茂名油页岩沉积有机质特征及古气候意义

The characteristics of organic matter in Maoming oil shales and their paleoclimate significance

基于海洋沉积确定的始新世晚期至渐新世早期(约33.5 Ma)的气候过渡期是新生代全球性气候事件,但长期以来相关的陆相沉积记录研究则比较缺乏。本文对形成于始新世晚期至渐新世早期的茂名油柑窝组油页岩样品进行了有机质丰度、烃类组成、单体烃碳同位素组成等分析,以期研究低纬度陆相沉积有机质对古近纪始新世—渐新世气候过渡期(EOT)的响应。研究结果显示,埋藏较浅的上部层位样品正构烷烃碳同位素组成显著正偏,为-19.1‰^-25.9‰,平均值为-22.2‰;而下部层位样品正构烷烃碳同位素组成在-23.7‰^-30.2‰,平均值为-26.3‰。两个样品正构烷烃碳同位素组成之差在1.5‰~8.1‰,平均值之差为4.6‰。正构烷烃碳同位素组成的显著差异指示了EOT低纬度陆相古气候变化引起的陆相有机质响应,而这种响应与当时大气CO_2浓度和海洋温度降低导致的气候和植物群落变化密切相关,即可能存在着C_3植物向C_4植物的转化、C_4植物的迅速增加并最终导致沉积有机质碳同位素组成的变化;也可能是由于大气CO_2浓度降低导致的大气CO_2碳同位素组成整体正偏的结果。很显然,进一步详细的连续剖面分析将对研究古近纪EOT低纬度陆相古气候变化提供重要科学依据。

The late Eocene to the early Oligocene （about 33.5 Ma） climate transition determined by the marine sediment studies was a global event based on marine sediments during the Paleogene, while there is few researches on the terrestrial sediment records of this climate transition. The Youganwo Formation oil shales from Maoming basin of Guangdong Province, deposited during the late Eocene to the early Oligocene, have been studied in terms of organic matter abundance, hydrocarbon components and the carbon isotopic composition of individual hydrocarbon compounds, and aiming to study the response of terrestrial organic matter to the Eocene-Oligocene transition （EOT） at the low latitude region. The results show that the b^3C composition of n-alkanes in the shallower buried sample display a significant positive excursion ranged from --19.1~0 and --25.9~0 with an average of --22.2~0. By contrast, the ~13C composition of n-alkanes in the deeper buried sample is much negative and varied from --23.7%~ and --30. 2~0 with averaged at --26.3%o. The difference between the ~a3 C compositions of corresponding n-alkanes in these two samples vary from 1.5N0 to 8. 1~0 with an average deviation of 4. 6~0. This significant shift in the 813C composition of n-alkanes signals that the terrestrial organism（s） at the low latitude land response to the paleoclimate changes which may closely related to the decrease in the concentration of atmospheric COg and the ocean temperature during EOT. Therefore, the discrepancy in the 633C composition of n-alkanes may suggest that there was an significant shift or increase from C3 plant to C4 plant at the low altitude continental region of Maoming basin, which resulted in a remarkable change in the 5a3C composition of sedimentary organic matter; while it is also possible that the positive excursion of organic matter might be due to the decrease in the concentration of atmospheric COg that gives rise to the 3a3C composition of atmospheric COg. Finally, this primary study suggests that more detailed research on the Maoming oil shale could provide some critical information to a better understanding of low latitude continental paleoclimate changes during EOT.