Using the nutrient ratio NO/PO as a tracer of continental shelf waters in the central Arctic Ocean

Abstract

Historical nitrate, phosphate, and dissolved oxygen data from the central Arctic Ocean are examined with particular emphasis on the conservative parameters NO (9 * NOz + O•. ) and PC (135 * POt + O•.). The NO/PC ratio is shown to increase with depth in the Canada Basin, being ~0.78 in Surface and Upper Halocllne Waters and ~1.0 in the Atlantic Layer and Deep Waters. Lower Halocline Water is marked by NO and PC mlnlma and intermediate NO/PC. NO/PC ratios from the Arctic sheri seas are examined to determine possible source regions for the various water masses. The NO/PC ratio of Canada Basin Deep Water implies an upper bound of ~11% shelf water contribution to this water mass. A slight oxygen maximum core in the Lower Halocline Water is identified at a salinity of 5=34.5 in the vicinity of the Alpha Ridge. This core appears to be diminished by diapycnal mlxing and does not extend into the Beaufort Gyre. BACKGROUND The distribution of chemicals in the ocean can provide insight into the locations, processes and rates of water mass formation. Direct observation of water mass formation is particularly difficult in the Arctic; hence Arctic oceanographers have focused extensively on deriving such information from chemical oceanographic measurements. In this paper we examine the available historical data from the Arctic Ocean using perhaps the most basic chemical tracers: nutrients and oxygen. Shelf Inputs to Halocline Water The Arctic Ocean's strong halocline separates a mixed surface layer from a deeper, relatively warm Atlantic-derived layer. Chemical oceanographers have sub-divided the halocline into Upper and Lower Halocline Water types (UHW and LHW [Jones and Anderson, 1986]). The UHW is identilled by a nutrient maximum which was originally explained as a signature of Pacific-derived waters coming in through the Bering Strait. These waters were recogniled to be altered in the Chukchi Sea [Coachman and Barnes, 1961; Kinnelt et al., 1970a]. Moore et al. [1983] attributed the high levels of all nutrients primarily to the Bering Sea source; however, they also suggested that "new" halocline waters formed over continental shelves might become enriched in nutrients through contact with sediments, and that the high nutrient content of halocline waters might therefore have a more widespread origin on other Arctic continental shelves. This idea was developed by Jones and Anderson [1986] who argued that nutrient regeneration within the confines of the Arctic Basin was primarily responsible for the maintenance of the nutrient maximum layer. Their reasoning implied that high nitrate and phosphate levels were less a function of proximity to the Bering Sea, but were more dependent on the physical nature of the continental shelf over which the UHW is formed. These authors attributed UHW to a Chukchi Sea origin on the basis of high silicate concentrations (attributed to a Bering Sea source) and hypothesiled longer shelf residence times in this region which permit a greater contribution of bottom-regenerated nutrients. Kinney et al. ...

The Arctic Ocean marine carbon cycle: evaluation of air-sea CO<sub>2</sub> exchanges, ocean acidification impacts and potential feedbacks

“…In the central basin, density stratification generally acts as a barrier to mixing between nutrient-poor surface waters and nutrient-rich deep waters (Aagaard et al, 1981;Jones and Anderson, 1986). Halocline waters generally have much higher pCO 2 and DIC content than surface waters (Jutterstrom and Anderson, 2004), with low rates of upward vertical diffusion between these waters and the surface mixed layer (Wallace et al, 1987;Wilson and Wallace, 1990;Bates, 2006). The mixed layer typically extends to 10-50 m, with depth heavily dependent on seasonality of winds and sea-ice cover .…”

“…The shelf surface waters and underlying upper halocline waters have a predominantly Pacific Ocean origin based on characteristic temperature, salinity (Aagaard et al, 1981), inorganic nutrient, dissolved oxygen, barium and alkalinity distributions (e.g. Wallace et al, 1987;Wilson and Wallace, 1990;Jones et al, 1991;Salmon and McRoy, 1994;Falkner et al, 1994; 1999; Rudels et al, 1996;Guay and Falkner, 1997;Jones et al, 2003). In surface waters of the Canada Basin, the river fraction can be up to 20% Cooper et al, 2005), while localized to the upper few metres, the sea-ice melt fraction can also exceed 25% (Cooper et al, 2005) during the summertime retreat of sea-ice.…”

Deep water circulation and composition in the Arctic Ocean by dissolved barium, aluminium and silicate

“…They have shown that freshwater content in all Arctic bottom waters is about 5 ‰ but riverine water is 2 ‰ in the DMBW while it is only 1 ‰ in the DEBW. Wilson and Wallace (1990) calculated the Canadian shelf water contribution to deep water of the Canada Basin by using NO/PO ratios finding 7 to 11 % and Östlund et al (1987) found 10 to 15 %. Because these calculations strongly depend on uptake and mineralization of nitrate and phosphate following Redfield ratios they have to be taken with care.…”

Modification of NO, PO, and NO/PO during flow across the Bering and Chukchi shelves: Implications for use as Arctic water mass tracers

“…Although this is the appropriate direction to explain the apparent decrease in NO/PO ratios observed from North Pacific values (0.87-0.91) to lower values observed in Ameriasian Arctic surface waters (~0.7), the magnitude of the observed change is too large to be explained solely on the basis of a decrease in those waters that are supersaturated with oxygen to levels of near saturation. For example, Wilson and Wallace [1990] point out that for typical Arctic shelf nutrient concentrations, a 100 gM decrease in dissolved oxygen, such as might happen as a result of gas exchange at the sea surface, results in a change of only 0.03 in the corresponding NO/PO ratio. This leads us to consider two biologically mediated factors that would affect the rates of use and the rates of recycling of nitrate and phosphate in the highly productive waters of the Bering and Chukchi shelf.…”