Exploring the potential of 17O-excess in ice to constrain post-depositional alteration and loss by sublimation.

目前，由于沉积后发生的同位素分馏反应的复杂性，使用中低纬度雪和冰沉积中水的共同稳定同位素比(D/H，18O/16O)来预算来自山脉冰川的水资源和从冰芯重建古气候是有限的。升华作用造成的沉积后损失可能是最广泛认识的改变冰雪同位素组成的过程。由于同位素分馏依赖于温度，这种通过升华以及熔融-再冻结循环等其他过程进行的沉积后改造对于使用传统的H和O同位素系统是很难校正的。从三氧同位素系统(16O-17O-18O)计算出的17O超额参数--类似于由DeltaD和Delta18O得出的d超额参数--对地球表面典型范围内的温度变化几乎不敏感。这使得可以识别动力学控制的同位素交换，例如在蒸发过程中，而不必考虑依赖于温度的平衡分馏。我们建议通过一系列精心设计的实验室实验来确定三重氧同位素分馏系数和17O过剩在升华和熔融冻结过程中的演化。然后，我们试图通过在德国阿尔卑斯山Schnefernerhaus研究站进行为期两周的高分辨率监测研究，并辅之以一年中定期的低分辨率积雪采样，来测试17O过量在自然高海拔环境中限制雪和冰的二次同位素变化的普遍适用性。最后，我们试图研究罗马尼亚斯卡里萨拉冰洞全新世洞穴冰沉积的最新冰，目的是在每年发生的熔融-再冻结循环期间，以17O过量限制冰的沉积后同位素变化，目前该旋回阻碍了稳定的H和O同位素比值作为该地区古气候指标的使用。

The use of common stable isotope ratios of water (D/H, 18O/16O) in low and mid-latitude snow and ice deposits for budgeting water resources from mountain glaciers and for paleoclimate reconstruction from ice cores is currently limited by the complexity of isotope fractionation reactions occurring after deposition. Post-depositional loss due to sublimation is perhaps the most widely recognized process that modifies the isotopic composition of snow and ice. Such post-depositional modification by sublimation - and by other processes, such as melting-refreezing cycles - are difficult to correct for using the conventional H and O isotope systems because of a dependency of isotope fractionation on temperature. The 17O-excess parameter - similar to d-excess derived from deltaD and delta18O - calculated from the triple O-isotope system (16O-17O-18O), is nearly insensitive to temperature changes in the typical range found on the earth's surface. This allows to identify kinetically controlled isotope exchange, for example during evaporation, without having to consider temperature dependent equilibrium fractionation. We propose to determine triple O-isotope fractionation factors and the evolution of 17O-excess during sublimation and melting-freezing in a series of carefully designed laboratory experiments. We then seek to test the general applicability of 17O-excess to constrain secondary isotopic alteration of snow and ice in the natural high-altitude environment - at Schneefernerhaus research station, German Alps - by a high-resolution monitoring study over a few two-week-long intervals, complemented by regular low resolution snow sampling over the course of a year. Finally, we seek to investigate the most recent ice from the well dated holocene cave ice deposit at Scarisoara ice cave, Romania, with the objective to constrain by 17O-excess the post-depositional isotopic alteration of ice during the annually occurring melting-refreezing cycles that currently hampers the use of stable H and O isotope ratios as paleoclimatic proxies at this location.