A lightweight vertical rosette for deployment in ice-covered waters

Deep-Sea Research I 58 (2011) 460–467

Contents lists available at ScienceDirect

Deep-Sea Research I journal homepage: www.elsevier.com/locate/dsri

Instruments and Methods

A lightweight vertical rosette for deployment in ice-covered waters William M. Smethie Jr.a,n, Dale Chayes a, Richard Perry a, Peter Schlosser a,b,c a b c

Lamont-Doherty Earth Observatory of Columbia University, USA Department of Earth and Environmental Sciences, Columbia University, USA Department of Earth and Environmental Engineering, Columbia University, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 31 August 2010 Received in revised form 15 December 2010 Accepted 23 December 2010 Available online 13 January 2011

A lightweight modular rosette system has been developed that can be launched and recovered from aircraft in ice-covered waters through a 12 in. diameter hole in the ice. The small diameter is achieved by the modular design, in which a CTD module is attached to the end of a conducting cable and water bottle modules (four 4-L bottles per module) are positioned vertically above it. A novel tripping mechanism based on melting a link of monofilament line is used to close the water bottles at the desired depths. After launching the rosette, the cast proceeds like a normal rosette cast with the traces of temperature, salinity, oxygen and other desired sensors being displayed on a computer screen during the down and up casts and tripping the bottles electronically at the desired depths on the up cast. A Seabird 19 + CTD and Seabird 43 oxygen sensor are mounted in the CTD module and data acquisition and bottle tripping are controlled using a Seabird 33 deck unit and Seabird’s SeaSave software run on a laptop computer. Deployment and recovery are done in a heated tent attached to the aircraft to prevent the water from freezing. After recovery the bottle modules are placed in coolers with bags of snow to stabilize the cooler temperature close to 0 1C, which is within 7 1.8 1C of the in situ temperature, and the modules are transported back to a base camp for subsampling and sample processing. This system has been used to collect over 250 water samples in the ice-covered Lincoln Sea and the quality of the samples for dissolved gases and other constituents has been excellent. & 2011 Elsevier Ltd. All rights reserved.

Keywords: Arctic Ocean Ice Rosette Water samples CTD Aircraft

1. Introduction During the last two decades icebreakers have sampled much of the Arctic Ocean, although at times with great difficulty. However, regions with heavy, typically multi-year sea ice and ice ridges have not been adequately sampled due to the difficulty of navigating the ice covering these regions even with powerful icebreakers. Additionally, it is difficult to conduct time series measurements in these regions from icebreakers because of the expense and the long time required to transit to specific locations. Whereas sea ice is a formidable barrier to sampling from ships, it does provide a platform on which aircraft can land and thus gain access to regions difficult or time consuming to reach by ship. Russian scientists pioneered the use of fixed wing aircraft to conduct hydrographic surveys in the central Arctic Ocean, conducting extensive surveys of temperature and salinity from the 1950s to the 1980s and these surveys provide much of the data used for the joint US—Russian Atlas of the Arctic Ocean (EWG, 1997). Fixed wing aircraft (De Havilland DHC-6 Twin Otters) have been used extensively during the North Pole Environmental

n

Corresponding author. Tel.: +1 845 365 8566; fax: + 1 845 365 8155. E-mail address: [email protected] (W.M. Smethie Jr.).

0967-0637/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr.2010.12.007

Observatory (NPEO) program to take hydrographic sections along nominally 901W, 1701W, 1601E and 901E from the North Pole southward to about 851N (Morison et al., 2006). An internally recording CTD with an oxygen sensor is deployed through a hole drilled in the sea ice and four to six 1.5-L Niskin bottles are hung on the wire and tripped mechanically. The winch is mounted in the airplane and a tent attached to the cargo door covers the hole next to the airplane. The tent and airplane are heated, which prevents water from freezing on the sensors or in the Niskin bottles. Typically casts are taken to 500–1000 m depth. The rosette system described here further expands the water sampling capability in ice-covered regions using aircraft. Water samples are required for measurement of many substances including nutrients and natural and anthropogenic tracers such as stable isotopes of water, chlorofluorocarbons (CFCs), tritium or helium isotopes that cannot be measured with in situ sensors and these substances provide valuable information on the amount of river water present, the composition of water masses, circulation patterns, ventilation rates, biological productivity and many other phenomena. The rosette operates in the same manner as standard rosettes used for oceanographic work from ships, in that the operator obtains a down trace of temperature, salinity and other variables for which there are sensors (for example, oxygen), decides on the depths to collect water samples, and trips the

W.M. Smethie Jr. et al. / Deep-Sea Research I 58 (2011) 460–467

bottles on the up cast. However, the orientation is different; the bottles are configured in modules hung above the CTD on a conducting wire to provide a small diameter package that can be deployed through a 12-in. diameter hole in the ice.

2. Design The rosette consists of modules that are connected together physically and electrically (Fig. 1). There are two types of modules, a CTD module and a water sample module. Both modules have the same dimensions: 1 m in height and 11 in. in diameter. The CTD module weighs 80 lb and the water sample module weighs 60 lb empty and 95 lb full. The CTD module (Plate 1) contains a Seabird 19+ CTD housed in a 7000 m pressure case, a Seabird 43 oxygen sensor, a Tritech PA200 altimeter, a Seabird Custom Release Mechanism (CRM), which is based on the standard SBE32 Carousel Water Sampler, and a Lamont-designed interface card (Fig. 2) to connect the CRM to the bottle tripping mechanism. The Seabird 19+ was chosen because of its compact design and its excellent track record for use in the Arctic. There is some additional space for sensors to be mounted in the CTD module. The Seabird CRM and the interface card are housed in the same pressure case with a rechargeable lithium ion battery that powers the interface card and is recharged between flights. The water sample module (Plate 2) contains four 4-L Niskin-type bottles, a battery pack to power the tripping mechanism and a Lamont-designed ‘‘melter’’ circuit board (Fig. 3) to route power to the selected bottle. The melter board and battery pack, containing three alkaline C cells wired in series, are contained in the same pressure housing. These batteries are replaced every three stations. The water sample bottles are constructed of either gray or clear Schedule 40 PVC, have a 4 in. OD, 3.5 in. ID and are 28 in. long. The clear bottles were constructed so we could visually observe if air leaked into the bottles during sampling and transport or if degassing occurred. The ends and end caps are machined from gray PVC for the gray bottles and Lucite for

461

the clear bottles. The openings at each end are 2.12 in. in diameter. End caps are connected with an epoxy coated stainless steel spring inside the bottle with a tension of 14 lb when the bottle is closed. The bottle is cocked by holding the end caps open with a link of monofilament fishing line (0.032 in. in diameter), which passes across a 26 gauge Nicrochrome wire within a melter unit (Plate 2). The bottle is tripped by heating the Nicrochrome wire, which melts the monofilament and releases the end caps to close the bottle under spring tension. The melt time is controlled by the melter card and is programmable. The Nicrochrome wire is housed in a small cavity through which the monofilament passes and the cavity is packed with vacuum grease (Plate 2) to provide thermal and electrical insulation from the seawater during the melting process. This design was chosen because it requires little space and minimizes lanyard lengths, which reduces foul ups (hung caps). The Seabird carousel tripping mechanism would not fit into the module geometry. The end caps seal with the ends of the bottle via an O-ring mounted in a grove in the end cap in the style of traditional Niskin bottles. Each bottle has an air vent at the top and a sampling port at the bottom. The sampling port is a Nylon quarter inch Ultra-Torr fitting, which is fitted with a quarter inch diameter PVC plug during a cast; the plug is replaced with a quarter inch Nupro stainless steel valve for sampling. During a cast, the CTD module is attached to the end of a single-conductor hydrowire and 1, 2 or 3 water sample modules are stacked above it, depending on the number of samples desired. The modules are connected physically to each other and to the hydrowire, and the Seabird CRM is connected electrically to (1) the melter cards on each sample module with water proof cables, (2) the Seabird 33 deck control box through the conducting hydrowire and (3) the SBE19+ CTD. A 3.2-kW gasoline generator connected to the SBE33 deck control box powers the CTD/rosette system, the winch and other topside electronics, and data are acquired on a laptop computer connected to the deck control box (Fig. 1) using Seabird’s SeaSave software. A photograph of the CTD module and 3 bottle modules connected together is shown in Plate 3.

Fig. 1. Schematic wiring diagram for the modular CTD/rosette system.

462

W.M. Smethie Jr. et al. / Deep-Sea Research I 58 (2011) 460–467

3. Operation

Plate 1. The CTD module for the modular rosette. The silver colored cylinder on the left side is the Seabird 19+ CTD unit. To the front and right of this unit, is the water pump and Seabird 43 oxygen sensor. To the right and back there is a gold colored cylinder, which is the pressure case that houses the Seabird 32 Custom Release Mechanism, the Custom Release Mechanism interface card that connects the Seabird electronics to the bottle tripping mechanism, and the battery that powers the interface card. For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.

Pressure, temperature, conductivity and oxygen data are transmitted through the conducting cable to the Seabird 33 deck unit and then to a laptop computer. The CTD and oxygen sensor are powered by the batteries in the CTD unit and the CRM is powered by the deck unit sending current down the single conductor hydrowire. The interface board in the CRM is optically isolated from the CRM and powered by its own battery. Bottles are tripped using the deck unit on the up cast at desired depths, determined from the CTDO down traces displayed on the computer. We deploy the system from a DeHaviland Twin Otter (DHC-6) aircraft with a tent attached to the cargo door and heated with an external kerosene heater as described in Introduction (Plate 4). A lightweight winch (400 lb) with 0.1-in. diameter conducting hydrowire is mounted in the airplane next to the cargo door. A 12-in. diameter hole is drilled in the ice next to the cargo door, a tripod (7 ft in height) is placed over the hole and is enclosed in the tent. The wire is fed through a sheave at the top of the tripod and into the hole. The CTD module is connected to the cable physically and electrically and lowered partially into the hole. Water sample modules are then stacked on top of the CTD module one at a time, connected electrically and mechanically to the module below and lowered into the hole to make room to connect the next module (Plate 5). The package is lowered at 30–40 m/min after passing through the high salinity gradient region in the upper 100 m at about 20 m/min. Higher lowering speeds have not been attempted to avoid exceeding the terminal velocity of the package. The bottles are tripped at the desired depths on the up cast. At each depth where a sample is collected, the package is raised and lowered a couple of meters to flush the bottle before tripping. Deeper sample bottle modules are tripped before upper modules. When the cast is complete, the modules are placed in Igloo coolers and the end caps clamped shut. Two 1-gal ziplock bags of snow are placed in the coolers with each bottle module to maintain a temperature near 0 1C. The modules are then transported back to a base camp where they are sampled and samples are processed. Generally four people participate in the flights, the pilot and co-pilot and two members of the scientific party. The flight crew assists the scientific party in the set up and breakdown of the equipment and the scientific party runs the cast. Station time for a 12-sample cast extending to 800 m is about 3 h.

Voltage Regulator Bottle 1

Bottle 2 RXD TXD

µProcessor Bottle 3 Bottle 4

Fig. 2. Block diagram of the Custom Control Module interface card that interfaces the Seabird electronics to the sample bottles. This unit routes the signal to trip a bottle from the Seabird 33 deck box to the appropriate bottle module.

W.M. Smethie Jr. et al. / Deep-Sea Research I 58 (2011) 460–467

We routinely sample the bottles for salinity, oxygen, nutrients, oxygen isotopes, CFCs, SF6 (not sampled in early years), helium isotopes, tritium, barium and iodine-129 using techniques described in the GO-SHIP Repeat Hydrography Manual (2010). The gas samples are taken first and are drawn within 5 min of opening the bottle. Oxygen and salinity are measured at the base camp using an automated Winckler titration system and Guildline salinometer, respectively, and other samples are returned to various labs for measurement. Nutrient samples are quick-frozen and returned to the lab frozen. CFC samples are flame sealed in glass ampoules under ultra pure nitrogen, SF6/CFC samples are collected in 250 cm3 ground glass-stopper bottles, which are stored in plastic jars filled with rinse water and refrigerated at

Plate 2. The water bottle module. The Ultra-Torr sampling port can be seen at the bottom of the bottle. The melter unit (see expanded view) is located just below the midpoint of the bottle. The smaller diameter cylinder is the pressure case that houses the battery pack (which provides power to melt the monofilament line) and the melter card that controls the melt time and routes the current to the appropriate melter when a bottle is tripped. Each bottle module has one such unit. The end plates of the module frames have holes larger than the bottle diameter aligned with the bottles to enhance bottle flushing. In the monofilament melter, the monofilament line is fed around a roller, then through a notch and through a brass chamber. Within the brass chamber the monofilament line runs across a Nicrochrome wire that is oriented perpendicular to the line. This is not seen in the photograph because the chamber is filled with vacuum grease, which serves as a thermal insulator between the water and the wire.

463

about 2 1C until measurement, and helium samples are collected in copper tubes. Other samples are collected in the appropriate glass or plastic bottle.

Plate 3. The assembled modular rosette system with the CTD module on the bottom and 3 bottle modules above it.

Voltage Regulator

Bank

Row

Optical Isolators (4) Optical Isolators (6)

Confirm

µProcessor

RXD

TXD

Optical Isolator (1)

Program Debug

Fig. 3. Block diagram of the melter PCB that routes the electrical current to appropriate melter and controls the melt time.

464

W.M. Smethie Jr. et al. / Deep-Sea Research I 58 (2011) 460–467

crew working day. The range for a two-station flight is about 350 nautical miles (700 nautical miles round trip). Longer ranges can be achieved by taking only one station and carrying extra fuel or using a fuel cache deployed earlier. These stations extend to about 800 m depth, but sampling could be carried out to 5000 m, the maximum amount of wire the winch will hold, since all of the components are rated to at least 6000 db pressure.

14

Plate 4. Twin otter aircraft with attached tent carrying out a station.

Number of modules

12 10 8 6 4 2 0 -1

0

1 2 3 4 Temperature (°C)

5

6

7

-1

0

1 2 3 4 5 Temperature (°C)

6

7

-1

0

1 2 3 4 5 Temperature (°C)

6

7

50

Number of samples

40

30

20

10

0

35

Plate 5. Deployment of the modular rosette through the ice from a twin otter aircraft.

4. Performance 4.1. Operational limitations The number of 12-sample stations that can be taken per day is two. For this operation, we take six water sample modules, a CTD module, a generator, a kerosene heater, fuel, a tent, a tripod with a sheave, a winch with 1500 m of wire, survival gear, an ice drill and augers, snow shovels, tools topside electronics and lunch. The total weight is about 1500 lb. The main limiting factors are weather, the time required to find a suitable landing site, the time to deploy and recover the system and the length of the flight

Number of samples

30 25 20 15 10 5 0

Fig. 4. Histogram of (a) rosette module temperature when sampling begins, (b) oxygen sample temperature, (c) difference between the oxygen sample temperature and the in situ temperature.

W.M. Smethie Jr. et al. / Deep-Sea Research I 58 (2011) 460–467

4.2. Bottle closure and mistrips The bottle closure rate is about 98%. This is not quite as good as a 24-place or 36-place rosette, which, from our experience, has a closure rate of 99–99.5%. The main reason for a bottle not closing is a bottle end cap hangs up in the confined space of the module. Incomplete melting of the monofilament can also prevent a bottle from closing, but this only occurs if the batteries are defective or discharged.

465

temperature was 6 1C. The difference between the oxygen sample temperature and the in situ temperature (Fig. 4c) was less than 5.5 1C for 98% of the samples and the average warming was 2.5 1C. This amount of warming is less than warming that typically occurs when collecting deep ocean samples in the tropics and subtropics using a rosette and many thousands of samples collected in these regions indicate that degassing is not a problem. 4.4. Data

4.3. Temperature control of the water samples

87

Rid

N

N

84

81

Lom

sov ono

87

Vertical profiles of various constituents for Station 2 from the Switchyard 2006 field season are presented as an example of the types of high quality data that can be obtained with the rosette. Station 2 is located over the continental slope directly north of Alert at 84.11N, 64.71W (Fig. 5). The CTDO down traces are shown in Fig. 6a along with bottle data for oxygen and salinity. Two traces are plotted for oxygen, the uncorrected data and the corrected data. The uncorrected plot used the Seabird 43 sensor calibration data supplied by Seabird. Revised calibration coefficients were calculated with Seabird software using the bottle oxygen data and the CTDO measurements obtained on the up cast at the bottle trip depths. The corrected profile is in excellent agreement with the bottle measurements. No corrections were needed for the CTD salinity calculation. The vertical profile of temperature reveals a mixed layer extending to 65 m, which is underlain by a temperature maximum of 1.55 1C at 85–90 m (Fig. 6a). The salinity of this maximum is about 34.5 psu and this temperature/salinity combination is consistent with the properties of summer Bering Sea Water, which enters the Arctic Ocean through Bering Strait (Coachman et al., 1975; Steele et al., 2004). There is an oxygen minimum and nutrient maximum (Fig. 6a, b) at the base of the temperature maximum with a salinity of about 33.1 psu, which is typical for water entering the Arctic Ocean through Bering Strait and being modified by nutrient regeneration on the Arctic continental shelves (Jones and Anderson, 1986; Wheeler et al., 1997). The presence of these features indicates advection of Pacific origin

ge

After the water is collected in the rosette bottles, it is critical to avoid cooling the water, which is close to the freezing point, to prevent freezing, and to minimize warming, which decreases the solubility of gases and can cause degassing. The heated tent solves the cooling problem because the modules are recovered in an environment above the freezing temperature and immediately transferred to an insulated cooler. The risk of warming is reduced by placing bags of snow in the coolers with the water bottle modules. The in situ temperature of the water samples ranges from about 1.8 to 1.5 1C and the bags of snow provide a stable temperature near 0 1C. When oxygen samples are drawn from the bottle, the temperature is measured, which provides information on warming between the time the water was collected and the time it was sampled. Warming occurs during the indoor sampling process at the base camp where the temperature is about 20 1C. The oxygen sample temperature for the first of the four bottles sampled gives a good estimate of the temperature in the cooler. This sample is taken within 5 min of removing the bottle module from the cooler. For samples collected during three field campaigns (2006–2009), this temperature ranged from 1.2 to 3.2 1C and 80% of the samples were less than 2 1C (Fig. 4a). Sampling the four bottles on a module requires about 30 min and during this time warming occurrs, with the fourth bottle heating the most. The oxygen sample temperature for all samples collected during three field campaigns is shown in Fig. 4b. Eighty-four percent of the samples had temperatures below 3 1C and the maximum

84

N

-2

N

N

81

0 12 W

30

W

N

RT

ALE

90 W -5500

-3000

60 W -2000

-1500

-1000

-500

0

Fig. 5. Location of Station 2 of the 2006 Switchyard expedition. Results from Station 2 are presented in Fig. 6.

466

W.M. Smethie Jr. et al. / Deep-Sea Research I 58 (2011) 460–467

water to the continental slope north of Ellesmere Island. Two possible pathways are cyclonic flow around the periphery of the Canadian Basin, which has been postulated for waters between 200 and 1700 m depth (Rudels et al., 1994) and waters in the upper 30 m (Jones et al., 1998), and anti cyclonic flow in the Beaufort Gyre and Transpolar Drift Stream postulated by Steele et al. (2004). Deeper in the water column there is a broad temperature maximum of 0.4 1C centered at about 370 m, which is the core of Atlantic water that entered the Arctic Ocean through Fram Strait and is observed throughout much of the Arctic Ocean (i.e. Rudels et al., 1994). The halocline is defined by the increase in salinity between the base of the mixed layer and about 250 m. The d18O profile (Fig. 6b) resembles the salinity profile, low in the surface mixed layer and increasing with depth in the halocline. Meteoric water is composed primarily of river water with a small contribution from precipitation and the d18O of meteoric

0

100

water is assumed to be the same as the average value for river water inflow to the Arctic Ocean, 18% (Ekwurzel et al., 2001). Seawater d18O values are close to 0.3% but when water freezes fractionation occurs. Following the convention of Ekwurzel et al. (2001) the d18O for sea ice and sea ice melt was calculated to be the surface water d18O plus the fractionation of 2.6%. The low d18O values in the surface water indicate the presence of river water at this location. As will be seen below, the fraction of river water can be determined from d18O and salinity data. The CFC concentrations (Fig. 6c) decrease with depth through the halocline, indicating poorer ventilated water in the lower halocline than in the upper halocline. The CFC concentration in the core of the Atlantic layer is about the same as at the base of the halocline. Tritium (Fig. 6d) has a smaller concentration range than the CFCs reflecting its input into the environment in the early 1960s compared to the increasing CFC input from the 1950s

0

0

100

100

CFC-11

Salinity

CFC-12

200

Potential Temperature

300

200 Pressure (db)

Pressure (db)

Pressure (db)

200

300

300 CFC-113

Oxygen Uncorrected

400

400

400

500

500

Oxygen

500

600 -2.0

-1.5

29

30

5.5

600

6.0

-1.0

-0.5

31 32 Salinity (psu) 6.5

0.0

0.5

1.0

0

5

10

15

20

25

30

33

34

35

0.50

0.75

1.00

1.25

1.50

1.75

9.5

-3.0

7.0 7.5 8.0 Oxygen (ml / l)

8.5

9.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

600 0

1

2

3 4 5 6 CFC-11 (pmol / kg)

7

8

9

2.00

0.0

0.5

1.0

1.5 2.0 2.5 3.0 CFC-12 (pmol / kg)

3.5

4.0

4.5

1.0

0.0

0.1

0.2

0.3 0.4 0.5 0.6 CFC-113 (pmol / kg)

0.7

0.8

0.9

0

0

0

100

100

100 Sea-Ice Melt

300

Pressure (db)

Pressure (db)

Pressure (db)

200

200

200

Meteoric Water

300

300

400

400

400

500

500

600 0.0

600 0.0

0.1

0.2

0.3 0.4 0.5 0.6 0.7 Fraction of Pacific Water

0.8

0.9

1.0

-0.08

-0.04 -0.00 0.04 Fraction of Meteoric Water

0.08

0.0

0.1

0.2

0.3 0.4 0.5 0.6 0.7 Fraction of Atlantic Water

0.8

0.9

1.0

-0.08

-0.04 -0.00 0.04 Fraction of Sea-Ice Melt

0.08

Pacific Water

500 Atlantic Water

0.0 0

2.5

5.0

7.5

0.5 5

10.0 12.5 15.0 17.5 20.0 22.5 25.0 1.0 1.5 Tritium (TU)

10

15

2.0 20

25

2.5

600

30

Fig. 6. Vertical profiles from Station 2 of the 2006 Switchyard expedition: (a) CTDO down traces of potential temperature, salinity and oxygen, and the oxygen and salinity measurements from bottle samples. Two traces are shown for the oxygen sensor, the uncorrected down trace and the corrected down trace using the bottle oxygen values to determine the oxygen sensor calibration. (b) Nitrate, phosphate, silicate and d18O. (c) CFCs 11, 12 and 113. (d) Tritium, 3He and the tritium/3He age. (e) Fraction of Atlantic and Pacific water. (f) Fraction of meteoric water and sea ice melt.

W.M. Smethie Jr. et al. / Deep-Sea Research I 58 (2011) 460–467

to the 1990s. The tritium:3He age is about 5 years for the mixed layer, 15 years for the Bering Sea water, 20 years for the mid halocline and 27 years for the core of the Atlantic layer. Fractions of Atlantic Water, Pacific Water, meteoric water and sea ice melt water were calculated using the phosphate/nitrate relationship reported by Jones et al. (1998) for Atlantic and Pacific waters and a 4-component mass balance calculation similar to the method described by Oestlund and Hut (1984). The mixed layer is composed nearly entirely of Pacific water, the Bering Strait water just beneath the mixed layer is a mixture of about 80–90% Pacific water and 10–20% Atlantic water and the Atlantic layer is about 90% Atlantic and 10% Pacific water (Fig. 6e). The meteoric water content of the mixed layer is about 7% and it decreases to zero at the base of the halocline (Fig. 6f). The sea ice melt fraction is negative indicating a net addition of salt to the water column due to brine rejection (which is equivalent to a net removal of freshwater) during the freezing process in winter. Vertical integration of the freshwater components reveals 10.7 m of freshwater relative to a salinity of 34.8, the ambient salinity of Arctic Ocean water, 7.2 m of meteoric water 6.1 m of sea ice melt and 9.5 m of freshwater from the Pacific Water inflow through Bering Strait, assuming a salinity of 32.5 for the inflow (Woodgate et al., 2005).

5. Conclusions We have developed a lightweight modular CTD/rosette system that can be deployed from aircraft through a12-in. diameter hole drilled in sea ice or lake ice. It is capable of collecting very high quality CTDO data and water samples for salinity, oxygen, nutrients, trace gases, oxygen isotopes and other constituents.

Acknowledgments We thank the Twin Otter pilots from Kenn Borek Air: Paul Rask, Brent Este, Troy McKerral, Travis Goetzinger, Jason Preston, Don Boe, Karen Morwood, Jim Haffey, Brian Scott, Jackie Bremner, Andrew Ysselmuiden, for taking us safely to and from our sampling locations and assisting us with the deployment of the equipment, the Canadian National Defense and Environment Canada for allowing us to use their facilities at CFS Alert, the 109th New York Air National Guard wing for transporting us and our equipment to and from the Arctic, Andy Heiberg for making the complex logistical arrangements, Anthony Dachille, Eugene

467

Gorman, Hoyle Lee, Bob Newton and Chuck McNalley for measuring the samples and processing the data, Guy Mathieu for his support at Alert in 2006, Jay Ardai for his guidance on many phases of the design and operation, Earl Vaughn for his valuable assistance coordinating cargo with the 109th, three anonymous reviewers for helpful suggestions for the revision, and Bob Williams for the excellent chemical measurements made at Alert, his broad field expertise, which contributed to solutions of problems we encountered, his enthusiasm for this project and his cheerful camaraderie. Bob passed away in the fall of 2009 and he will be sorely missed on future trips to the Arctic. We thank the US National Science Foundation for providing funding to carry out this work, which was supported by two grants, OPP02-30238 and ARC06-33878. This is Lamont-Doherty Earth Observatory contribution number 7424. References Coachman, L.K., Aagaard, K., Tripp, R.B., 1975. Bering Strait: The Regional Physical Oceanography. University of Washington Press. Ekwurzel, B.P., Schlosser, R.A., Mortlock, R.G., Fairbanks, 2001. River runoff, sea ice meltwater and Pacific water distribution and mean residence times in the Arctic Ocean. Journal of Geophysical Research 196, 9075–9092. Environmental Working Group (EWG), 1997. Joint U.S.-Russian Atlas of the Arctic Ocean, Oceanography Atlas for the Winter Period. CD-ROM, National Ocean Data Center. GO-SHIP Repeat Hydrography Manual: A Collection of Expert Reports and Guidelines, IOCCP Report No. 12, ICPO Publication Series No. 134, /www.go-ship. orgS, 2010. Jones, E.P., Anderson, L.G., 1986. On the origin of the chemical properties of the Arctic Ocean halocline. Journal of Geophysical Research 91, 10759–10767. Jones, E.P., Anderson, L.G., Swift, J.H., 1998. Distribution of Atlantic and Pacific waters in the upper Arctic Ocean: implications for circulation. Geophysical Research Letters 25, 765–768. Morison, J., Steele, M., Kikuchi, T., Falkner, K., Smethie, W., 2006. Relaxation of central Arctic Ocean hydrography to pre-1990s climatology. Geophysical Research Letters 33, L17604. doi:10.1029/2006GL026826. Oestlund, G., Hut, G., 1984. Arctic Ocean water mass balance from isotope data. Journal of Geophysical Research 89, 6373–6381. Rudels, B., Jones, E.P., Anderson, L.G., Kattner, G., 1994. On the origin and circulation of the Atlantic layer and intermediate depth waters in the Arctic Ocean. In: Johannessen, O.M., Muensch, R.D., Overland, J.E. (Eds.), The Polar Oceans and Their Role in Shaping the Global Environment, Geophysical Monograph Series, vol. 85. AGU, Washington, D.C., pp. 33–46. Steele, M., Morison, J., Ermold, W., Rigor, I., Ortmeyer, M., Shimada, K., 2004. Circulation of summer Pacific Halocline water in the Arctic Ocean. Journal of Geophysical Research 109, C02027. doi:10.1029/2003JC002009. Wheeler, P.A., Watkins, J.M., Hansing, R.L., 1997. Nutrients, organic carbon and organic nitrogen in the upper water column of the Arctic Ocean: implications for the sources of dissolved organic carbon. Deep-Sea Research II 44, 1571–1592. Woodgate, R.A., Aagaard, K., Weingartner, T.J., 2005. Monthly temperature, salinity, and transport variability of the Bering Strait through flow. Geophysical Research Letters 32, L04601. doi:10.1029/2004GL021880.