MYSTERY OF ICICLE FORMATION: AN APPROACH BASED ON STABLE ISOTOPE STUDIES

Icicles are a picturesque feature commonly seen in regions that experience cold winter weather. These cone-shaped features form whenever cold water, typically snowmelt, continuously flows over an overhang under subfreezing temperatures. The dripping water freezes as latent heat is lost to the colder atmosphere. Icicles may be found suspended from the roofs of buildings, ledges, tree branches, and power lines. Despite their frequent occurrence, relatively little attention has been paid to the physical processes that lead to the formation and subsequent growth of icicles. A better understanding of the mechanisms of icicle formation can advance our knowledge of the general process of ice formation and the melting of ice. It has also been suggested that icicles may add to the understanding of groundwater hydrology. We present a systematic triple isotope (δ¹⁷O, δ¹⁸O, δ²H) analysis of icicles that formed during the winter of 2015 in South West Michigan, USA. Our experimental data provide evidence for the few theoretical models that have been suggested for the rapid growth of icicles. Furthermore, our data is in agreement with laboratory experiments describing the isotopic fractionation that occurs during freezing, where diffusion induced kinetic fractionation occurs through a boundary layer between the forming ice and remaining water or, in the absence of a boundary layer, a Rayleigh type process is favored. Experimental evidence was attained by analyzing several icicles by stepwise melting at a constant temperature and analyzing the isotopic composition of various melt fractions. It is assumed that the melting process inversely mimics the formation of solid icicle from snow melt.