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Renewal and circulation of intermediate waters in the Canadian Basin observed on the SCICEX 96 cruise

 

W.M. Smethie, Jr1, P. Schlosser1,2, T.S. Hopkins3, and G. Boenisch1

1 Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964

2 Department of Earth and Environmental Sciences, Columbia University, New York, NY

3 Department of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, NC

June 9, 1999

ABSTRACT

During the summer of 1996 the nuclear submarine, USS Pogy, occupied a line of stations extending through the middle of the Canadian Basin between about 88°N, 44°W (Lomonosov Ridge) and about 78°N, 144°W (center of the Canada Basin). CTD/Niskin bottle casts extending to 1600 m were carried out at 8 stations providing the first high quality temperature, salinity, CFC, tritium, and 3He data obtained from this region, although XCTD data had previously been collected in this region. These data, along with data from stations at the basin boundary to the south and west, reveal the presence of well ventilated intermediate water beneath the halocline in the center of the Canada Basin, indicating renewal times of the order of 1-2 decades. The least ventilated intermediate water was observed at the northern end of the Canada Basin along the southern flank of the Alpha Ridge. Intermediate water is derived from the Atlantic Ocean and enters the Arctic Ocean through Fram Strait and the Barents Sea. It flows around the Arctic basins in boundary currents and splits in the eastern Amundsen Basin with one branch crossing the Lomonosov Ridge and flowing along the East Siberian continental slope and the other flowing along the Eurasian flank of the Lomonosov Ridge. From the SCICEX 96 observations we conclude that the branch that flows along the East Siberian continental slope transports this water to the Chukchi Rise where it apparently enters the central Canada Basin with some flow continuing along the boundary to the southern Canada Basin. The Fram Strait Branch Water mixes extensively with waters from the Canadian Basin during its transit along the East Siberian continental slope, being diluted by a factor of about 5 by the time it reaches the central Canada Basin. The Barents Sea Branch Water does not undergo such extensive mixing and is diluted by only a factor of about 2 when it reaches the central Canada Basin.

INTRODUCTION

The Arctic Ocean is thought to be very sensitive to global climate change and, at the same time to potentially have a significant influence on global climate (e.g., Hansen et al., 1988; Stouffer et al., 1989). It is also perhaps the least understood ocean basin because of the difficulty in making observations due to the perennial ice cover. Understanding the circulation of the Arctic Ocean and exchange of water between the Arctic and the ocean basins to the south is critical, not only for understanding the earth’s climate, but also for developing mitigation strategies for pollution events that have occurred there during the past several decades (e.g., Yablokov, 1993; OTA 1995).

Until the late 1980s and 1990s, when a series of expeditions successfully sampled the water column of the Eurasian sector of the Arctic Ocean from the shelf seas to the interior at high spatial resolution (e.g., EOS, 1988; Anderson et al., 1989, 1994), the circulation of the deep and intermediate waters in the Arctic Ocean was not well known. The new data revealed some of the main features of the circulation, particularly within the Eurasian Basin, as well as the exchange between the individual basins of the Arctic Ocean and the GIN seas (e.g., Rudels et al., 1994; Jones et al., 1995; Boenisch and Schlosser, 1995; Schlosser et al., 1994a,b). Presently, a complete survey of the Canadian Basin is still lacking and it is not possible to describe the circulation patterns and mean residence times in this basin to the extent possible for the Eurasian Basin. Work is underway to close the data gaps and here we report on one phase of this work, i.e., hydrographic and tracer observations from the heavily ice covered central Canadian Basin. These observations were obtained from a nuclear powered US Navy submarine, the USS Pogy, in the framework of the SCICEX 96 expedition (SCICEX: Scientific Ice Expedition). Submarines can easily access any region of the Arctic Ocean by transiting beneath the ice and can surface through the ice to conduct hydrographic stations. Thus, we have obtained a section of stations covering the water column between the surface and 1600 m depth in the interior Canadian Basin. These data are used to describe new elements of the circulation of the intermediate waters in the Canadian Basin.

BACKGROUND

The Arctic Ocean is vertically well stratified into several distinct water masses. The shallowest is the surface mixed layer which typically is 10-50 m thick. It is a mixture of waters from the Atlantic and Pacific oceans with the addition of river runoff and sea ice meltwater which results in a relatively low salinity (less than 30 to about 34 psu in the interior basins). This mixed layer is underlain by the halocline, which is a region of strong vertical density stratification caused by the strong increase in salinity with depth and it extends to about 200 m depth. The halocline waters are believed to form on the extensive shelf regions surrounding the Arctic Ocean from Pacific and Atlantic source waters that are modified on the shelves by mixing with river water and by the sea ice formation/melt cycle (e.g., Aagaard et al., 1981; Steele et al., 1995). On the basis of geochemical data, Jones and Anderson (1986) distinguish between the upper halocline water (salinity: about 33.2) derived from Pacific water that enters the Arctic Ocean through the Bering Strait and is modified on the Chukchi, East Siberian and Laptev sea shelves, and lower halocline water (salinity: about 34.2) derived from Atlantic water that is modified by mixing with river runoff and sea ice melt during winter convection north of the Barents Sea (Rudels et al., 1996). The shelf processes produce waters with a wide range of densities and inflow of shelf water is an important process contributing to the formation of all the water masses in the Arctic Ocean extending from the surface mixed layer to the bottom. Beneath the halocline we observe the intermediate waters which extend down to roughly the depth of the Lomonosov Ridge (about 1500 to 1700 m). These waters are too dense to have been derived directly from the shallow Pacific water that enters the Arctic through Bering Strait. Therefore, Atlantic water is the major source, although it is modified by water flowing into the Arctic Basin from the shelf regions. Below 1500 m we find the deep and bottom waters, which are different for the Eurasian sector (potential temperature: about -0.95°C, salinity: about 34.93) and Canadian sector (potential temperature: about -0.55°C; salinity: about 34.95) and isolated from each other by the Lomonosov Ridge.

Atlantic water enters the Arctic Ocean directly through Fram Strait. It is warmer, saltier and denser than the upper waters in the Arctic Ocean and sinks to a depth beneath the halocline, creating a temperature maximum that is prevalent in varying degrees throughout the Arctic Ocean (Coachman and Barnes, 1963; Timofeyev, 1961). This was thought to be the main source of intermediate water although there was strong evidence for flow of Atlantic water into the Barents Sea as well (i.e., Blindheim, 1989; Rudels, 1987; Loeng et al., 1993; Pfirman et al., 1994). Detailed hydrographic surveys extending to the interior of the Eurasian Basin were obtained in the early 1990s. From hydrographic and tracer data collected during the Oden 91 expedition Rudels et al. (1994) inferred that intermediate water was formed from two branches of Atlantic water: one branch entering directly through Fram Strait, and another branch that enters through the Barents Sea where it is extensively modified. These authors also reported rough estimates of the mean renewal times of the intermediate waters, about one decade in the Eurasian sector and two decades in the Canadian sector, and postulated a circulation scheme. The circulation scheme consisted of boundary currents in three large cyclonic cells, one flowing around the Eurasian Basin along the Barents, Kara and Laptev sea slopes and the Lomonosov Ridge, the second around the Makarov Basin along the East Siberian Sea slope, the Alpha/Mendeleyev Ridge and the Lomonosov Ridge, and the third around the Canada Basin along the Chukchi Sea slope, the north slopes of Alaska and Canada, and the Alpha/Mendeleyev Ridge. The Makarov and Canada basin cells were fed by water flowing across the eastern end of the Lomonosov Ridge.

Much of the Rudels et al. (1994) circulation scheme for intermediate water has now been confirmed. Detailed hydrographic/tracer surveys along short sections across the Barents and Laptev shelf/slope regions taken in 1993, 1995, and 1996 confirm the existence of two branches of Atlantic water feeding the intermediate waters: Fram Strait Branch Water (FSBW) and Barents Sea Branch Water (BSBW) (Schauer et al., 1997; Frank et al., 1998). BSBW is formed by Atlantic water being freshened by mixing with river water and sea ice melt and cooled by winter time air sea interaction during its transit through the Barents Sea. It enters the Arctic Ocean between the Barents Sea and the Laptev Sea, probably through the Santa Anna Trough, displacing the FSBW towards the center of the Nansen Basin and also mixing with it along isopycnal surfaces. Its core along the Laptev Sea slope at the eastern end of the Eurasian Basin has a distinctive signal of relatively low salinity and high CFC and tritium concentrations and a density range of s Q = 27.94-28.02 (Frank et al., 1998). The core of FSBW is distinguished by a temperature maximum at a density of s Q= 27.90. Evidence for cyclonic flow of intermediate water along the Lomonosov Ridge is found in the Oden 91 data (Rudels et al., 1994) and in data collected in 1993 from the US Navy submarine USS Pargo (Morison et al., 1998). The flow of intermediate water across the Lomonosov Ridge and along the East Siberian Sea slope is strongly supported by hydrographic and tracer distributions along the 1994 Arctic Ocean Section expedition (AOS 94) (Swift et al., 1997; Ekwurzel, 1998; Schlosser et al., 1997).

In addition to information about circulation patterns, data from the SCICEX 93 and AOS 94 cruises, and a cruise by the CCGS Henry Larsen to the Canadian Basin in 1993, reveal that the Atlantic water has warmed and penetrated further into the Canadian Basin than in the recent past (Morison et al., 1998; Carmack et al., 1995; Carmack et al., 1997). The front between Atlantic and Pacific halocline water has shifted from the Lomonosov Ridge to the Alpha/Mendeleyev Ridge (McLaughlin et al., 1996). This change appears to have occurred during the early 1990s (Carmack et al., 1997).

The cyclonic intermediate water circulation in the Makarov and Canada basins proposed by Rudels et al. (1994) has not yet been confirmed. If these gyres exist, it would mean that the oldest and least ventilated intermediate water would be found in the center of the Canada and Makarov basins. However, as will be shown below using the SCICEX 96 data, this is not the case. The oldest water is found over the southern flank of the Alpha Ridge in the northernmost region of the Canada Basin and relatively young water is found in the center of the Canada Basin.

METHODS

Sample collection

 

Figure 1. Location of SCICEX 96, ARK IX/4 (Schauer et al., 1997) and AOS 94 (Swift et al., 1997) stations used in this study. The color and shape of the station symbols are used in Figures 3, 4 and 5 to represent data points from these stations.


The SCICEX 96 samples were collected along a line of stations extending through the middle of the Canadian Basin between about 88°N, 44°W (Lomonosov Ridge) and about 78°N, 144°W (center of the Canada Basin), as well as at one station over the Alaskan north slope and one adjacent to the Chukchi Rise (Figure 1). Hydrographic stations were taken by surfacing the USS Pogy, either in open water or through the ice, and performing CTD/Niskin bottle casts extending to 1600 m depth. For this purpose, a light weight winch equipped with kevlar cable was used. CTD data were collected with an internally recording Sea-Bird 19 probe attached to the end of the cable and water samples were collected in 10-l Niskin bottles attached to the cable above the Sea-Bird 19 and tripped mechanically. Generally, two 8-bottle casts were taken. These casts were preceded by a 300 m CTD cast to determine the Q /S structure of the surface and halocline layers and these data were used to plan the Niskin bottle sample depths for the upper layers. The casts were performed from the deck of the submarine if it surfaced in a lead or in thin ice. For thicker ice, the casts were performed from a small ice camp set up adjacent to the submarine. Samples for measurement of tritium, 3He, and CFCs, as well as for salinity, oxygen and nutrients were collected from the Niskin bottles. The station line was originally planned to extend to the Alaskan slope at the southern end of the Canada Basin, but the Sea-Bird 19 and some of the Niskin bottles were lost at station 39 when the ice field shifted during the station. Unstable ice conditions prevented casts from being taken in the southern part of the Canada Basin except for station 44. Stations 44 and 68 did not have a CTD and hence do not have temperature data.

CTD data processing

 

The data processing and calibration for the SCICEX 96 Sea-Bird 19 surface casts is summarized and described in Hopkins, et al. (1998) and only briefly described here. The initial processing was done using the Sea-Bird Software routines, which involved editing, filtering and averaging into 1 dbar pressure bins. The pre- and post-calibrations for pressure and temperature sensors obtained from the manufacturer were accepted, as quality in-situ controls were not possible. The conductivity sensor, which is generally less reliable, was calibrated with the bottle salinity values analyzed on board with a Guildline Portasal salinometer which was calibrated with standard sea water. This revealed a small change in the readings of the conductivity sensor after station 35. CTD salinities before station 34 deviated by a mean of 0.001 psu from the bottle values and no correction was applied. For those after station 34, a linear correction was applied, which brought the standard deviation of the difference between the sensor and bottle values to + 0.0012 psu. After salinity was corrected, additional parameters including density, potential temperature, Brunt-Våisålå frequency, integrated density, steric height and depth were derived.

Tracers

 

About 150 Tritium/3He samples were collected in copper tubes (volume of about 40 cm3) sealed on each side by stainless-steel pinch-off clamps. The samples were analyzed at the Lamont-Doherty Earth Observatory (L-DEO) noble gas laboratory (NGL) following procedures described by Ludin et al. (1998). In short, the gases were extracted from the water samples in a vacuum extraction system and transferred into a liquid nitrogen cooled glass ampoule containing about one gram of activated charcoal to adsorb the permanent gases. After completion of the gas transfer, the glass ampoule was flame-sealed for storage before the mass spectrometric measurement. The degassed water was flame-sealed in a pretreated glass bulb for about 6 months to allow ingrowth of 3He from tritium decay. Helium isotopes and tritium were measured on two fully automated, dedicated mass spectrometric systems (one for He isotopes and one for tritium). Before He isotope measurement, the gases trapped in the glass ampoule were dried and He and Ne were purified by liquefying all other gases at 24°K. He was then separated from Ne on a charcoal-filled cold trap at 12°K. The He isotope data are reported as 4He concentrations (cm3 STP g-1) and d 3He values where d 3He is the percent deviation of the 3He/4He ratio of the sample from that of atmospheric air (d 3He = (Rs – Ra)/Ra * 100 %, where Rs is the 3He/4He ratio of the sample and Ra is that of air; 1.384*10-6; Clarke et al., 1976). Tritium was measured in a similar fashion (for details, see Ludin et al., 1998 or Bayer et al., 1989). The precision of the helium isotope data was typically ± 0.2 to 0.3 %, that of the 4He concentration ± 0.5 %. Tritium was measured at a precision of about ± 2 % or 0.03 TU, whichever is larger (one TU means a tritium to hydrogen ratio of 10-18).

On the first SCICEX cruise, USS Pargo in 1993, CFC samples were collected from Niskin bottle casts performed as described above. Water was drawn into 200 cm3 glass syringes from the Niskin bottles which were then taken inside the submarine where the water was transferred into 60 cm3 glass ampoules and flame sealed under continuously flowing nitrogen creating a nitrogen headspace (Busenberg and Plummer, 1992). All of the samples were severely contaminated with CFC-12 and we learned after the cruise that CFC-12 concentrations inside the submarine had reached levels a million times greater than those in outside air. Thus, special precautions were taken to avoid contamination during the SCICEX 96 cruise. The CFC samples were collected by filling 60 cm3 glass ampoules directly from the Niskin bottles using a stainless steel filling tee (Bulsiewicz et al., 1998) and then flame sealing the ampoules under continuously flowing nitrogen. This was done outside the submarine, either on the deck or in a tent on the ice. The Niskin bottles and the equipment required for flame sealing the ampoules were stored in gas-tight aluminum boxes that were opened only outside the submarine to avoid contact with the submarine’s atmosphere. The empty ampoules were stored in gas-tight paint cans prior to use. The nitrogen was passed through a molecular sieve 13X cleanup column located between the nitrogen tank regulator and the ampoule flame sealing system. This proved to be a weak link in the procedure. The cleanup trap had to be baked out periodically and this could be done only inside the submarine. Although this was done with nitrogen flowing through the trap and the trap was capped on both ends immediately after the bakeout was completed, it still became contaminated with CFC-12.

CFC samples were not collected at stations 28 and 30 and all of the CFC samples collected after station 34 were contaminated with CFC-12, but not CFC-11 or CFC-113 (the CFC-12 results are not reported). About 100 samples were obtained and returned to L-DEO, where they were stored in the dark at a temperature of about 4°C for several months prior to analysis. The ampoules were opened under an atmosphere of ultra-pure nitrogen using Busenberg and Plummer’s (1992) technique and introduced into a purge and trap system interfaced to a Shimadzu 8A gas chromatograph with an electron capture detector (Smethie et al., 1988). The gas chromatography was carried out using a 40 inch x 1/8 inch diameter precolumn of porasil B and a 60 inch x 1/8 inch diameter main column of carbograph-1AC which provides excellent separation of CFCs 11, 12 and 113 as well as separation of CFC-113 from methyl iodide. The gas chromatograph was calibrated against a known gas standard and concentrations are reported on the SIO93 scale. The precision of this technique for previous studies was the larger of ± 1% or 0.01 pmol kg -1 (Mensch et al., 1998).

RESULTS

Vertical sections of the potential temperature, salinity and tracer data are shown in Figure 2. The Atlantic water temperature maximum is a prominent feature extending across the entire section between 200 and 600 m depth. The highest temperature is close to 1.5°C at about 250 m depth over the Eurasian flank of the Lomonosov Ridge and decreases to less than 0.4°C at about 500 m depth in the northern Canada Basin. Salinity is lowest at the surface due to the presence of river runoff and sea ice meltwater. Lowest values in the surface water and halocline occur in the center of the Canada Basin and increase toward the Lomonosov Ridge. In the underlying intermediate water there is a prominent hump in the salinity distribution with salinity being highest at station 35 at the northern end of the Canada Basin and decreasing to the north and south. The CFC and tritium distributions are similar to the salinity distributions, but inversely correlated. Highest concentrations are observed in the surface water due to input from the atmosphere for CFCs and from river water for tritium, and concentrations decrease with depth through the halocline, monotonically for the CFCs. There is some structure in the lateral distribution of tritium in the surface water and halocline with lowest concentrations occurring at stations 33 and 34 over the Alpha Ridge, which may indicate less river water at this location. In the intermediate water, the isopleth doming is much more dramatic than for salinity with lowest CFC and tritium concentrations occurring at stations 34 and 35 and concentrations increasing to the north and south. d 3He is lowest in the surface water because surface waters are generally renewed more rapidly than deep waters and because of gas exchange with the atmosphere. There is a d 3He maximum that extends across the sections in the lower part of the halocline. The major source of 3He in this section is the decay of tritium and the dome structure of the tritium distribution in intermediate water is mirrored in the d 3He distribution, but with a double dome below 800 m.


Figure 2. Vertical sections of CTD potential temperature (°C), CTD salinity , CFC-11 (pmol/kg), CFC-113 (pmol/kg), tritium (TU), d 3He (%), s Q (kg/m3), and tritium/3He age (years) along the SCICEX 96 cruise track through the middle of the Canadian Basin.


Q /S plots for the SCICEX 96 stations and stations from the ARK IX/4 cruise of the Polarstern conducted in the summer of 1993 (Schauer et al., 1997) and the AOS expedition conducted in the summer of 1994 (Aagaard et al., 1996) are plotted on three scales in Figures 3, 4 and 5. The ARK IV/4 and AOS 94 data will be compared to the SCICEX 96 data below. On the coarse scale (Figure 3), the most prominent features are the cold halocline with salinities ranging from 29 to about 34.2 psu, and the Atlantic water potential temperature maximum (0.4 to 1.7oC; salinity: 34.83 to 34.89). The temperature maximum is much stronger at station 28 (1.7oC) than at the other SCICEX stations which is not unexpected since it is located on the Atlantic side of the Lomonosov Ridge. The intermediate scale plot (Figure 4) covers the intermediate waters from the top of the temperature maximum layer to 1600 m depth and the fine scale plot (Figure 5) covers the lower intermediate water below the Atlantic water temperature maximum. There is a clear trend in both the intermediate and fine scale plots. Salinity increases from station 28 at the Lomonosov Ridge to station 35 at the northern end of the Canada Basin and then decreases from station 35 to station 38 located in the center of the Canada Basin.

Figure 3. Overall CTD Q /S plot for stations used in this study. Data points are 1 db averages for SCICEX 96 and ARK IX/4 stations and 10 db averages for AOS 94 stations. The boxes represent the regions of the intermediate and fine scale plots shown in Figures 4 and 5. See Figure 1 for station locations. The logic behind the color scheme is as follows. For the SCICEX 96 stations, red is used to represent the stations with the highest intermediate water salinities which are taken to represent the ambient Canadian Basin water. There is a gradation in color from red to blue to green to magenta representing a decrease in salinity of intermediate water in the Canadian Basin due to mixing with intermediate water from the Eurasian Basin. Eurasian Basin intermediate water (ARK IX/4 station 32) is represented by purple. AOS stations 7, 9, and 21 are represented by black.


Figure 4. Intermediate scale Q /S plot for (a) SCICEX 96 stations 28, 30, 33, 34, 35 and ARK IX/4 station 32 and (b) SCICEX 96 stations 35, 36, 37, 38; ARK IX/4 station 32 and AOS 94 stations 7, 9 and 21. The box represents the region for the fine scale plot shown in Figure 5. See Figure 1 for station locations and Figures 1 and 5 for symbol key.


Vertical profiles of bottle salinity (measured on water samples collected from the Niskin bottles), CFCs and tritium are presented in Figure 6 to further contrast the differences between the northern end and the center of the Canada Basin and to compare the central Canada Basin to its western and southern margins. Below 400 m depth the highest salinity is observed at the northern end of the Canada Basin (sta. 35). The salinities adjacent to the Chukchi Rise (sta. 68) and at the southern end of the Canada Basin (sta. 44) are slightly lower than those observed at station 38 in the center of the basin. There is a sharp contrast in CFC and tritium concentrations between the northern end of the basin and the center with higher transient tracer concentrations being present in the center as observed in the vertical sections. Stations 44 and 68 have similar or slightly higher transient tracer concentrations if compared to station 38. At station 28 on the Atlantic side of the Lomonosov Ridge, salinity and tritium concentrations are similar to those at stations 44 and 68 below 600 m depth.

Figure 5. Fine scale Q /S plot for (a) SCICEX 96 stations 28, 30, 33, 34, 35 and ARK IX/4 station 32 and (b) SCICEX 96 stations 35, 36, 37, 38; ARK IX/4 station 32 and AOS 94 stations 9 and 21. See Figure 1 for station locations.


DISCUSSION

Circulation pathways

 

The sea ice circulation in the Canadian Basin is wind driven and dominated by the anticyclonic Beaufort Gyre, which sometimes occupies nearly the entire basin. There is now evidence that a cyclonic wind driven circulation regime also exists and alternates with the anticyclonic regime every 5-7 years (Proshutinsky and Johnson, 1997). The depth to which this circulation extends beneath the ice is not known, but Jones et al. (1998) present evidence that the upper 30 m mixed layer does not follow the ice drift. There is very little information on the circulation beneath the surface mixed layer in the Canadian Basin. The early work of Coachman and Barnes (1963) suggested a general cyclonic circulation pattern for the Atlantic temperature maximum layer and Rudels et al. (1994) have postulated that the intermediate water in the Canadian Basin circulates in two cyclonic gyres as discussed previously. The density structure below 500 m along the SCICEX 96 section reflects the hump in the salinity structure (Figure 2) with higher density water at the northern end of the Canada Basin just south of the Alpha Ridge. This suggests that intermediate water flows towards the west across the northern end of the Canada Basin and towards the east across the Makarov Basin. Although one could connect these flows to form an anticyclonic gyre, the spatial resolution of our data across the complex topography of the Alpha and Lomonosov ridges is much too sparse to resolve flows in this region and there could be multiple reversals in circulation along the section. As will be discussed below, the tracer and hydrographic data indicate that water of Atlantic origin flows along the Lomonosov Ridge and along the Siberian margin of the Canadian Basin, entering the central Canada Basin in the vicinity of the Chukchi Plateau.

The relatively high salinity intermediate water at the northern end of the Canada Basin (SCICEX stations 34 and 35, Figure 2) is associated with low transient tracer concentrations. In the central Canada Basin (SCICEX station 38) and the Makarov Basin (SCICEX station 30) the intermediate water is relatively low in salinity and high in tracers suggesting an Atlantic origin as will be discussed below. This Atlantic derived intermediate water must be a significant source of intermediate water for the Canadian Basin, but it would be expected to be modified by mixing with dense water that flows off the shelves (Aagaard et al., 1985; Rudels et al., 1994). We do not know the endmember characteristics of intermediate water in the Canadian Basin, but the water at station 35 is the most different from intermediate water in the Eurasian Basin. Therefore, we will use this as an endmember in the analysis that follows and designate it ambient Canadian Basin water.

Q /S plots for Ark IX/4 station 32 at the Laptev Sea continental slope and AOS 94 stations 7, 9 and 21 are shown in Figures 3-5 for comparison with the SCICEX data. Swift et al’s. (1997) analysis of the AOS 94 section, which extends along the East Siberian margin of the Canada Basin and crosses the Makarov Basin, demonstrates that water of Atlantic origin, flows over the Lomonosov Ridge and along the East Siberian margin. AOS 94 stations 9 and 21 are located in the flow of this water and station 7 is located inshore of this flow. Ark IX/4 station 32 is located just upstream of where flow of Atlantic water splits with one branch flowing along the Eurasian flank of the Lomonosov Ridge and the other crossing the ridge and flowing along the East Siberian Sea continental slope. The water observed at station 32 contains both FSBW and BSBW components which have been altered from their original endmember characteristics by mixing with each other (Schauer et al., 1997). Its temperature maximum marks the core of the FSBW and the broad salinity minimum between about 0.2 and -0.2°C (about 700 to 1100 m depth) marks the core of the BSBW.

Along the East Siberian Sea margin the strength of the FSBW component weakens along the flow path. This is evident from the decrease in the maximum temperatures from station 32 to station 21 to station 9 (Figure 4b) indicating extensive mixing with waters of Pacific or Canadian Basin origin. This mixing could be with ambient Canadian Basin water found at the northern end of the Canada Basin (SCICEX 96 stations 35 and 36) but water with a similar density is also observed at AOS 94 station 7 at the shelf break between about 200 and 250 m (Figure 4b). This location suggests a shelf source in the East Siberian or Chukchi Sea. This water type is also found between 200 and 300 m at AOS 94 stations 17 and 18.

For BSBW, the Q /S structure for AOS 94 station 21 is nearly identical to that of Ark IX/4 station 32 for densities less than 28.02 s Q (Figure 5b), indicating little alteration due to mixing during the transport of water from the Eurasian Basin. However, at AOS 94 station 9 there is a small increase in salinity indicating some mixing with ambient water in the Canadian Basin (Figure 5b).

Figure 6. Vertical profiles of bottle salinity, CFC-11, CFC-113 and tritium at select stations used in this study. See Figure 1 for station locations.


Two SCICEX 96 stations are at the Canada Basin margin, station 68 adjacent to the Chukchi Rise and station 44 at the southern end of the basin. We do not have temperature data for these stations but the relatively low salinity and high tritium and CFC concentrations in the intermediate waters (Figure 6) clearly show the extension of the Atlantic water boundary flow to the southern Canada Basin and indicate that the boundary current, or at least a branch of the boundary current, flows around the Chukchi Rise.

Vertical profiles of bottle salinity, tritium and CFCs at SCICEX 96 station 38 in the center of the Canada Basin are nearly the same as the vertical profiles at stations 44 and 68, with lower salinities and much higher tritium and CFC concentrations below the halocline compared to stations 34 and 35 in the northern Canada Basin (Figure 6). The Q /S plot for station 38 shows extensive reduction of the FSBW component relative to Ark IX/4 station 32 at the Laptev slope and AOS 94 stations 9 and 21 (Figure 4b); the temperature maximum at 27.90 s Qhas been completely eroded. There is still a temperature maximum which apparently comes from the lower part of FSBW, but it occurs at a greater density. The BSBW component is not as dramatically affected by mixing as is the FSBW component, but salinity in the BSBW core is higher than that at the boundary stations (Figure 5b) indicating mixing with ambient Canadian Basin water. The salinity of both the FSBW and BSBW components increases monotonically from station 38 in the center of the Canada Basin to station 35 at the northern end of the Canada Basin, indicating larger proportions of ambient Canadian Basin water to the north. This pattern and the similarity in the vertical profiles at stations 38, 44 and 68 indicate that Atlantic water is transported away from the boundary to the interior in the vicinity of the Chukchi Rise. From the SCICEX 96 data it is not possible to determine if the Atlantic water is being transported to the center of the basin by direct flow, by eddies or by horizontal mixing along layers as proposed by Carmack et al. (1997), but it is clear that the central Canada Basin is much better ventilated than the northern end of the basin.

Proceeding northward along the SCICEX 96 cruise track from station 35, the trend in the Q /S diagram reverses and salinity decreases monotonically in the intermediate water to station 28 (Figures 4a, 5a) as tritium and CFC concentrations increase (Figure 2), showing a clear trend toward Atlantic water. There is a dramatic difference in the Q /S structure for FSBW between stations 30 and 28. The 27.90 s Q potential temperature maximum is not present at station 30 but is present at station 28 where it is nearly as strong as at Ark IX/4 station 32 (1.49° compared to 1.73°C). This clearly indicates flow of FSBW along the Eurasian flank of the Lomonosov Ridge as previously observed by Swift et al. (1997) and Morison et al. (1998). The potential temperature maximum measured on the AOS 94 expedition at a location on the Eurasian flank of the ridge about 300 km to the east was 1.65°C (Swift et al., 1997). For the BSBW density horizon the Q /S characteristics at station 28 are similar to those at station 38 in the center of the Canada Basin and the tritium concentration is somewhat higher; this indicates that BSBW also flows along the Lomonosov Ridge.

Figure 7. Concentration of CFC-11 and CFC-113 in the Northern Hemisphere troposphere verses time (Walker et al., in press).


Circulation timescales and dilution

 

The tritium, 3He and CFC data provide information on the timescale of flow of intermediate water into the Canadian Basin.The presence of tritium and CFCs along the margin and in the interior of the Canadian Basin indicate that the water is being renewed on a time scale of no more than 3-4 decades since this is how long these transient tracers have been entering the ocean in significant quantities . Ages based on tracer data can be affected by mixing. From the discussion above it can be seen that both FSBW and BSBW were altered by mixing along their flow paths into the Canadian Basin, but FSBW was altered to a much greater extent. Here we will use the tritium/3He ratio to estimate ages (e.g., Jenkins and Clarke, 1976; Östlund et al., 1982; Wallace et al., 1992; Schlosser et al., 1990, 1995; Frank et al., 1998) for FSBW and BSBW. Since 3He is produced by the decay of tritium, the ratio increases in a known way as a function of time once a water parcel is isolated from the surface (the tritium/3He clock is set to zero at the surface due to the loss of 3He to the atmosphere by gas exchange). Mixing with water free of tritium and 3He will not affect this ratio and in this case the age represents the age of the youngest component of the mixture. However, the ratio and hence the age will be affected by mixing with water containing tritium and 3He. In this case the age represents an integrated value of the various components of the mixture and is an upper limit for the youngest component. When hydrographic data indicates mixing between two or more water masses, it is not always possible to determine if more than one component contains transient tracers from tritium and 3He data alone. However, CFCs entered the Arctic Ocean on the same time scale as tritium and provide information to address this issue. Here we examine the effect of mixing on the tritium/3He age by calculating a dilution factor from the CFC data.

Tritium/3He ages

 

Figure 8. Vertical profiles of the tritium/3He age at select stations used in this study. See Figure 1 for station locations.


In the Canadian Basin below the Atlantic water temperature maximum (between 250 and 500 m depth), the tritium/3He age generally increases with depth to the deepest water layers sampled during the SCICEX 96 cruise (between 1300 and 1600 m depth; Figures 2 and 8). The tritium/3He ages of the FSBW and BSBW cores at each station were determined from linear interpolation of the vertical age profiles. The core of FSBW was taken to be the 27.90 s Q surface, which is the density of the temperature maximum at the Laptev Sea and the AOS 94 boundary stations. For SCICEX 96 stations 44 and 68, which did not have temperature data, the depth of the 27.90 s Q surface was assumed to be the same as its depth at station 38. Although this density surface may be shallower at the boundary, the vertical tritium/3He age gradient at stations 44 and 68 between 200 and 500 m is very small (Figure 8). Thus, this assumption introduces little error. The core of BSBW was taken to be s Q = 28.00, the middle of the BSBW density horizon at Ark IX/4 station 32 (Figure 5). As for FSBW the depth of this isopycnal at stations 44 and 68 was assumed to be the same as for station 38. These ages are presented in Table 1.


Table 1. Tritium/3He ages and dilution factors for the cores of FSBW and BSBW at SCICEX 96 stations. 3H/3He ages and dilution factors were linearly interpolated to the 27.90 s Q density surface for FSBW and the 28.00 s Q density surface for BSBW and the 3H/3He ages were rounded to the nearest 0.5 year. The error on the age is estimated to be no greater than 1.5 years. Dilution factors were not calculated for CFC concentrations less than 0.02 pmol/kg.

______________FSBW_______________ ______________BSBW________________

 

Station

Depth

(m)

3H/3He

Age

(years)

é/S

dilution

factor

CFC-11

dilution

factor

CFC-113

dilution

factor

Depth

(m)

3H/3He

Age

(years)

é/S

dilution

factor

CFC-11

dilution

factor

CFC-113

dilution

factor

44

380

16.5

800

18.5

68

380

15.0

1.2

1.2

800

16.5

1.0

1.0

38

380

15.0

5

1.1

1.2

800

18.5

2.0

1.6

1.1

37

360

18.5

10

1.2

1.0

740

23.0

6.0

36

350

19.0

10

1.3

1.1

740

22.0

6.0

2.5

1.7

35

330

*

670

28.0

*

6.5

34

330

26.5

*

8.0

690

30.0

8.0

5.5

33

300

21.5

20

690

20.0

4.0

30

300

17.5

7

710

21.5

3.0

28

290

13.5

1.15

760

14.5

2.5


The error in the tritium/3He age resulting from the analytical measurement errors of tritium and 3He is less than 0.5 years except for the very low tritium concentrations measured at SCICEX 96 stations 34 and 35, where the error is about 1 year. The tritium/3He ages in Table 1 were interpolated to the depths of the 27.90 and 28.00 s Q surfaces which introduces some additional error, and rounded to the nearest half year. From the fairly small vertical tritium/3He age gradients in the depth ranges of these density surfaces (300-400m for 27.90 and 650-800m for 28.00) (Figure 8) we estimate the total error of the interpolated tritium/3He age to be no greater than 1.5 years.

The tritium/3He age of the FSBW density horizon along the boundary was 15.0 years at station 68 and 16.5 years at station 44, increasing slightly in the downstream direction. At station 38 the tritium/3He age was 15.0 years, suggesting rapid inflow or mixing from the boundary. The age of the FSBW density horizon increased along the SCICEX 96 section toward the north reaching a maximum of 26.5 years at station 34 and then decreased, continuing north, to a value of 13.5 years at station 28 over the Eurasian flank of the Lomonosov Ridge. The tritium/3He age of the BSBW density horizon was 16.5 years at station 68 and 18.5 years at stations 38 and 44. As in the case of the FSBW tritium/3He age, the BSBW age increased in the northward direction along the section to a maximum at station 34 of 30.0 years. The tritium/3He age then decreased toward the Lomonosov Ridge to a value of about 14.5 years at station 28.

Dilution factors

 

Dilution factors were calculated as follows. The time of formation of the water mass was determined from the tritium/3He age. The CFC concentration at this time was then calculated from the atmospheric time history (Figure 7) and the CFC solubility which is a function of temperature and salinity (Warner and Weiss, 1985; Bu and Warner, 1995). For this calculation, we assumed that the water was at 85% saturation with respect to the atmospheric CFC concentration when it became isolated from the surface. Frank et al. (1998) presented strong evidence of this saturation level for CFC-11 from measurements made along the margin of the Laptev Sea. The CFC concentration at the time of formation was then compared to the measured CFC concentration to estimate how much the original water had been diluted by mixing along the flow path from its source region, assuming that the mixing was with tracer free water. Plots of dilution factors versus depth are shown in Figure 9 and dilution factors for the FSBW and BSBW density horizons at each station are listed in Table 1. Dilution factors were not calculated for CFC concentrations less than 0.02 pmol kg -1 because uncertainties in the concentration due to blank corrections at this level result in very large errors.

Figure 9. Vertical profiles of dilution factors at select stations used in this study. See Figure 1 for station locations.


The dilution factor was generally 1 or slightly less for depths shallower than about 400 m except for SCICEX 96 stations 34 and 35, which had much higher values. This depth range includes the FSBW density horizon. The high dilution factors at stations 34 and 35 are expected since these are the oldest waters as well as the purest Canadian Basin waters. The dilution factor of 1 in FSBW along the boundary and at station 38 suggests that FSBW has been transported from its source region to these locations without significant mixing with tracer free water along the way. However, as discussed previously, the Q /S plots for these stations (Figure 4) indicate that extensive mixing has occurred. Taking ARK/4 station 32 as the FSBW endmember and SCICEX 96 station 35 as the ambient Canadian Basin water endmember and assuming isopycnal mixing along the 27.90 s Q surface, the Q/S data indicate that FSBW is diluted with ambient Canadian Basin water by a factor of 2 at AOS 94 station 9 and by a factor of 5 at SCICEX-96 station 38. Therefore, the FSBW must have mixed with tracer bearing water after entering the Canadian Basin.

As discussed in the previous section, water from the East Siberian and Chukchi shelf regions has been postulated to enter the Canadian Basin (Aagaard et al., 1985; Rudels et al., 1994). Water observed at the shelf break at AOS 94 station 7 has similar Q /S characteristics as ambient Canadian Basin water and is dense enough to enter the FSBW density horizon (Figure 4b). This water also has a high CFC-11 concentration (5.25 pmol kg-1) (Figure 10). Thus the FSBW density horizon in the Canadian Basin appears to consist of three water types: (1) FSBW from the Eurasian Basin, (2) water originating on the East Siberian or Chukchi shelf, and (3) older water from the central Canadian Basin, which may also have East Siberian and/or Chukchi shelf origins since it has Q /S properties similar to the shelf water. Since the shelf water has a high CFC-11 and presumably high tritium concentration (tritium was not measured at AOS 94 station 7), the tritium/3He age of the mixture does not represent the age of the FSBW component. The ambient Canadian Basin water has very low tracer concentrations (Figures 2, 10) and the ages reported in Table 1 for FSBW represent a mixture of the ages for the FSBW and shelf water components of the FSBW density horizon.

Figure 10. CFC-11 vs salinity for select stations from the SCICEX 96, AOS 94 (Carmack et al., 1997) and ARK IX/4 (Frank et al., 1998) cruises.


For the BSBW component the dilution factor is also close to 1 at the boundary (SCICEX 96 station 68, Table 1). However, the Q /S plots for the BSBW density horizon at the AOS 94 boundary stations (Figure 5) are similar to the plot for the Laptev Sea station indicating that not much dilution has occurred. At SCICEX 96 station 38 in the center of the Canada Basin the dilution factor for BSBW is between 1 and 2. Carrying out the same Q /S analysis as was done for FSBW, but along the 28.00 s Q core density surface for BSBW, reveals that BSBW at SCICEX 96 station 38 has been diluted by a factor of 2 with ambient Canadian Basin water which is consistent with the tracer derived dilution factor. This suggests that BSBW flows from the Laptev Sea slope region along the Siberian margin of the Canadian Basin with little dilution and is then transported to the center of the Canada Basin, mixing with ambient Canadian Basin water in roughly a 1:1 ratio. The ambient Canadian Basin water for the BSBW density horizon has very low tracer concentrations (Figures 2, 10) and the tritium/3He age does represent the age of the youngest component of the mixture, i.e. the BSBW. Therefore, this age can be used to calculate the transit time for the BSBW to flow from its region of formation to the central and southern Canada Basin.

Transit times

 

Tritium/3He ages were determined for the eastern Eurasian Basin from stations taken on the Ark IX/4 cruise in 1993 by Frank et al. (1998). For station 32 and nearby stations, the tritium/3He age for BSBW (for the 28.00 s Q density surface) was between 7 and 8 years. The difference between the SCICEX 96 tritium/3He ages for BSBW and these tritium/3He ages represents the transit time required for BSBW to be transported from the eastern end of the Eurasian Basin to the central Canada Basin and along the Lomonosov Ridge. Thus, about 11 years is required for the transit of BSBW from the Eurasian Basin to the central and southern Canada Basin and about 7 years for the transit along the Lomonosov Ridge to SCICEX 96 station 28. These transit times represent an integrated value over roughly the last decade and correspond to an average spreading rate of about 0.6 cm s-1 for the Canadian Basin branch and 0.7 cm s-1 for the Lomonosov Ridge branch. As discussed above, the tritium/3He age of FSBW in the Canadian Basin represents a composite age of a mixture of FSBW and waters originating from the Chukchi and/or East Siberian shelf regions, and thus transit times from the Eurasian Basin cannot be determined. However, the FSBW at station 28 has not been diluted much by mixing (about 15%, Figure 4) and its tritium/3He age of 13.5 years does represent its transit time from its source region. The tritium/3He age of FSBW at the Ark IX/9 station 32 and nearby stations was 5.0-5.5 years (Frank et al., 1998). Thus, the transit time from the eastern Eurasian Basin along the Lomonosov Ridge to station 28 is about 8.0 years corresponding to an average spreading rate of about 0.6 cm s-1.

The data presented here were taken during or immediately after a major warming of Atlantic water and a shift in the boundary between Atlantic derived and Pacific derived halocline waters (Carmack et al., 1995; 1997; McLaughlin et al., 1996; Morison et al., 1998). These changes are clearly seen in the SCICEX 96 data and the circulation patterns and time scales we derived from these data may be influenced by these changes. It is not possible to determine if the circulation pathways and transit times we observe for FSBW and BSBW in the Canadian Basin are influenced by these changes or are truly representative of a longer time period.

SUMMARY AND CONCLUSIONS

The SCICEX 96 data provides evidence that the flow of Atlantic water from both the Fram Strait and Barents Sea sources splits into two branches in the eastern end of the Eurasian Basin with one branch flowing along the Eurasian flank of the Lomonosov Ridge and the other branch crossing the Lomonosov Ridge into the Makarov Basin. This branch continues along the East Siberian margin of the Makarov and Canada Basins reaching at least as far as the Alaskan continental slope. Although these flow paths, originally proposed by Rudels et al. (1994), have been confirmed by earlier observations, the SCICEX 96 data provides evidence for their existence further along the flow paths than previous studies. The most striking feature of the SCICEX 96 data is the well ventilated Atlantic water in the center of the Canada Basin, indicating renewal times of the order of 1-2 decades. These data demonstrate that Atlantic water is transported rapidly from the boundary to the interior of the Canada Basin in the vicinity of the Chukchi Rise, although it cannot be determined if this is caused by direct flow or rapid lateral mixing. The oldest and least ventilated water in the Canadian Basin was observed at its northern end over the southern flank of the Alpha Ridge.

Figure 11. Circulation pathway of Barents Sea Branch Water with tritium/3He ages based on the results of this study and previous studies ( Rudels et al., 1994; Schauer et al., 1997; Swift et al., 1997; Frank et al., 1998).


FSBW mixes extensively with Canadian Basin waters after entering the Canadian Basin, but BSBW appears to be diluted only to a small degree along the East Siberian boundary. In the center of the Canada Basin, BSBW is diluted by a factor of about 2, compared to a factor of about 5 for FSBW. Tritium/3He ages of FSBW are strongly affected by mixing with tracer bearing waters from the Chukchi/East Siberian shelf region and represent a composite age of the mixture rather than the transit time of FSBW from its source region. In contrast, BSBW mixes with water that has low tracer concentration along its flowpath and the tritium/3He age is representative of its transit time. A map showing the circulation pathway and transit times (or ages) for BSBW derived from the potential temperature, salinity and tracer distributions is shown in Figure 11. The flowpath for FSBW after the two branches merge in the eastern Nansen Basin is the same, but the transit times in the Canadian Basin cannot be determined from the tritium/3He ratio because of mixing with tracer bearing Canadian Basin shelf water.

The presence of relatively young and well ventilated water in the center of the Canadian Basin indicates that intermediate water does not flow around the Canada Basin in a single large cyclonic gyre. The presence of the oldest and least ventilated intermediate water at the northernmost end of the Canada Basin suggests the existence of a small gyre that isolates this water from more recently formed water. Alternatively, this distribution may be caused by a change in the circulation pattern associated with the recent increase in flow of Atlantic water into the Canadian Basin.

ACKNOWLEDGMENTS

Obtaining the data reported on in this paper in the heavily ice covered central Canadian Basin was made possible by using a US Navy nuclear submarine which could reach these locations by transiting beneath the ice. We are very grateful to Commander James Reilly and the crew of the USS Pogy for taking us to these locations and for their assistance in collecting the samples. We would like to thank members of the Arctic Submarine Lab for their assistance in handling the logistics and in carrying out the scientific mission of the cruise. We also would like to thank the members of the scientific party, Ray Sambrotto (chief scientist), Jay Ardai, Jay Simpkins, and Mark Cook, who worked on numerous projects for different investigators, for their assistance in obtaining the samples. We especially would like to thank Jay Ardai who was responsible for collecting the tritium, helium isotope and CFC samples. We also thank Eugene Gorman for analyzing the CFC samples, M.Klas, D. Breger and C. McNally for preparing the tritium and helium samples, Carol Kinder for processing the CTD data and Hoyle Lee for assisting with data analysis and preparing the figures. Three anonymous reviewers provided valuable suggestions for improving the manuscript. The L-DEO helium isotope laboratory was established with the help of a generous grant by the W.M. Keck Foundation. This work was supported by NSF Grant OPP 95-29834. L-DEO contribution no. xxxx.

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* ambient Canadian Basin water endmember

 

 


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