Closing the Isotope Hydrology at Summit: Measurements of Source Regions, Precipitation and Post-deposition Processes

The stable isotopic records from the Greenland Ice Sheet are the gold standard for understanding climate variations in the Arctic on decadal to millennial scales. While the basic tenets that underlie interpretation of isotopic information appear robust in a mean sense, meteorological and glaciological processes can confound simple interpretations. Processes of concern are variations in moisture sources, cloud processes, surface ablation, blowing snow and vapor diffusion in the firn. The project objectives are to resolve fundamental uncertainties in the controls on the isotopic composition of the ice sheet through a 3-year measurement campaign at Summit, Eureka and Reykjavik. The project will use measurements and modeling to evaluate 1) the degree to which oxygen isotopic composition and deuterium excess of snow capture variations in moisture sources versus cloud microphysical conditions, and 2) the degree to which blowing snow and vapor diffusion within the firn confound accurate interpretation of variability in the isotopic record. Continuous measurements of the isotopic composition of water vapor and daily measurements of the isotopic composition of freshly-fallen and blowing snow will be made at Summit, Eureka and Reykjavik. These will be combined with measurements of the amount, size distribution, and approximate habit of falling and blowing snow, turbulence measurements to evaluate snow lofting, surface latent heat flux (ablation and frost) and energy balance, and remote sensing of polar clouds and atmospheric structure. High-resolution firn cores will be drilled to reconcile the detailed isotopic measurements and modeling with glaciological records. The new isotope measurements will jump start an emerging international pan-Arctic cooperative network of isotope measurements, which complements Arctic observations under existing Arctic Observing Network activities. The advanced measurements at Summit enhance the site as a comprehensive observatory for monitoring and understanding Arctic change.

格陵兰冰盖（Greenland Ice Sheet）的稳定同位素记录是理解北极十年至千年尺度气候变异性的金标准。尽管同位素信息解译的核心原理在平均态层面看似稳健，但气象与冰川学过程仍可能干扰对其的简单解译。需要关注的过程包括水汽源变化、云物理过程、表面消融、吹雪以及粒雪层（firn）内的水汽扩散。本项目旨在通过在萨米特（Summit）、尤里卡（Eureka）、雷克雅未克（Reykjavik）开展的三年期测量观测计划，解决冰盖同位素组成控制机制中的核心不确定性问题。本项目将结合测量与建模手段，评估两大核心问题：其一，积雪的氧同位素组成与氘盈余（deuterium excess）在多大程度上能够反映水汽源变化与云微物理条件；其二，吹雪与粒雪层内的水汽扩散在多大程度上会干扰同位素记录变异性的精准解译。研究团队将在上述三个站点开展水汽同位素组成的连续观测，以及新鲜降雪与吹雪的同位素组成的每日监测。这些观测数据将与以下多类测量数据融合：降雪及吹雪的总量、粒径分布与近似雪晶形态观测，用于评估雪粒扬起过程的湍流观测，表面潜热通量（消融与霜形成）及能量平衡观测，以及极地云与大气结构的遥感监测。本项目还将钻取高分辨率粒雪冰芯，以将精细化的同位素测量与建模结果与冰川学记录进行匹配校准。这批全新的同位素观测数据将助力新兴的泛北极跨国同位素测量合作网络，该网络可补充现有北极观测网络（Arctic Observing Network）框架下的北极观测工作。在萨米特站点开展的先进观测将进一步将该站点打造为监测与理解北极变化的综合观测台站。