Yuzhen Yan

Blue ice areas (BIAs) are regions where ablation exceeds accumulation. In Allan Hills BIA, East Antarctica, ancient ice is brought up to the surface by ice flow guided by a rising bedrock (Figure 1; Whillans and Cassidy, 1983; Bintanja, 1999). This unique glaciological setting makes Allan Hills BIAs one of the most studied meteorite concentration sites in Antarctica (Whillans and Cassidy, 1983). Intriguingly, one meteorite (ALH88019) collected in the Allan Hills BIAs has a terrestrial age of 2 million years (Scherer et al, 1997), suggesting the existence of ice of similar age here. Still, there are many mysteries around the BIAs that call for answers before they can be used a paleoclimate archives. One question has been: Does the glacial motion and negative mass balance at the ice surface impact its climate record, especially the stable water isotopes?

This project was inspired by one peculiar observation made in and near Allan Hills BIAs, where the deuterium excess (d-excess = δD - 8*δ¹⁸O) values are negative (note: only in Allan Hills BIAs and the Dry Valleys nearby is persistently negative d-excess found!). Negative d-excess values are rare in Antarctic precipitation (Masson-Delmotte et al, 2008). They could arise from moisture sourced from high-latitude, polar ocean. Alternatively, extensive sublimation under dry, windy conditions could lead to isotope fractionation, although what physical mechanisms could register fractionation in a solid phase is unclear.

To answer these questions, I collaborated with Dr. Jun Hu, Dr. Sylvia Dee, and Dr. Laurence Yeung at Rice to investigate the origin of this rather unusual glaciological phenomenon. We utilized isotope-enabled general circulation model to evaluate under what conditions negative d-excess can form in precipitation. We found that in order to produce negative d-excess in precipitation over Dry Valleys and Allan Hills, an unrealistically high fraction of moisture from the high-latitude Southern Ocean is required. This premise is both against the water-tagging experiment from the same model and against observations.

Therefore, sublimation must have contributed to the isotopic fractionation of the surface snow, challenging the prevailing notion that no isotope fractionation occurs during sublimation. Our calculation further shows that solid-phase-diffusion in ice grains is sufficiently fast to allow the isotopic fractionation to be recorded by the whole grain. Our model could explain why in other low-accumulation sites such as Dome C and Vostok sublimation is not observed, because temperatures there are too low to allow isotopic homogenization by solid-phase diffusion. Allan Hills and the Dry Valleys represent a peculiar spot where two seemingly contradictory conditions are met: (1) the temperature is warm enough to allow solid-state diffusion and (2) there is a strong sublimation flux. A manuscript led by Dr. Hu and me (contributed equally) has recently been published on JGR-Atmospheres (DOI: 10.1029/2021JD035950).

Bintanja, R., 1999. On the glaciological, meteorological, and climatological significance of Antarctic blue ice areas. Reviews of Geophysics, 37(3), pp.337-359.

Masson-Delmotte, V., Hou, S., Ekaykin, A., Jouzel, J., Aristarain, A., Bernardo, R.T., Bromwich, D., Cattani, O., Delmotte, M., Falourd, S. and Frezzotti, M., 2008. A review of Antarctic surface snow isotopic composition: Observations, atmospheric circulation, and isotopic modeling. Journal of climate, 21(13), pp.3359-3387.

Scherer, P., Schultz, L., Neupert, U., Knauer, M., Neumann, S., Leya, I., Michel, R., Mokos, J., Lipschutz, M.E., Metzler, K. and Suter, M., 1997. Allan Hills 88019: An Antarctic H‐chondrite with a very long terrestrial age. Meteoritics and Planetary Science, 32(6), pp.769-773.

Whillans, I.M. and Cassidy, W.A., 1983. Catch a falling star: Meteorites and old ice. Science, 222(4619), pp.55-57.