Inverse altitude effect disputes the theoretical foundation of stable isotope paleoaltimetry

Inverse altitude effect disputes the theoretical foundation of stable isotope paleoaltimetry
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When talking about the Tibetan Plateau, the first impression you may have is that it is cold and desolate. I used to think so as well. But when I went to the southern Tibetan Plateau to collect leaf samples, I found that I was totally wrong. With the glaciers, forests, rivers, and yaks revealing themselves to me, I was breathless by the incredible scenery which I had never seen before.

 

Figure 1. A snapshot of the beautiful scenery in the southern Tibetan Plateau (photo credit W Yu)

 As a Ph.D. student in the Institute of Tibetan Plateau Research, Chinese Academy of Sciences, I was conducting fieldwork to explore the variations of stable isotopes in leaf water at different altitudes. Although I was expecting some new findings, the results were not surprising. Similar to other water samples (snow, rainfall, river water, etc.), the stable isotopes in leaf water samples we collected showed a so-called “altitude effect”. The altitude effect refers to the phenomenon in which the stable water isotopes decrease with increasing altitude and accompanying lower temperature. Based on this isotope-altitude relationship, scientists have developed a method called paleoaltimetry to reconstruct the uplift history of famous mountains and massifs around the world, like the Tibetan Plateau, the Alps, and the Rocky Mountains.

 Just when I was feeling down, my supervisor Prof. Wusheng Yu reminded me that the stable isotopes in several surface isotopic carriers like snow, rainfall, and river water in the northern Tibetan Plateau show an “inverse altitude effect” (IAE), i.e., the stable isotopes can increase with increasing altitude. Obviously, this IAE directly contradicts the theoretical foundation of stable isotope paleoaltimetry. The reasons for the IAE had puzzled scientists, who usually explained it from local and surface perspectives. But the explanations are diverse and difficult to be reconciled. This enigma of the IAE deeply intrigued me.

 So, I started to pay attention to the issue of the IAE, and by chance, I found that it also occurred in water vapor isotopes observed by Ehhalt et al. (2005), although the authors did not refer to it by name. When I excitedly discussed this with my supervisor, we realized that we can do something different. Thus, Prof Yu decided to change my research topic from leaf water stable isotopes to water vapor stable isotopes and suggested that I should focus on the key scientific question of whether the IAE is present in global water vapor isotopes.

 Following his suggestion, I analyzed the satellite-observed stable isotopes in water vapor on a global scale. We were surprised to find that the IAE also occurs in water vapor in the mid-troposphere over certain regions, which motivated us to explore the mechanism in water vapor. We found the two indispensable factors for the occurrence of the IAE in water vapor, i.e., the supply of moisture with enriched isotopic values from distant source regions, and the signals of the enriched isotopic values can be transported to the target regions from the source regions.

 More surprisingly, the global-scale spatial distributions of the IAE in water vapor are mostly consistent with its occurrence in precipitation and other surface isotopic carriers. This finding inspired us to link the IAE in water vapor to that in surface isotopic carriers. Hence, we proposed that the IAE already appears in water vapor before the precipitation event occurs. As water vapor is the “mass source” of precipitation, the IAE in water vapor isotopes is deeply imprinted on precipitation.

 As we know, the gradual uplift of mountainous regions and large massifs such as the Tibetan Plateau leads to changes in atmospheric circulation patterns within the broader region, which in turn alters moisture source regions and moisture transport pathways and their inherent patterns in isotopic values. These changes complicate the application of stable isotope paleoaltimetry in such regions. Therefore, we caution that the influence of moisture source and moisture transport pathway on stable isotopic values during different mountain uplift stages requires careful consideration before using stable isotope palaeoaltimetry.

In addition, our findings also provide insights into understanding isotopic records from ice cores recovered from different elevations. One of our co-authors, Prof. Thompson, noticed that the average oxygen isotopic value in an ice core from the summit (6700 m) of the Guliya ice cap in the northern Tibetan Plateau was higher than the average value from an ice core drilled 500 m lower. The findings from our research may potentially help to resolve this apparent contradiction. As Prof. Thompson said, “the IAE raises an important issue for the interpretation of isotopes from proxy records that cover very long-time scales.”

 

Reference:

Ehhalt D H, Rohrer F, Fried A. Vertical profiles of HDO/H2O in the troposphere. Journal of Geophysical Research: Atmospheres, 2005, 110(D13). https://doi.org/10.1029/2004JD005569