BARIUM AS A TRACER OF ARCTIC HALOCLINE AND RIVER WATERS

Dissolved barium (Ba) concentrations are reported for water samples co llected during six oceanographic cruises to the Arctic in 1993 and dur ing the 1994 Arctic Ocean Section. Upper Arctic (< 200 m) values range d widely (19-168 nmol Ba l(-1)) in a manner geographically consistent with identified sources and sinks. Although surface waters in the east ern part of the Bering Strait and the southern Chukchi Sea were highly enriched in Ba (reflecting the influence of the Yukon River), surface waters over most of the Chukchi Sea were generally depleted in Ba due to the intense seasonal biological activity which occurs in the area. The highest Ba concentrations (greater than or equal to 75 nmol Ba l( -1)) observed in the surface mixed layer of the Arctic interior (i.e. beyond the 200 m isobath) occurred in the Canada Basin. As a result of the biological removal of Ba from surface waters of the Chukchi Sea a nd the tendency of the Bering inflow to enter the Arctic interior at d epths below the relatively fresher mixed layer, we hypothesize that th e Mackenzie River is the dominant source of the high Ba observed in th e surface waters of the Canada Basin. Features characterized by local salinity minima and Ba maxima in surface waters down to depths of 30-5 0 m were observed over the Arlis Plateau and Mendeleyev Ridge in both 1993 and 1994, and over the Makarov Basin and Lomonosov Ridge in 1994. The physical and chemical properties of these waters suggest that the y have been significantly influenced by fluvial discharge. It could no t be determined from available data whether these features arose from offshore transport of Eurasian river discharge from the Russian Arctic seas, meanders of the Mackenzie River-influenced Beaufort Gyre, or a branch of Bering inflow that crossed the Chukchi shelf and entrained r iver water from North American and/or Eurasian sources. In both 1993 a nd 1994 Ba concentrations in the upper halocline layer (as defined by a core salinity of 33.1) ranged between 42 and 77 nmol Ba l(-1), with highest values observed in the Canada Basin. The Ba maximum and the re lated nutrient maxima and oxygen minimum that characterize the upper h alocline in the Canada Basin suggest sources in the Chukchi and Beaufo rt seas, and that the mechanism by which these waters are formed is li nked with the influx of water through the Bering Strait. The Ba distri butions for 1993 and 1994 delineate the lateral extent of upper halocl ine waters with Canada Basin character and show them to be chemically distinct from upper halocline waters in the Eurasian Basin. The front separating these water types ran roughly along the Mendeleyev and Alph a ridges, consistent with reports that the boundary between Atlantic a nd Pacific water mass assemblies in the Arctic has recently been displ aced from its historical position over the Lomonosov Ridge. Lateral gr adients in both Ba and the NO parameter were observed within the lower halocline layer (defined by an NO minimum and core salinity of 34.2-3 4.4) along the 1994 Arctic Ocean Section, indicating that lower halocl ine waters receive additional inputs as they transit to the Canadian B asin from their region of formation in the Eurasian sector of the Arct ic. (C) 1998 Elsevier Science Ltd.