Incorporation of particulates into accreted ice above subglacial Lake Vostok , Antarctica

The nature of microscopic particulates in meteoric and accreted ice from the Vostok ice core, is assessed in conjunction with existing ice core data to investigate the mechanism by which particulates are incorporated into refrozen lake water. Melted ice samples from a range of ice core depths were filtered through 0.2 μm polycarbonate membranes and secondary electron images were collected at ×500 magnification using a scanning electron microscope. Image analysis software was used to characterise the size and shape of particulates. Similar distributions of major axis lengths, surface areas and shape factors (aspect ratio and compactness) for particulates in all accreted ice samples suggest that a single process may be responsible for incorporating the vast majority of particulates for all depths. Calculation of Stokes settling velocities for particulates of various sizes implies that 98 % of particulates observed could ‘float’ to the ice-water interface with upward water velocities of 0.0003 m s where they could be incorporated by growing ice crystals at the ice-water interface, or by rising frazil ice crystals. The presence of particulates that are expected to sink in the water column (2 %) and the uneven distribution of particulates in the ice core further implies that periodic perturbations to the lake’s circulation, involving increased velocities, may have occurred in the past. Introduction Lake Vostok is the largest of more than one hundred lakes identified beneath the Antarctic ice sheet (Siegert and others, 1996; Priscu and others, 2003). The lake is up to 260 km long and 80 km wide (Tabacco and others, 2002), with a volume calculated to be 5,200 km (Studinger and others, 2003). The liquid water results from melting of the overlying ice sheet in the north where it is sufficiently thick for the basal ice to reach the pressure melting point (PMP). Melting is thought to be approximately balanced by the re-freezing of lake water to the