早上新世非洲季风与地中海表层生产力变化的岁差节律

Early Pliocene Precession Rhythm of African Monsoon and Mediterranean Sea Surface Productivity

地质历史上留下了许多反映非洲季风的记录,意大利南方晚新生代的海相地层便是其中著名的一例.Cape Sper-tivento剖面位于意大利卡拉布里亚半岛,属于早上新世(5.3～4.8Ma)地中海泥灰岩-灰岩沉积物.在借鉴前人工作的基础上,重新为该剖面建立一个精度更高的天文年代标尺.反映古生产力的指标显示,早上新世在北半球夏季辐射量增大期间,地中海有机碳含量及其堆积速率增大,C/N出现高值,碳酸盐含量及其堆积速率减小,G.obliquus稳定氧碳同位素出现负偏移,这是非洲夏季风带来的降雨增强的结果.尼罗河泛滥让更多陆源营养物和淡水输入地中海.各个古环境指标的频谱分析结果存在强烈的岁差周期和丰富的半岁差周期,说明早上新世非洲季风主要受低纬过程控制.

The Profile Cape Spertivento, located in the Calabria peninsula of Italy, is composed of the Early Pliocene （5. 3-4. 8 Ma） Mediterranean marl-clay sediments. Based on the former work, a more accurate astronomically tuned timescale has been reconstructed for the sequence Cape Spertivento in this paper. Paleoproductivity proxies show that during the Early Pliocene, the increases of the Northern Hemisphere summer insolation always concurred with the increases of the organic carbon MAR, the high C/N ratios, the decreases of the carbonate MAR, and the negative excursions of G. obliquus δ^18O and δ^13C, which were probably caused by the enhanced rainfall, resulting from the strengthened African summer monsoon. Moreover, the strong African summer monsoon led to the flooding of the Nile River, increasing the discharge of continental nutrients and fresh water into the Mediterranean Sea. Strong precession and abundant semi-precession cycles are found in the spectrums of the proxy records, implying the tropical forcing of the African monsoon variability during the Early Pliocene.