CryoSat-2/ICESat-2 integrated time series and comparison of shoreline evolution in Antarctic active subglacial lakes

Subglacial water systems and deeper groundwater networks beneath the Antarctic ice sheet connect glacial and oceanic systems, modulate ice dynamics, and remain a major physical uncertainty of future ice sheet predictions. Subglacial water collects in subglacial lakes, some of which are "active" lakes that episodically fill and drain inferred from ice-surface uplift and subsidence respectively. Active lake drain-fill cycles generate time-varying evolution of subglacial water distribution, transport mechanisms, and freshwater flux into sub-ice-shelf cavities and the Southern Ocean. When these lakes were initially discovered in the 2000s, the coarse spatial resolution and short temporal duration of available ICESat observations limited comprehensive knowledge about how lake geometry changed through fill-drain cycles. This limited view resulted in uncertain and stationary lake boundaries representative of a brief observational window when lakes were initially discovered. Now almost two decades later, the time series surface observations demonstrate shoreline position is another dynamic aspect of active subglacial lakes. Here, we combine multi-mission radar and laser altimetry records from the CryoSat-2 and ICESat-2 missions, into a 12-year time series of ice-surface deformation over active subglacial lakes in Antarctica to apply a robust, reproducible shoreline delineation method. This method generates time-varying lake outlines to quantify lakeshore migration and time-variable water fluxes. We find general agreement of temporal trends observed using this method compared to using static outlines; however, the amplitude of volume flux estimates is doubled or tripled in some cases using our refined approach due to lake activity adjoining the static outlines. These improved estimates of interconnected subglacial lake activity using variable shorelines will inform transient subglacial water models with more precise inferences of water flux through the subglacial system, across the land-ocean interface at the grounding zone, and into sub-ice-shelf 'estuaries' and the greater Southern Ocean.