Using the δ18O composition in landfast ice as a record of arctic estuarine processes

Estuaries that produce substantial amounts of ice differ from those that do not. First, ice formation and melting make important contributions to salt and freshwater budgets and thereby influence water circulation. Second, ice cover provides a physical barrier which severely restricts the air–sea exchange of properties and energy. Unfortunately, the logistical problems of oceanographic sampling in the Arctic during winter have resulted in sparse data sets. Here, we show how the oxygen isotope composition (δ18O) in landfast ice at the end of winter provides a record of surface water properties throughout winter. Two arctic estuaries are contrasted: the Mackenzie estuary which faces directly onto a broad, open shelf and the Husky Lakes estuary which comprises a series of basins that exchange with one another and the shelf through narrow channels. The method of converting records of δ18O in ice cores to surface water salinity as a function of time throughout winter is outlined. The Mackenzie estuary has a large winter inflow which spreads beneath the ice as a plume overwhelming brine production by sea-ice formation and thereby shutting down convection. Using only ice records, confident estimates can be made of the rate and direction of plume spreading. In contrast, the Husky Lakes estuary is supported only by a local, truly Arctic drainage basin which becomes frozen in winter. The landfast ice in this system records a large, and relatively static, horizontal gradient in surface salinity. Small variations in δ18O with time evident in all ice cores are found to correlate between adjacent sites suggesting coherent displacements of surface water by 10–15 km as might be produced by seiching.