NOAA/WDS Paleoclimatology - Vostok, Antarctica Ice Core 4G-2 61-58KYr Methane and d15N Data

We investigate for the first time the loss and alteration of past atmospheric information from air trapping mechanisms under low-accumulation conditions through continuous CH4 (and CO) measurements. Methane concentration changes were measured over the Dansgaard-Oeschger event 17 (DO-17, - 60 000 yr BP) in the Antarctic Vostok 4G-2 ice core. Measurements were performed using continuous-flow analysis combined with laser spectroscopy. The results highlight many anomalous layers at the centimeter scale that are unevenly distributed along the ice core. The anomalous methane mixing ratios differ from those in the immediate surrounding layers by up to 50 ppbv. This phenomenon can be theoretically reproduced by a simple layered trapping model, creating very localized gas age scale inversions. We propose a method for cleaning the record of anomalous values that aims at minimizing the bias in the overall signal. Once the layered-trapping-induced anomalies are removed from the record, DO-17 appears to be smoother than its equivalent record from the high-accumulation WAIS Divide ice core. This is expected due to the slower sinking and densification speeds of firn layers at lower accumulation. However, the degree of smoothing appears surprisingly similar between modern and DO-17 conditions at Vostok. This suggests that glacial records of trace gases from low-accumulation sites in the East Antarctic plateau can provide a better time resolution of past atmospheric composition changes than previously expected. We also developed a numerical method to extract the gas age distributions in ice layers after the removal of the anomalous layers based on comparison with a weakly smoothed record. It is particularly adapted for the conditions of the East Antarctic plateau, as it helps to characterize smoothing for a large range of very low-temperature and low-accumulation conditions.

本研究首次通过连续甲烷（CH4）及一氧化碳（CO）测量，探究了低积累条件下空气捕集机制对古大气信息的损耗与改造作用。针对南极沃斯托克（Vostok）4G-2冰芯中丹斯果德-厄施格事件17（Dansgaard-Oeschger event 17，DO-17，距今60000年）期间的甲烷浓度变化，本研究开展了相关测量。测量采用连续流分析结合激光光谱技术完成。研究结果显示，冰芯沿线分布着大量厘米级异常层位，且分布并不均匀。异常甲烷体积混合比与相邻层位的数值最大偏差可达50 ppbv。该现象可通过简单的层状捕集模型在理论上复现，进而产生局域性极强的气体年龄标尺倒转现象。本研究提出一种针对异常值的记录校正方法，旨在最小化整体信号的偏差。在去除层状捕集作用引发的异常信号后，DO-17的甲烷记录相较于高积累条件下的西南极冰盖分水岭（WAIS Divide）冰芯对应记录更为平滑。这一结果符合预期，因为在低积累条件下，粒雪层的沉降与致密化速率更为缓慢。但令人意外的是，沃斯托克冰芯中现代与DO-17时期的记录平滑程度却十分相近。这表明，南极东高原低积累点位的冰川痕量气体记录，所能提供的古大气成分变化时间分辨率要高于此前的预期。本研究还开发了一种数值方法，通过与弱平滑记录进行对比，在去除异常层位后提取冰芯内的气体年龄分布信息。该方法尤其适配南极东高原的环境条件，可用于表征大范围极低温、低积累条件下的记录平滑特征。