南海南部中上新世以来沉积有机碳与古生产力变化

CHANGES OF ORGANIC CARBON AND PALEOPRODUCTIVITY IN THE SOUTHERN SOUTH CHINA SEA SINCE MIDDLE PLIOCENE

对南海南部ODP1143站上部150m的1005个沉积物样品进行了有机碳分析,求取中上新世3.5Ma以来表层生产力长期的演化趋势和短期的变化规律。有机碳含量、有机碳堆积速率和输出生产力在2.85～1.58Ma和0.9～0Ma两个时期呈现相对高值,反映具较高的古生产力,推测分别与上新世晚期北极冰盖形成和“中更新世革命”之后北极冰盖扩大事件密切相关。与有孔虫δ18O记录对比,有机碳含量、有机碳堆积速率和输出生产力都显示在冰期增加,间冰期降低,反映了冰期具较高的表层生产力。冰期时较高的沉积物堆积速率和C/N比值反映低海平面时期陆源营养物质供应增加,是造成冰期表层生产力高的重要原因。同时推测冰期时低氧含量的底层水有利于有机物质的保存。频谱分析揭示,中上新世以来有机碳含量变化复杂,说明其变化除主要受到地球轨道周期变化的驱动外,还受到陆源物输入等其他因素的影响;而有机碳堆积速率和输出生产力主要受地球轨道周期变化的驱动。

Total organic carbon content (TOC%) of 1 005 sediment samples from the upper 150 m section of Site 1143 in the southern South China Sea (SCS) was measured for understanding long- and short-term changes in paleoproductivity during the last 3.5 Ma. The values in TOC%, TOC accumulation rate (TOC AR) and estimated export productivity (Pexp) show distinct increases of the two periods in 2.85~1.58 and 0.9~0 Ma, which may be related to the North Hemispherical Glaciation (NHG) in the late Pliocene and the enhancement of the NHG since the middle-Pleistocene Revolution respectively. A comparison with the benthic foraminiferal δ^(18)O record shows that TOC %, TOC AR and Pexp are usually higher in the glacial periods and lower in the interglacial periods, indicating higher productivity condition in the glacial periods during the last 3.5 Ma. Higher sediment mass accumulation and C/N rates also show a glacial/interglacial changes with usually higher values in the glacial periods, indicating that the paleoproductivity is mainly controlled by the terrigenous inputs associated with sea-level fluctuations. Moreover, the bottom water with lower oxygen content is thought to play a significant role in the organic carbon preservation in the glacial periods. Spectral analyses reveal that organic carbon content is probably forced by changes of orbital forcing but other factors such as terrigenous input also play an important role, while TOC AR and estimated export productivity are mostly controlled by changes of orbital forcing.