Historical Carbon Dioxide Record from the Vostok Ice Core (417,160 - 2,342 years BP)

In January 1998, the collaborative ice-drilling project between Russia, the United States, and France at the Russian Vostok station in East Antarctica yielded the deepest ice core ever recovered, reaching a depth of 3,623 m (Petit et al. 1997, 1999). Ice cores are unique with their entrapped air inclusions enabling direct records of past changes in atmospheric trace-gas composition. Preliminary data indicate the Vostok ice-core record extends through four climate cycles, with ice slightly older than 400 kyr (Petit et al. 1997, 1999). Because air bubbles do not close at the surface of the ice sheet but only near the firn-ice transition (that is, at ~90 m below the surface at Vostok), the air extracted from the ice is younger than the surrounding ice (Barnola et al. 1991). Using semiempirical models of densification applied to past Vostok climate conditions, Barnola et al. (1991) reported that the age difference between air and ice may be ~6000 years during the coldest periods instead of ~4000 years, as previously assumed. Ice samples were cut with a bandsaw in a cold room (at about -15°C) as close as possible to the center of the core in order to avoid surface contamination (Barnola et al. 1983). Gas extraction and measurements were performed with the "Grenoble analytical setup," which involved crushing the ice sample (~40 g) under vacuum in a stainless steel container without melting it, expanding the gas released during the crushing in a pre-evacuated sampling loop, and analyzing the CO2 concentrations by gas chromatography (Barnola et al. 1983). The analytical system, except for the stainless steel container in which the ice was crushed, was calibrated for each ice sample measurement with a standard mixture of CO2 in nitrogen and oxygen. For further details on the experimental procedures and the dating of the successive ice layers at Vostok, see Barnola et al. (1987, 1991), Lorius et al. (1985), and Petit et al. (1999).

1998年1月，俄罗斯、美国与法国在南极洲东部俄罗斯沃斯托克（Vostok）科考站开展的联合冰钻项目，获取了迄今最深的冰芯，钻探深度达3623米（Petit等，1997、1999）。冰芯因其包裹的空气内含物而具备独特研究价值，可直接记录大气痕量气体组成的历史演化过程。初步数据显示，沃斯托克冰芯记录覆盖了四个完整气候周期，所包含冰体的年代早于40万年（Petit等，1997、1999）。由于空气气泡并非在冰盖表面闭合，而是仅在粒雪-冰过渡带（firn-ice transition）附近闭合（沃斯托克站该过渡带位于地表下约90米处），因此从冰体中提取的气体年龄比周围包裹的冰体更年轻（Barnola等，1991）。通过将致密化半经验模型应用于沃斯托克站古气候条件，Barnola等（1991）指出，在最冷气候阶段，冰体与其中包裹气体的年龄差可达约6000年，而非此前假设的约4000年。为避免冰样受到表面污染，研究人员在温度约为-15℃的冷室内，使用带锯在冰芯尽可能靠近中心的位置切割获取冰样（Barnola等，1983）。气体提取与浓度测量采用“格勒诺布尔分析装置（Grenoble analytical setup）”完成，具体流程为：将约40克冰样置于真空环境下的不锈钢容器中粉碎且不使其融化，将粉碎过程中释放的气体导入预先抽真空的采样回路，随后通过气相色谱法分析二氧化碳（CO₂）浓度（Barnola等，1983）。除用于冰样粉碎的不锈钢容器外，每套冰样测量前均需使用氮气与氧气中的二氧化碳标准混合气对分析系统进行校准。有关沃斯托克站冰层层序定年与实验流程的更多细节，可参阅Barnola等（1987、1991）、Lorius等（1985）及Petit等（1999）的相关研究。