Composition and modification of water masses in the Mackenzie shelf estuary

Abstract

The distributions of/i180, salinity, temperature, and nutrients have been used to quantify water sources to the Mackenzie shelf in the Beaufort Sea. Comparison of water mass analyses with satellite imagery confirms that the meteoric (runoff) water is associated with the Mackenzie plume. The seasonally variable surface layer for the shelf is viewed as cycling between a "reverse estuary" in winter, when the polar mixed layer (PML) is formed, and a positive estuary in summer when the shelf waters respond to freshwater inputs (runoff and ice melt). We infer a standing stock of 3.7 m fresh water at the end of summer 1986, of which 30% owes its origin to the melting of sea ice; our data coupled with river flow imply a freshwater flushing time for the Mackenzie shelf at about 150 days. To re-form the PML during winter requires the removal of this seasonal fresh water through the combined processes of flushing and ice formation: once this fresh water has been removed, continued ice growth can produce "new" brine which would be observed as a deeper and saltier PML from the previous year. A simple geochemical model shows that autumn conditions (freshwater accumulation) and the rate of flushing are important controls on the potential of the shelf to produce "new" brine and that winter runoff, were it to distribute evenly across the shelf, is sufficient to inhibit brine production. INTRODUCTION Arctic Ocean studies have recently focused on freshwater budgets [Aagaard and Coachman, 1975; Hanzlick and Aagaard, 1980; Anderson et al., 1983] and particularly on the two main sources, runoff (including Pacific Ocean sources) and ice melt [e.g., Tan and Strain, 1980; Ostlund and Hut, 1984]. The reason for this interest is simple: fresh water plays a key role in maintaining the Arctic halocline and, by inference, controls ice cover and deep water formation, both of which have climatic implications [Carmack, 1989]. The Arctic Ocean can be considered as a large, complex estuary where seasonal runoff impinges on wide shelves synchronously around the basin edge; in fact, the combined inflow (about 3500 km3yr -• [Treshnikov, 1985]) is surpassed in magnitude only by the Amazon. Additional runoff (estimated at 1670 km3yr -1 when referenced to salinity of 34.8 practical salinity units (psu) [Aagaard and Carmack, 1989]) enters indirectly through Bering Strait. Unique to cold region estuaries is a melt-freeze cycle which can cause positive or negative estuarine forcing [Carmack et al., this issue]. The logical place to look for reverse estuarine behavior (surface production of brine) is in the shear zone between landfast ice and the moving pack and in polynyas where water is kept open for long periods in winter. There seasonal processes have been implicated in producing brine drainage and hence maintaining the halocline [Melling and Lewis, 1982; Aagaard et al., 1981]. We expect therefore that the important estuarine processes (both negative and positive) occur over the shelves, are strongly seasonal, and are unevenly distributed within the Arc...

Tracing the transport of colored dissolved organic matter in water masses of the Southern Beaufort Sea: relationship with hydrographic characteristics

“…This water is characterized by a temperature maximum around the centered salinity (Shimada et al, 2001). Below the PSW, Pacific winter waters (PWW: salinity centered at 33.1) form upper halocline waters (UHW) from 100 m to 180 m depth Macdonald et al, 1989;Shimada et al, 2005). This water mass has a temperature minimum (≈-1.4 • C) and clear maxima of nitrate and silicate concentrations (Codispoti et al, 2005;Shimada et al, 2005).…”

“…Below the UPML, the LPML is found from 20 to 50 m and is characterized by a temperature minimum around a salinity of 30.5. The temperature minimum indicates that waters from the previous winter stay at those depths Macdonald et al, 1989). Below the LPML, Pacific summer waters (PSW; salinity centered at 31.5) from the Bering Sea enter the Chukchi Sea during summer, and are then located from approximately 50 m to 100 m depth Macdonald et al, 1989;Shimada et al, 2001).…”

“…The temperature minimum indicates that waters from the previous winter stay at those depths Macdonald et al, 1989). Below the LPML, Pacific summer waters (PSW; salinity centered at 31.5) from the Bering Sea enter the Chukchi Sea during summer, and are then located from approximately 50 m to 100 m depth Macdonald et al, 1989;Shimada et al, 2001). This water is characterized by a temperature maximum around the centered salinity (Shimada et al, 2001).…”

“…Below those layers, cold and dense Arctic bottom waters (ABW) are present (e.g., Aagaard, 1989). Based on the physical properties of the water masses Macdonald et al, 1989;Shimada et al, 2001Shimada et al, , 2005 and two chemical variables, nitrate and oxygen concentrations (Codispoti et al, 2005;Shimada et al, 2005), we grouped water masses into 6 categories (see Table 1). …”

Mesoscale Distribution of Springtime Waters and Nutrients near the Continental Shelf Break, Beaufort Sea, Alaska~!2010-05-13~!2010-06-25~!2010-09-08~!

“…10), may have been generated either if dense waters were being formed inshore or if other sources of higher salinity waters existed inshore and were leaking out across the shelf [60][61][62]. We did not sample further inshore than the stations shown, so conditions further inshore are unknown.…”