THE MACKENZIE SHELF - BUDGETS AND PROBLEMS
R.W. Macdonald
Institute of Ocean Sciences, P.O. Box 6000, Sidney, B.C., V8L 4B2
K.C. Ruttenberg
Wood Hole Oceanographic Institution, Woods Hole, Mass., 02543
C. Gobeil
Institute Maurice Lamontagne, B.P.1000, Mont Joli, P.Q., G5H 3Z4
1. INTRODUCTION
Continental shelves are often viewed as boxes between land and ocean (Gordon et al., 1995). These boxes are vulnerable to global change, especially in the arctic where there is the added potential to affect ice climate. The greatest leverage for change will arise from active processes on the shelf rather than simple, passive transmission of properties. For example, a small change in the annual riverine inflow would have only a small direct effect on freshwater budgets but potentially a very large effect on convection (Melling, 1993; Macdonald et al., 1995). Shelves clearly take much more than a passive role in cycles for carbon (Hedges, 1992), nitrogen (Christensen, 1994) and phosphorus (Ruttenberg, 1993).
The Mackenzie shelf is unique in the North American side of the Arctic Ocean in that it is a wide shelf dominated by input from a large, sediment-rich river. Within the Arctic Ocean, this shelf probably provides the best opportunity to construct material budgets because 1) inputs can be reasonably well constrained, 2) a large amount of data exist with which to estimate water and sediment properties over the shelf and 3) the important processes are known qualitatively if not quantitatively. Despite efforts to study this shelf, there remain difficulties in closing budgets for non-conservative properties such as sediments and nutrients, and processe like brine production. We proceed from detailed water, sediment and carbon budgets (Macdonald et al., 1995, 1998) to produce budgets for inorganic phosphorus and nitrogen.
2. BUDGETS
2.1 Freshwater
The freshwater budget (Fig. 1) shows that the Mackenzie River dominates the shelf with an annual yield (inflow/shelf area) of > 5 m. Ice export, either as ice or icemelt, contributes significantly to freshwater exchange. This export is mirrored elsewhere in the system by the brine rejected during ice growth. How this ice export affects budgets and/or stratification depends crucially on whether the brine is separated from the ice by deep convection or whether it remains coupled with the ice in the polar mixed layer (PML) as discussed next.
2.2 Salt
Using the freshwater budget, salt exchanges (Fig. 2) can be estimated from representative salinity distributions. Shelf-edge exchange dominates. Although ice, at about 5 psu, transports only small amounts of salt, the brine rejected from the ice is a significant component of the salt budget. This brine can be exported near the surface if, for example, the water fails to convect deeply in winter. When sufficiently dense water is formed, brine may sink and transit the shelf bottom picking up other properties (e.g., regenerated nutrients) to be exported into deeper waters of the interior ocean (cf. Melling and Lewis, 1982). If 60 km3 of ice is exported, a further 1500 × 109 kg of salt must also be exported in surface or deeper water to maintain salt balance. The ice/brine budget is poorly determined because ice export is difficult to measure (Melling and Reidel, 1996) as is brine export (Macdonald et al., 1995). This lack of closure makes it difficult to estimate other parameters (nitrate, phosphate) transported with the brine.
Corresponding author address, R.W. Macdonald, IOS, P.O.
Box 6000, Sidney B.C., V8L 4B2; e-mail macdonaldrob@dfo-mpo,gc.ca
