Stable water isotopes measured along two snow trenches sampled at Kohnen Station, Dronning Maud Land, Antarctica in the 2014/15 field season

The isotopic composition of water in ice sheets is extensively used to infer past climate changes. In low-accumulation regions their interpretation is however challenged by poorly constrained effects that may influence the initial isotope signal during and after deposition of the snow. This is reflected in snow-pit isotope data from Kohnen Station, Antarctica, which exhibit a seasonal cycle but also strong inter-annual variations that contradict local temperature observations. These inconsistencies persist even after averaging many profiles and are thus not explained by local stratigraphic noise. Previous studies have suggested that post-depositional processes may significantly influence the isotopic composition of East Antarctic firn. Here, we investigate the importance of post-depositional processes within the open-porous firn (> 10 cm depth) at Kohnen Station by separating spatial from temporal variability. To this end, we analyse 22 isotope profiles obtained from two snow trenches and examine the temporal isotope modifications by comparing the new with published trench data extracted 2 years earlier. The initial isotope profiles undergo changes over time due to downward-advection, firn diffusion and densification in magnitudes consistent with independent estimates. Beyond that, we find further modifications of the original isotope record to be unlikely, or small in magnitude (<< 1 per mil RMSD). These results show that the discrepancy between local temperatures and isotopes most likely originates from spatially coherent processes prior to or during deposition, such as precipitation intermittency or systematic isotope modifications acting on drifting or loose surface snow.

冰盖中的水体同位素组成被广泛用于反演古气候变化。然而在低积累区域，对其的解读面临诸多挑战：降雪沉积过程中及沉积后可能影响初始同位素信号的效应约束不足。这一点在南极科嫩站（Kohnen Station）的雪坑同位素数据中体现得尤为明显：这些数据既表现出清晰的季节循环特征，也存在与当地温度观测结果相悖的强烈年际变化。即便对多条剖面进行平均处理后，这种不一致仍持续存在，因此无法用局地层序噪声来解释。既往研究指出，沉积后过程可能对东南极粒雪层的同位素组成产生显著影响。本研究通过分离空间变异性与时间变异性，探究了科嫩站开放孔隙粒雪层（深度>10cm）内沉积后过程的重要性。为此，我们分析了来自两条雪堑的22条同位素剖面，并通过将新获取的数据与2年前公开的雪堑数据进行对比，检验了同位素的时间演化特征。初始同位素剖面会随时间发生变化，其变化幅度由向下平流、粒雪扩散及致密化过程主导，与独立估算结果相符。除此之外，我们发现原始同位素记录不太可能发生进一步的改造，或者改造幅度极小（远小于1‰的均方根偏差（RMSD））。上述结果表明，当地温度与同位素记录之间的矛盾，大概率源于沉积前或沉积过程中具有空间一致性的过程，例如降水间歇性，或是作用于漂移或松散表层积雪的系统性同位素改造过程。