Interpreting ancient ice in a shallow ice core from the South Yamato (Antarctica) blue ice area using flow modeling and compositional matching to deep ice cores

[1] We explore methods of dating a 101 m ice core from a bare ice ablation area in the Yamato Mountains, Dronning Maud Land, East Antarctica. There are two unknowns, the age of the ice at the surface and the age spanned by the core. The ice crystal growth rate was used to estimate the age span of the core at about 5 kyr. CO2, CH4 and N2O data on the core were compared with well-dated records from deep cores, leading to two plausible matches, both within isotope stage C. Detailed comparison of high-resolution DEP records from this core and the Dome Fuji core support the 55–61 kyr BP fit best. Oxygen isotope values in the core were then used to constrain the source elevation of the snow in the core, and hence the velocities in a simple flow line model. We inverted the ice core surface age, age span and origin site and their confidence intervals in a sensitivity study of flow model parameter space. The flow line model predicts good matches to the core by reducing glacial flow rates to 70% of present-day, accumulation rates by 45% with 10% confidence intervals. The modeled surface age for the whole meteorite field yields maximum surface ages of about 90 kyr, which is consistent with known, but poorly constrained, tephra dating, meteorite terrestrial ages and the frequency of meteorite discoveries. This approach can be used quite generally to link deep ice cores to surface outcrops on blue ice fields.

[1]我们探索了对东南极洲毛德皇后地大和山脉一处裸冰消融区的一根101米长冰芯进行定年的方法。存在两个未知量，即表面冰的年龄以及冰芯所跨越的年龄。利用冰晶生长速率估计冰芯的年龄跨度约为5000年。将该冰芯上的二氧化碳、甲烷和氧化亚氮数据与来自深部冰芯的精确定年记录进行了比较，得出了两种合理的匹配结果，都在同位素阶段C内。对该冰芯和富士圆顶冰芯的高分辨率电偶极子（DEP）记录进行详细比较后，发现公元前55000 - 61000年的匹配度最佳。然后利用冰芯中的氧同位素值来确定冰芯中雪的源海拔高度，进而确定一个简单流线模型中的速度。在对流动模型参数空间进行的敏感性研究中，我们反演了冰芯表面年龄、年龄跨度和起源地点及其置信区间。流线模型通过将冰川流速降低到现今的70%，积累速率降低45%（置信区间为10%），预测出与冰芯的良好匹配。整个陨石场的模拟表面年龄得出的最大表面年龄约为90000年，这与已知但约束较差的火山灰定年、陨石陆地年龄以及陨石发现频率是一致的。这种方法可以相当普遍地用于将深部冰芯与蓝冰场上的地表露头联系起来。