Possible displacement of the climate signal in ancient ice by premelting and anomalous diffusion

The best high-resolution records of climate over the past few hundred mille nnia are derived from ice cores retrieved from Greenland and Antarctica(1-3 ). The interpretation of these records relies on the assumption that the tr ace constituents used as proxies for past climate have undergone only modes t post-depositional migration. Many of the constituents are soluble impurit ies found principally in unfrozen liquid that separates the grain boundarie s in ice sheets. This phase behaviour, termed premelting, is characteristic of polycrystalline material(4,5). Here we show that premelting influences compositional diffusion in a manner that causes the advection of impurity a nomalies towards warmer regions while maintaining their spatial integrity. Notwithstanding chemical reactions that might rx certain species against th is prevailing transport, we rnd that-under conditions that resemble those e ncountered in the Eemian interglacial ice of central Greenland (from about 125,000 to 115,000 years ago)-impurity fluctuations may be separated from i ce of the same age by as much as 50 cm. This distance is comparable to the ice thickness of the contested sudden cooling events in Eemian ice from the GRIP core.