Earth' s past temperatures in the polar regions are often calculated from the relative abundances of heavy hydrogen or oxygen atoms (isotopes) in the polar ice. Implicit to this approach is the assumption that once the snow has fallen from the sky, its isotopic composition no longer changes. Yet, this conventional notion is incompatible with some observations. In and near the McMurdo Dry Valleys in Antarctica, for example, the surface snow and ice show an unusually large depletion in heavy hydrogen isotopes relative to heavy oxygen isotopes. Deficits of such a magnitude are rare in precipitation and therefore hint at sublimation causing the ice to change its isotopic composition (to "fractionate") after deposition; if true, this process would challenge the conventional wisdom about snow and ice as paleoclimate indicators. Here, we investigate whether these unusual heavy-isotope deficits could originate from Antarctic precipitation. We determine that the moisture arriving at Antarctica today does not have these deficits. Sublimation of the snow that falls, however, can quantitatively explain the observed range of heavy-hydrogen depletions. We conclude that sublimation does fractionate isotopes in and near the Dry Valleys, and may affect the past temperatures reconstructed from the hydrogen and/or oxygen isotopes.
颜余真，美国莱斯大学地球科学系博士后。2019年博士毕业于普林斯顿大学地球科学系（导师Michael Bender/John Higgins）。研究方向为古气候和同位素地球化学，主要手段为冰芯包裹气体的元素和同位素组成分析。于2015年参与了东南极地区Allan Hills蓝冰区为期十周的科学考察，所发现的270万年冰芯入选《科学》杂志2017年十大科学突破之一。