Freezing and Open-System Water Exchange in Subglacial Lake Vostok

Lake Vostok, the largest of the Antarctic subglacial lakes, is a major unexplored system. Evidence has been advanced for a balanced, possibly closed system with melting of the overlying ice sheet into the lake along the western margin being approximately equal to the freezing of the lake water onto the ice sheet base. If the melting and freezing into Lake Vostok are balanced then little need for the influx of water from the surrounding catchment is necessary and biologic samples from the Vostok ice core only reflect the lake environment. We use ice-penetrating radar data over the lake to evaluate both the origin of the accreted ice recovered from the base of the Vostok core and to evaluate the balance of water through the system. We have reconstructed the ice flow trajectory for the Vostok ice core site across the width of the lake from the complex grounding line in the west, where the ice sheet enters the lake, to the regrounding line in the east. The Vostok core trajectory traverses a shallow embayment, regrounds on a bedrock ridge then goes afloat over the main lake. The accreted ice forms rapidly at both of these grounding lines. Accretion is enhanced where the incoming topography thins the ice sheet close to the grounding line. The accreted basal ice forms rapidly along both the western grounding line, deforms close to the regrounding line and is transported to the east of the lake. Most biological and geochemical analyses of accreted ice have been conducted on this rapidly developed grounding line ice accreted to the ice sheet 15,000 years ago and not representative of the open lake system. Development of accretion ice along the western margin accounts for the large number of inclusions recovered in the ice core and the short residence times estimated from the He3 measurement. Our analysis documents that accreted ice is exported along the southeastern margin of the lake. The export of 255 m of accreted lake ice through the Vostok ice core flowline, the dominance of export mechanism along the eastern margin and the absence of any melting in areas previously thought to be melting indicates that a substantial upstream subglacial water catchment is supplying Lake Vostok with freshwater.