Comment on the comment by M. J. Siegert on “A numerical model for an alternative origin of Lake Vostok and its exobiological implications for Mars” by N. S. Duxbury et al.

Abstract

[1] There is some debate on the origin of subglacial Lake Vostok and whether the water within the present subglacial lake system contains biota that survived the buildup of the Antarctic ice sheet. One theory suggests that Lake Vostok existed as a preglacial lake before glaciation of the continent at around 15 Ma ago, survived the subsequent period of ice sheet growth, and remained stable beneath the thick ice cover to the present day [Duxbury et al., 2001]. Another hypothesis challenges this view by stating that the early phase of build-up would have resulted in ice grounding throughout the trough which the lake now occupies [Siegert, 2004]. [2] According to Duxbury et al. [2001], the lake could have survived the growth of the ice sheet as long as the preglacial lake was more than 53 m deep. In their model, subglacial Lake Vostok is considered as an approximate closed system. However, if Lake Vostok existed as a preglacial lake, the closure of the Lake Vostok system must have happened between the initiation of the ice sheet and the establishment of a stable ice cover in East Antarctica [Siegert, 2004]. Therefore the dynamic evolution of the ice sheet and its interaction with the preglacial lake should be accounted for in determining whether subglacial Lake Vostok is a direct remnant of a preglacial surficial lake. [3] Siegert [2004] demonstrates that the preglacial lake could not have survived the buildup of the Antarctic ice sheet: water flow beneath an ice mass is controlled by the hydraulic potential gradient, Pg = rigas + (rw ri) gab, where ri and rw are the density of ice and water, and as and ab are the surface and basal slope, respectively [Shreve, 1972]. If the magnitude of surface slope is larger than one tenth the basal slope, water can be driven out of a topographic depression and flow ‘‘uphill.’’ Two icesheet model studies [Huybrechts, 1993; DeConto and Pollard, 2003] show that during the inception and growth of the Antarctic ice sheet, the ice sheet margin was situated close to Lake Vostok. The steep margin of the ice sheet across Lake Vostok implies high subglacial hydraulic potential gradients, which led Siegert [2004] to conclude that basal water is evacuated as the steep ice sheet margin progresses over the lake. However, none of the above models take into account the interaction of the ice sheet with the underlying lake, whether it be a preglacial or a subglacial lake, and only consider an ice sheet more or less frozen to the underlying bedrock. [4] Below I present a physical mechanism that allows for subglacial Lake Vostok to survive the buildup of the ice sheet, by taking into account the interaction of the ice sheet with the preglacial/subglacial lake. This hypothesis is supported by the fact that the surface of a preglacial lake as well as the interface between a subglacial lake and the overriding ice sheet can be regarded as a slippery spot.