Gröcke Darren R
Department of Geology, Royal Holloway, University of London, Egham Hill, Egham, Surrey TW20 0EX, UK.
Philos Trans A Math Phys Eng Sci. 2002 Apr 15;360(1793):633-58. doi: 10.1098/rsta.2001.0965.
Carbon isotope ratios in higher-plant organic matter (delta(13)C(plant)) 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 CO(2). 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 continental-margin marine gas-hydrate reservoirs (clathrates). Methane has a very negative carbon isotope composition (delta(13)C, ca. 60 per thousand ) 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 CO(2), causing the isotopic composition of CO(2) to become more negative from its assumed background value (delta(13)C, ca. -7 per thousand ). 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 delta(13)C 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 CO(2) 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 CO(2) during the Mid-Cretaceous had a background value of 3 per thousand, but fluctuated rapidly to more positive (ca. +0.5 per thousand ) and negative values (ca. 10 per thousand ) 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 CO(2) would compromise our use of palaeo-CO(2) proxies that are dependent on constant carbon isotope ratios of CO(2).
多项研究表明，高等植物有机质中的碳同位素比值（δ¹³C(植物)）与海洋 - 大气碳库的碳同位素组成密切相关，尤其是与CO₂的同位素组成相关。这些研究主要集中在地质时期，在这些时期，海洋碳库发生了重大扰动，如有机碳和碳酸盐相中所记录的那样（例如二叠纪 - 三叠纪和三叠纪 - 侏罗纪边界、早托阿尔期、早阿普特期、森诺曼阶 - 土伦阶边界、古新世 - 始新世极热事件（PETM））。除了森诺曼阶 - 土伦阶边界外，所有这些事件都记录了负碳同位素偏移，许多作者推测这种偏移的原因是大陆边缘海洋天然气水合物储层（笼形水合物）的大量释放。与高等植物、海洋有机质和碳酸盐相比，甲烷具有非常负的碳同位素组成（δ¹³C，约为 -60‰）。甲烷在海洋 - 大气储层中的停留时间很短（约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（δ¹³C，约为 -7‰）变得更负。然而，迄今为止，只有早托阿尔期、早阿普特期和PETM是受良好约束的计时序列，能够将笼形水合物释放归因于造成这种快速负δ¹³C偏移的可行原因。尽管如此，对高等植物有机质（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）关联海洋和非海洋地层序列，以及（iii）确认海洋碳扰动在范围上并非纯粹是海洋学的，而是影响整个海洋 - 大气系统。来自英国怀特岛的一个案例研究表明，白垩纪中期古大气CO₂的碳同位素组成的背景值为 -3‰，但在碳循环扰动（如碳埋藏事件、碳酸盐台地淹没、大火成岩省形成）期间迅速波动到更正（约 +0.5‰）和更负的值（约 -10‰）。因此，古大气CO₂碳同位素组成的波动会影响我们对依赖于CO₂恒定碳同位素比值的古CO₂代理指标的使用。
