Pervasive diffusion of climate signals recorded in ice-vein ionic impurities

Abstract

A theory of vein impurity transport conceived two decades ago predicts that signals in the bulk concentration of soluble ions in ice migrate under a temperature gradient. If valid, it would mean that some palaeoclimatic signals deep in ice cores (signals from vein impurities as opposed to matrix or grain-boundary impurities) suffer displacements that upset their dating and alignment with other proxies. We revisit the vein physical interactions to find that a strong diffusion acts on such signals. It arises because the Gibbs–Thomson effect, which the original theory neglected, perturbs the impurity concentration of the vein water wherever the bulk impurity concentration carries a signal. Thus, any migrating vein signals will not survive into deep ice where their displacement matters, and the palaeoclimatic concern posed by the original theory no longer stands. Simulations with signal peaks introduced in shallow ice at the GRIP and EPICA Dome C ice-core sites, ignoring spatial fluctuations of the ice grain size, confirm that rapid damping and broadening eradicates the peaks by two-thirds way down the ice column. Artificially reducing the solute diffusivity in water (to mimic partially connected veins) by 103 times or more is necessary for signals to penetrate into the lowest several hundred metres with minimal amplitude loss. Simulations incorporating grain-size fluctuations on the decimetre scale show that these can cause the formation of new, non-migrating solute peaks. The deep solute peaks observed in ice cores can only be explained by widespread vein disconnection or a dominance of matrix or grain-boundary impurities at depth (including their recent transfer to veins) or signal formation induced by grain-size fluctuations; in all cases, the deep peaks would not have displaced far. Disentangling the different signal contributions – from veins, the ice matrix, grain boundaries, and grain-size fluctuations – will aid robust reconstruction from ion records.