separation and allows for the simultaneous determination of 

 both the total -'"Pb and the -'^Pb-supported level. Excess -'"Pb 

 was calculated by subtracting the -'^Pb-supported level from 

 the total -'"Pb activity. 



Results and Discussion 



Stable Oxygen Isotopes of Seawater and Tunicate Cellulose 

 During August 1988, the stable oxygen i.sotope (6 "O) 

 signature values were determined for the major water masses 

 in the northern Bering Sea (Fig. 2). In the northwestern section 

 of the study area, the Anadyr Current (a bifurcation of the 

 Bering Slope Current further south) travels clockwise in the 

 Gulf of Anadyr. Most of this high nutrient, saline water exits 

 northward through Anadyr Strait (Fig. 1), but some of this 

 Anadyr water travels southeast of St. Lawrence Island 

 (Coachman etai, 1975; Walsh etai, 1989) . The 5 ''O value 

 for deep Bering Sea water was -0.8'Voo, while Bering Slope 

 Current water and Anadyr water were -1.2 to -1.3"/(x, and 

 -1.4 to -LS^/ijo, respectively, showing the early stages of 

 freshwater dilution as deep Bering Sea water is advected up 

 onto the shelf (Fig. 2). The cold pool in the Gulf of Anadyr, 

 southwest of St. Lawrence Island, had a 5 '"O value of -2.0'Voo 

 and the Alaska Coastal water to the east had the most depleted 

 8 "*0 values (-3.0 to -5.0"/,„). This west to east depletion of "O 

 in bottom seawater parallels the west to east decreasing gradient 

 in salinity occurring across the shelf in spring/summer due to 

 freshwater dilution of Bering Sea water (Coachman et ai, 

 1975: Schumacher et ai, 1983). 



deep Bering 

 !•) Sea waler 

 (3215 m depth) 



Bottom water 



"SMOW 



Siberian Coastal Waler 



Gult ol Anadyr 

 waler 



tronlal station 

 BSW-ACW 



Alaska Coastal Water 



Wollstiore Yukon River ll4mdeptri) 



Bottom water salinity {7oo) 



Fig. 2. Relation between bottom water '"O values and salinity for stations 

 occupied in the northern Bering and Chukchi Seas in 1987 and 1988. 

 Water masses are defined by both salinity and location (Grebmeier 

 elal. 1990). 



The Bering Shelf water (BSW) is formed south of 

 St. Lawrence Island in the summer as a mixture of water from 

 the Bering Sea moving northward, mixing with the less saline 

 cold pool of resident water formed in the polynya south of 

 St. Lawrence Island over winter, but it is the least understood 

 water mass in the northern Bering Sea (Coachman etal..\915). 

 5 "*0 values for BSW north of St. Lawrence Island in August 

 range from -1.6 to -2.1"/fj(i (Fig- 2). The resident cold pool 

 measured in the central Gulf of Anadyr southwest of 

 St. Lawrence Island in August 1988 was composed of less 



saline water (32.4'V,j„) with a 5 '"Oof -2.0"/oo. Coachman ef a/. 

 (1975) propose that this cold pool results from less saline water 

 off the Alaska coast being advected into the area between 

 St. Matthew and St. Lawrence Islands. It is subsequently 

 cooled and salinated in the winter and then is isolated from 

 surrounding shelf waters in the summer, although some mixing 

 occurs along its boundary with northward flowing water from 

 the southeast Bering Sea Shelf to form the modified Bering 

 Shelf water advected north of St. Lawrence Island in the 

 summer. 



A significant correlation was found between the stable 

 oxygen isotope composition of bottom seawater and salinity in 

 the study area, enabling determination of 8 "*0 values for the 

 major water masses in the region dependent on freshwater 

 dilution of the most saline Bering Sea basin core water as it 

 moves onto the shelf (Fig. 2). The water most depleted in '"O 

 was found offshore of the Yukon River (Figs. 2,3), with 

 intermediate concentrations found in Bering Shelf water. The 



Fig. 3. Distribution of bottom waters 6 '"O values for stations occupied 

 during the 1988 RA' Akmlemik Korolev cruise 47 ( • ) and the 1987 

 R/V Thomas Thompson cruise 214 (o). Isotope ration units: 

 (Grebmeier fro/., 1990). 



waters most enriched in '"O were found to the west in high 

 salinity Anadyr water. In addition, the 8 '"O-salinily dilution 

 line shows the influence of brine rejection during ice formation 

 enhancing salinity of the southeast-flowing Siberian Coastal 

 water in the Chukchi Sea, even though the 8 "O remained 

 constant in relation to the northwest-flowing Bering Shelf- 

 Anadyr water, indicating the Siberian Coastal water sampled 

 probably originated from south of Bering Strait. 



246 



