影响湖相沉积岩中有机碳分布的主要因素——以三水盆地为例

Factors Influencing Organic Carbon Distribution in Lacustrine Sediments

湖相沉积中的有机质含量随有机质堆积条件变化而发生波动。本文以三水盆地下第三系土布心组二段地层为例 ,对其作高分辨率岩芯取样并结合地球化学方法 ,分析生物学因素对有机碳分布的影响。在较干燥的气候条件下 ,水生浮游生物是有机质的主要来源 ,由于此时湖泊与海洋沟通 ,出现半咸水环境 ,硫酸盐还原作用显著 ,不利于有机质保存。所取岩芯段的中上部即是这种情况 ,以低TOC和高SRI值为特征。而当气候湿润时 ,植物繁荣 ,湖水淡化 ,陆源有机质丰富。缺氧条件下的有机质降解主要是效率极低的发酵作用。有利于有机质保存。这种情形出现于岩芯段的下部 。

Organic matter content in lacustrine sediments fluctuates with the changes of organic accumulation conditions. Here a core section from the Buxin Formation (Lower Tertiary) of the Sanshui Basin was studied as an example. High resolution sampling combined with geochemical approaches was used to analyze the influence of primary biologic productivity and biochemical degradation on organic fluctuations. During the Early Tertiary, the Sanshui Basin was a tropical lake with anaerobic or near anaerobic bottom waters. When the amount of rainfall decreased on adjoining landmasses, the principal source of organic matter was aquatic phytoplankton, which was easily oxidized. Owing to the intermittent communications between the basin and the sea, brackish condition occurred and sulphate reduction was stronger. Most organic matter provided by the surface water layers was degraded during this process. This case was presented in the middle and upper parts along the core section, which were characterized by lower TOC and higher SRI values. During the interval of humid climate, terrestrial organic matter from streams and rivers increased and the lake waters became freshening. Sulphate reduction was limited by low sulphate concentration. The major anaerobic metabolism was fermentation, which was of less efficiency and favorable for the preservation of organic carbon. This case could be observed in the lower part of the core section, which was marked by higher TOC and lower SRI values. In short, climate changes and intermittent communications between the basin and the sea resulted in the fluctuations in the supply of organic matter and accumulation conditions, which controlled in turn the changes in organic carbon content.