The carbon isotope composition of ancient CO2 based on higher-plant organic matter
The carbon isotope composition of ancient CO2 based on higher-plant organic matter
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DOI：
10.1098/rsta.2001.0965
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发表时间：
2002
期刊：
影响因子：
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通讯作者：
Darren R. Grocke
中科院分区：
文献类型：
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作者：
Darren R. Grocke
一些研究表明，高等植物有机物中的碳同位素比率（δ C植物）与海洋-大气碳库的碳同位素组成密切相关，特别是与CO2的同位素组成密切相关。这些研究主要侧重于海洋碳储层发生重大扰动的地质间隔，如有机碳和碳酸盐相所记录的（例如二叠纪-三叠纪和三叠纪-侏罗纪边界，早托阿西期，早阿普第期，Cenomanian-Turonian边界，古新世-始新世热极大期）。所有这些事件，不包括Cenomanian-Turonian边界，记录负碳同位素偏移，许多作者假设这种偏移的原因是大陆边缘海洋天然气水合物储层（包合物）的大规模释放。与高等植物和海洋有机质以及碳酸盐相比，甲烷的碳同位素组成非常负（δ 13 C，约为− 60%%）。甲烷在海洋-大气储层中的停留时间很短（约1000年）。10年），并迅速氧化为CO2，导致CO2的同位素组成从其假定的背景值（δ 13 C，约-7%%）变得更负。然而，迄今为止，只有早期Toarcian，早期Aptian和PETM是严格约束的计时序列，可以将包合物释放归因为产生如此快速负δ 13 C偏移的可行原因。尽管如此，高等植物有机物（如木炭、木材、树叶、花粉）的同位素分析有能力（i）在地质记录中记录古大气CO2的同位素组成，（ii）将海洋和非海洋地层序列相关联，（iii）确认海洋碳扰动在其范围内并不纯粹是海洋学的，并影响整个海洋-大气系统。英国怀特岛的一个案例研究表明，白垩纪中期古大气CO2的碳同位素组成背景值为-3%%，但波动迅速，变得更正值（约+）。0.5在碳循环扰动期间（例如碳埋藏事件、碳酸盐台地淹没、大火成岩省形成），出现负值（约-10%%）。因此，古大气CO2碳同位素组成的波动将损害我们对依赖于CO2恒定碳同位素比的古CO2替代指标的使用。
Carbon isotope ratios in higher-plant organic matter (δCplant) have been shown in several studies to be closely related to the carbon isotope composition of the ocean– atmosphere carbon reservoir, and, in particular, the isotopic composition of CO2. These studies have primarily been focused on geological intervals in which major perturbations occur in the oceanic carbon reservoir, as documented in organic carbon and carbonates phases (e.g. Permian–Triassic and Triassic–Jurassic boundary, Early Toarcian, Early Aptian, Cenomanian–Turonian boundary, Palaeocene–Eocene Thermal Maximum (PETM)). All of these events, excluding the Cenomanian–Turonian boundary, record negative carbon isotope excursions, and many authors have postulated that the cause of such excursions is the massive release of continentalmargin marine gas-hydrate reservoirs (clathrates). Methane has a very negative carbon isotope composition (δ13C, ca.−60 % % ) in comparison with higher-plant and marine organic matter, and carbonate. The residence time of methane in the ocean– atmosphere reservoir is short (ca. 10 yr) and is rapidly oxidized to CO2, causing the isotopic composition of CO2 to become more negative from its assumed background value (δ13C, ca.−7 % % ). However, to date, only the Early Toarcian, Early Aptian and PETM are well-constrained chronometric sequences that could attribute clathrate release as a viable cause to create such rapid negative δ13C excursions. Notwithstanding this, the isotopic analysis of higher-plant organic matter (e.g. charcoal, wood, leaves, pollen) has the ability to (i) record the isotopic composition of palaeoatmospheric CO2 in the geological record, (ii) correlate marine and non-marine stratigraphic successions, and (iii) confirm that oceanic carbon perturbations are not purely oceanographic in their extent and affect the entire ocean–atmosphere system. A case study from the Isle of Wight, UK, indicates that the carbon isotope composition of palaeoatmospheric CO2 during the Mid-Cretaceous had a background value of −3 % % , but fluctuated rapidly to more positive (ca.+0.5 % % ) and negative values (ca.−10 % % ) during carbon cycle perturbations (e.g. carbon burial events, carbonate platform drowning, large igneous province formation). Hence, fluctuations in the carbon isotope composition of palaeoatmospheric CO2 would compromise our use of palaeo-CO2 proxies that are dependent on constant carbon isotope ratios of CO2.