Firn microstructure in the lock-in zone for three polar ice cores@en

In order to interpret the paleoclimatic record stored in the air enclosed in polar ice cores, it is crucial to understand the fundamental lock-in process. Within the porous firn, bubbles are sealed continuously until the respective horizontal layer reaches a critical porosity. Present-day firn models use a postulated temperature dependence of this value as the only parameter to adjust to the surrounding conditions of individual sites. However, no direct measurements of the firn microstructure could confirm these assumptions. Here we show that the critical porosity is a universal constant by providing a statistically solid data set of µm-resolution 3D X-ray computer tomographic measurements for ice cores representing different extremes of the temperature and accumulation ranges. We demonstrate why indirect measurements yield misleading data and substantiate our observations by applying percolation theory as a theoretical framework for bubble trapping. Incorporation of our results does significantly influence the dating of trace gas records, changing gas age-ice age differences by up to more than 1000 years. This will help resolve inconsistencies, such as differences between East Antarctic d15N records (as a proxy for firn height) and model results. We expect our findings to be the basis for improved firn air and densification models, leading to lower dating uncertainties. The reduced coupling of proxies and surrounding conditions may allow for more sophisticated reinterpretations of trace gas records in terms of paleoclimatic changes and will foster the development of new proxies, such as the air content as a marker of local insolation.为解读封存于极地冰芯包裹空气中的古气候记录，理解核心的锁定过程（lock-in process）至关重要。在多孔粒雪（firn）中，气泡会持续被封存，直至对应水平冰层达到临界孔隙率（critical porosity）。当前的粒雪模型仅将该临界孔隙率的预设温度依赖关系作为唯一参数，以适配各观测点的实际环境条件。然而，目前尚无针对粒雪微观结构的直接测量结果能够验证这些假设。本研究通过对覆盖温度与积累量不同极端区间的冰芯开展微米级分辨率三维X射线计算机断层扫描（3D X-ray computer tomographic）测量，获得了统计学上可靠的数据集，证明临界孔隙率为一普适常数。本研究阐明了间接测量为何会得到误导性数据，并以渗流理论（percolation theory）作为气泡捕获的理论框架，佐证了观测结果。将本研究结果纳入现有模型后，将显著影响痕量气体记录的定年工作，气体年龄与冰龄的差值最大可改变逾1000年。这将有助于解决现有矛盾，例如东南极δ15N记录（作为粒雪层高度的代用指标）与模型结果之间的偏差。我们预计，本研究发现将成为优化粒雪空气与密实化模型的基础，进而降低定年不确定性。代用指标与环境条件之间耦合性的减弱，将允许我们针对古气候变化开展更精细的痕量气体记录重新解读，并推动新型代用指标的开发，例如以空气含量作为局地日射量的标志物。