Early Pliocene Precession Rhythm of African Monsoon and Mediterranean Sea Surface Productivity
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摘要: 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.Abstract: 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 δ18O and δ13C, 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.
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Key words:
- Pliocene /
- African monsoon /
- the Mediterranean Sea /
- sea surface productivity /
- precession
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图 1 Cape Spertivento剖面的地理位置(a); Cape Spertivento剖面的野外露头照片(b)
Fig. 1. Location map of the Profile Cape Spertivento (a); outcrop of the Profile Cape Spertivento (b)
图 2 Cape Spertivento剖面时间控制点的选取.夏季辐射量为六月份和七月份太阳辐射量的平均值
Fig. 2. Selection of the time control points for the Cape Spertivento Profile.Note that summer insolation is the monthly mean value of June and July
图 3 Cape Spertivento剖面有机碳、碳酸盐百分含量和G. obliquus δ18O在岁差周期上的滤波曲线(实线) 与岁差(虚线) 的对比.采用Blackman-Turkey带宽滤波方法.岁差周期上的滤波中心频率为0.047 62 ka-1, 带宽为0.015 ka-1
Fig. 3. Comparison of filtering records of Corg%, carbonate% and G. obliquus δ18O at precession band (solid lines) with orbital precession (dashed line). The Blackman-Turkey method was used. The central frequency and band-width are 0.047 62 ka-1 and 0.015 ka-1, respectively
图 4 Cape Spertivento剖面(a) 有机碳、(b) 碳酸盐百分含量、(c) G. obliquus δ18O和(d) G. obliquus δ13C与30oN夏季辐射量的交叉频谱分析结果.图中上部实线代表 30oN夏季辐射量的频谱, 虚线代表各个古环境指标的频谱.下部点划线代表相关系数的频谱.水平实线代表 80%的检验标准.交叉频谱分析的软件为ARAND
Fig. 4. Cross spectral analyses of 30oN summer insolation with (a) Corg%, (b) Carbonate%, (c) G. obliquus δ18O and (d) G. obliquus δ13C of Cape Spertivento, respectively. The solid lines denote the spectrum of the 30oN summer insolation and the dashed lines denote those of the proxy records. The dotted lines denote the coherency, and the horizontal solid lines denote the 80% confidence level. We used the "ARAND" package from Brown University to perform the cross spectral analyses
图 5 Cape Spertivento剖面气候替代性指标与岁差的比较
Fig. 5. Comparison of the proxy records of Cape Spertivento profile with the precession variability
图 6 Cape Spertivento剖面各个气候替代性指标的频谱分析.实线和虚线分别代表 80%和90% 的检验标准.图中周期的单位为ka.频谱分析的软件为Redfit35 (Schluz and Mudelsee, 2002)
Fig. 6. Spectral analyses of proxy records from the Cape Spertivento Profile. The solid and dashed lines represent 80% and 90% confidence level, respectively. The unit for the periods is ka. We used "Redfit35" (Schluz and Mudelsee, 2002) to perform spectral analyses
图 7 Rossello综合剖面2.0~2.5 Ma G. ruber δ18O记录(a) 及其频谱分析(b).OOP659站2.0~2.5 Ma风尘通量记录(c) 及其频谱分析(d). (c)、(d) 中实线和虚线分别代表 80%和90% 的检验标准.图中周期的单位为ka.频谱分析的软件为Redfit35 (Schluz and Mudelsee, 2002)
Fig. 7. 2.0-2.5 Ma G. ruber δ18O record of the Rossello composite (a) and its spectral analyses (b). 2.0-2.5 Ma dust flux record of the OOP659 Site (c) and its spectral analyses (d). The solid and dashed lines represent 80% and 90% confidence level in (c) (d), respectively. The unit for the periods is ka. We used "Redfit35" (Schluz and Mudelsee, 2002) to perform spectral analyses
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