、输出生产力的演化历史，检验了这些古海洋参数在中更新世转型时期对暖池收缩的响应敏感度，提出黑潮源区上部水体结构及其相关的古环境信息主要响应于暖池在东西方向上的收缩。本研究的年代模式根据底栖有孔虫的氧同位素记录与LR04曲线进行对比而建立，可靠的年代底界为1.67Ma。研究显示，1.67Ma以来，温跃层和营养跃层不仅具有长期的变化趋势，而且存在明显的冰期-间冰期变化特征。由浮游有孔虫氧同位素差值定性的指示温跃层自1Ma左右开始快速加深，直至0.8Ma左右，达到较为稳定的深度。尽管浮游有孔虫碳同位素差值指示上部水体的营养梯度时存在一定的局限性，但是其长期的变动趋势与温跃层变动趋势具有相似性，即在1Ma左右开始，出现快速的营养梯度减小的特征，直到0.8Ma，营养梯度达到较低的状态，即营养跃层在此时达到较深的状态。根据下透光带颗石藻种属F.profunda相对百分含量不光可以定性的指示研究区营养跃层的变动，还可以定量的反映该海区初级生产力的变动特征。结果显示F.profunda%自0.95Ma左右出现快速增加，指示营养跃层不断的加深，到0.8Ma营养跃层达到较深的状态，进一步支持了中更新世转型时期黑潮源区的水体结构发生了重大的变动。将MD06-3050岩心的各古环境替代性指标与暖池其他站位的记录进行对比，发现黑潮源区水体结构发生重大变动的时间晚于西太暖池在南北方向的收缩，而与热带太平洋在东西方向温度梯度快速增加的时间相对应。因此，认为黑潮源区的上部水体古环境变化主要响应于西太暖池在东西方向上的收缩，现代西太暖池的最终建立时间为0.8Ma左右。0.8Ma之前，西太暖池的上部水体结构与现代暖池差别较大，显示出暖池核心区温跃层较深，而暖池北部边缘温跃层较浅的特征。