The distribution of aluminum in Beaufort Sea and the development of a sequential injection method for the determination of aluminum in natural waters




Giesbrecht, Timothy

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Here we report vertical profiles of dissolved (0.2 um filtered) Aluminum (Al) for eight stations in the Beaufort Sea in the Canadian Arctic, six of which are along a transect extending from the coastal shelf northeast of the Mackenzie River delta out to the Beaufort Sea. Sampling was performed aboard the CCGS Sir Wilfrid Laurier in September 2007 and all analyses were performed in a Class 100 clean space at the University of Victoria. Vertical profiles of dissolved Al in the water column displayed surface maxima, subsurface minima and a general increase in concentration with depth as is characteristic of a “scavenged” trace element in seawater. Concentrations of dissolved Al for the upper 1000 m were generally low ranging from the < 1 nmol kg-1 observed in the sub-surface minimum corresponding to the Pacific inflow layer and increasing to ~6-10 nmol kg-1 with depth. The surface maxima at stations for Al was associated with relatively fresh surface water (26-30 PSS) that is believed to be the result of seasonal sea-ice melt. This correlation suggests that the melting of sea-ice with entrained sediments may be an important mechanism for the delivery of Al and associated trace metals to the water column of the Beaufort Sea. We also report measurements of “total Al” (unfiltered and acidified to pH 1.7 for two year prior to analysis) for the Arctic Ocean which indicate that a significant proportion of Al in the water column is present in the > 0.2 um fraction. These measurements and the hydrographic data along the transect indicate the transport of a cold, saline, metal enriched water mass off of the continental shelf into the Canada Basin. This water mass appears to reflect the return of cold, high salinity slope water originally emplaced on the shelf by upwelling favourable winds. Alternatively, this water mass may be the product of brine exclusion from sea-ice formation during the previous winter that was unable to vacate the shelf due to the persistent upwelling observed throughout 2007. This finding suggests that the convection of cold, dense shelf water may be a mechanism for supplying the deep waters of the Arctic with an injection of water containing a significant Al content. In addition, a low volume sequential injection analysis (SIA) method is proposed for determination of elevated concentrations of Al, like those typically observed in coastal and river waters. A thorough optimization of the chemistry and instrumental parameters was performed along with an extensive investigation into potential interferents. The method was found to be largely free of interferents at environmentally relevant concentrations and was determined to have a detection limit of 24 nM. The precision of the method was reported to be 2% at 75 nmol kg-1 and analysis of the SLRS-4 certified reference material validated the accuracy of the method. Analysis of several samples that were previously analyzed via flow injection analysis (FIA) and standardized with consensus values of an open ocean reference material indicated the method returned comparable values for the Al concentration in the samples. Development and optimization of the SIA has resulted in an accurate and precise low-cost method of analysis that is both sensitive and relatively free from interference for the detection of nano-molar levels of Al in coastal and natural waters.



Aluminum, Beaufort Sea, Sequential Injection Analysis, Geochemical Tracer, Oceanography, Trace metals