The physical conditions within subglacial lakes are critical to evaluating how and where life may best exist.

Here, we propose that Earths geothermal flux provides efficient stirring of Antarctic subglacial lake water.

We demonstrate that most lakes are in a regime of vigorous turbulent vertical convection, enabling suspension of spherical particulates with diameters up to 36 micrometers.

Thus, dynamic conditions support efficient mixing of nutrient- and oxygen-enriched meltwater derived from the overlying ice, which is essential for biome support within the water column.

We caution that accreted ice analysis cannot always be used as a proxy for water sampling of lakes beneath a thin ( INTRODUCTION The Antarctic continent is covered with ice, growing and shrinking over periods of tens to hundreds of thousands of years, since at least the last 14 million years ( 1 ).

Over 250 hydrologically stable subglacial lakes (in which water inputs are constantly balanced by outputs) trapped between the bed and the ice are known to exist at and close to the ice sheet center ( 2 ).

They comprise a wide variety of sizes and glaciological and topographic settings ( 3 ) and have been hypothesized as potential habitats for the in situ development of microbial organisms ( 4 ).

A further 130 hydrologically active lakes, which experience rapid water discharges and large volume changes, exist toward the margin of the ice sheet ( 6, 7 ).

While these may contain microbial life ( 8 ), they are not considered as isolated habitats where microbes can adapt independently over long periods due to the flushing of water in and out of their systems and their potentially ephemeral nature.

The Antarctic ice and bed material carry lifes building blocks, with oxygen and minerals held within dust in the former, and minerals trapped inside sediments and bedrock in the latter.

Numerical models and radar observations have shown that the ice sheet base above subglacial lakes typically melts where the ice is thickest and freezes where it is thinnest ( 4, 9, 10 ).

Thus, oxygen and minerals are released at the top of the water column.

The rate at which this happens is key to assessing the possibility of having a biome, but remains largely uncertain.

Although microbial life is anticipated at the floors of subglacial lakes, where sediments are known to exist ( 11 ), dynamic flows and mixing of bottom water within the water column are essential for life to be widespread and detectable, avoiding, for example, anoxic conditions if oxygen-rich surface water is unable to access deeper parts of the lake.

Subglacial lakes are isolated from winds and solar heating but can experience vertical convection flows due to the upward geothermal flux at a background level of roughly 50 mWm 2; ( 12 ), and horizontal convection flows due to the ubiquitousalbeit variabletilt of their ice ceiling (about 10 times and in opposite direction to the ice surface slope).

Previous work has estimated that velocities of few tenths of a millimetersecond are generally required to suspend sediments in the water column ( 13 ).

This is of the same order of magnitude as that predicted by ocean modeling for a handful of subglacial lakes, including Lake Vostok ( 14 ), Lake Ellsworth ( 15 ), and Lake Concordia ( 9 ).

However, uncertainties are large, and velocities remain unknown for most subglacial lakes, including Lake CECs, which might be the first stable lake to be drilled into in a clean way in the coming years ( 16 ).

As a result, plans for direct sampling can be helped by establishing hydrological conditions in subglacial lakes, and their variation between lake settings, to recognize where microbial life is most likely to thrive.

Thus, our work complements previous studies on convection in lakes at atmospheric pressure (i.e., open) or with thin ice covers ( 17 ), and, more specifically, previous efforts that aimed to predict the hydrological conditions of individual subglacial lakes, including Lake Vostok ( 18 ).

We demonstrate that most subglacial lakes have large supercritical convective parameters, i.e., the geothermal flux is much larger than the minimum critical heat flux required to trigger convective flows, such that they are in a regime of vigorous turbulent convection.

We show that vertical convection is as important as horizontal convection and that the convective dynamics vary considerably based on the ice thickness, water depth, and ceiling slope.

For simplicity, we restrict our attention to freshwater, because salt concentration is typically low in isolated subglacial lakes ( 4 ).

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