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IARC Technical Report # 1

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7.1.3.5. Preliminary Results (I.Dmitrenko and I. Polyakov, IARC, D.Walsh, NRL, and L.Timokhov, AARI)

The high quality of MMP data is demonstrated by the good agreement found between the SBE19plus CTD record at station KD0602 carried out before the mooring deployment on September 1, 2002 and the first MMP profile carried out one day later (Figure 7.19). The temperature and salinity continuous MMP records allow us to examine temporal variability of the AW layer in the depth range between 164 and 820 m over almost six months of observations (September 1, 2002 - February 20, 2003, Figure 7.20).

Figure 7.19: Vertical temperature (°C) distribution on station KD0602 (CTD SBE19plus data, 09/01/02) and the MMP second run on 09/02/02 (black).

The depth of the AW core (maximum temperature) over the record length changed little, varying between 234 and 244 m, while the temperature in the core of the AW layer varied from 0.97°C at the beginning of observations to 1.33°C on December 4, 2002. The upper profiling depth limit was just beneath the upper boundary of the AW layer, defined by the 0°C isotherm (Figure 7.20). Therefore the variability of the AW upper boundary was not traced except for a short period on February 15-16, 2003.

CTD records at stations KD0602 and KD0103 carried out at mooring locations before deployment and after mooring recovery provided nearly the same depth of the 0°C (compare 157 m at the beginning and 160 m at the end of the record). Temperature records from the surface floating CTD SBE-37 (depth 136 m, Figure 7.21) and SBE 16 (161 m depth, Figure 7.22) are in very good correspondence with the temperature of the AW layer. The lower AW layer boundary varied over a wide range between 530 - 650 m; in contrast, during the CTD profile carried out before recovery (station KD0103) it was found at 470 m.

Figure 7.20: Temperature (°C, upper panel), and salinity (psu, lower panel) within the AW layer during 175 days of measurements from September 2, 2002 to February 20, 2003 from McLane MMP Profiler CTD data. Blank vertical columns represent missing data. Isotherm 0 °C (white curve) shows the boundaries of the AW layer.

Figure 7.21: Sea level, salinity, and temperature from the surface floating CTD SBE37 (about 136 m depth). Only the relative pressure variations are considered to be reliable. The records from day 160 through day 220 are suspicious.

Four major events of warm AW intrusions were recorded during the nearly half year period of observations: 09/19 - 10/18/2002 (A), 11/02 - 11/15/2002 (B), 11/20/2002 - 01/03/2003 (C), 01/07 - 02/15/2003 (D), Figure 7.20. The typical scale of temperature and salinity variations associated with these warm water intrusions is about 0.4-0.5°C and 0.02 psu respectively. The same warmer events have also been recorded by SBE 37 (136 m, Figure 7.21) and SBE 16 (161 m, Figure 7.22). This good agreement between MMP and SBE CTD data allows to conclude that most of the temperature maxima shown in Figures 7.21 and 7.22 are connected to warm temperature intrusions advected within the AW layer.

The seasonal trend is well captured in surface SBE 37 and SBE 16 temperature records. The maximum temperature at the upper boundary of the AW layer was recorded during winter while during summer the temperature was lower by 0.6 - 0.8°C. Small-scale steps in stratification (Figure 7.19) were typical features found in our observations. The thickness of steps varies within 2-4 m. A striking feature of these records is that these steps maintain their depth throughout the record (Figure 7.20). The intrusions of warmer water within the AW layer during A-D events do not disturb this spatial structure significantly. The temperature from the near-bottom record varies between -0.778 and -0.768°C (Figure 7.23). A gradual decrease of bottom salinity by 0.004 psu is within the accuracy of our conductivity sensor (Figure 7.23). A bottom salinity decrease of 0.2 psu was also recorded by an Anderaa RCM-7 conductivity sensor (Figure 7.24).

Figure 7.22: Salinity and temperature from the surface CTD SBE16 (about 161 m depth).

Figure 7.23: Sea level, salinity and temperature records from the bottom CTD SBE37 (about 2742 m depth).

The Anderaa RCM-7 current records are shown in Figure 7.24. Although there is no sign that the data are erroneous, the very low current speed is suspicious and we plan to check the record using data from the mooring recovery in 2004. Pressure sensors from surface and bottom SBE-37 installations recorded the tidal sea-level oscillations (Figure 7.21 and 7.23). The results of tidal harmonic analysis [Foreman, 1977] for the main tidal constituents are presented in Table 7.7 (surface SBE 37).

Figure 7.24: Current and salinity records from the bottom Anderaa RCM-7 (2670 db depth) from September 2, 2002 to September 1, 2003. The current speeds may be underestimated.

Table 7.7: Tidal Analysis Results for surface SBE37 recorded sea-level oscillations.

tide

Frequency

cycles/hour

Amplitude

cm

Error

cm

Phase

degrees

Error

degrees

MF

0.00305

5.23

7.4

254.29

98.06

Q1

0.037219

0.32

0.2

286.19

31.38

O1

0.038731

3.21

0.2

154.78

3.06

P1

0.041553

1.76

0.2

172.85

6.19

K1

0.041781

5.62

0.2

43.03

1.84

N2

0.078999

2.82

1.2

250.83

21.92

M2

0.080511

15.79

1.1

147.87

3.72

S2

0.083333

7.15

1.2

191.71

8.57

K2

0.083562

2.17

1.0

240.79

28.64

7.2. Chemical Observations (N.Tanaka, IARC, and M.Nitishinskiy, AARI)

7.2.1. Objectives

Exchange and interaction of water masses in the Laptev Sea have significant implications for the Arctic climatic system. In the past, dissolved oxygen (DO) and nutrients were used to identify specific water masses in the Arctic Ocean, although those chemical parameters are highly non-conservative due to biological activities in sea water and underlying sediments. Although there are substantial observation data for nutrients and DO on the shelf in the Laptev Sea, offshore hydrochemistry data in the middle Laptev Sea are still scarce and gaps need to be filled. As an unambiguous measure for river and sea ice melt water, seawater and sea ice samples will be analyzed for a stable oxygen isotope. Specific objectives on NABOS-03 cruise observation are:

a.    Fill observation gaps for hydrochemistry distributions in the Laptev Sea and provide a comprehensive picture of the synoptic distributions in the entire Laptev Sea, leading to a better understanding of the hydrochemical budget and major water mass mixing in this region; and

b.    Based on stable isotope systematics, fresh waters from river and sea ice melt are to be separately identified. The results will lead to understanding the behavior of fresh water from each separate source in this region, and therefore to a better evaluation of the role of fresh water in the vertical stability of the Laptev Sea.

7.2.2. Methods and Equipment

All seawater samples for chemical analysis were collected by CTD/Rosette Niskin bottle sampling systems. Detail of the sampling system and sampling procedures are described in section 7.1.2 of this preliminary report. Sampling locations are also shown in Fig. 7.3 in section 7.1.2. Water samplings were carried out at 26 hydrographic observation stations including within the Russian EEZ. The specification of the CTD and DO sensors are also given in Table 7.2, 7.3 in section 7.1.2. The system was also equipped with a Sea Tec fluorometer for in situ chlorophyll a measurement. The accuracy and precision is not known because calibration data are lacking.

Analytical methods

1)    DO: Dissolved oxygen contents in seawater were analyzed on board by using a modified Winkler titration method [Strickland and Parsons, 1968]. The typical precision of this method is 0.05 mlSTP/liter. The detection limit is around 0.6 mlSTP/liter. The analysis was completed within 24 hours after sampling. There were several samples collected from major intrusion structures in water column. The results from titration analysis of DO are then compared with DO sensor results. DO sensor results tend to filter out the high frequency variation due to the relatively slow response time of the sensor.

2)    Nutrients: Reactive phosphate, silicate, nitrate and nitrite will be measured by using an auto-analyzer Skalar Sun Plus system installed in the Otto Schmidt Laboratory for Marine and Polar Research at the Arctic and Antarctic Research Institute. The typical precision of those nutrient analyses are comparable to conventional analyses done by hand as described by Strickland and Parsons [1968]; the results are 0.02 micro-at P/kg, 0.25micro-at Si/kg and 0.5micro-at N/kg, respectively. The samples collected on board were immediately frozen at -20 degree C. all samples are now waiting for the analysis.

3)Stable oxygen isotope analysis: Stable oxygen isotope compositions of seawater samples will be determined by using a Finnigan MAT 252 ratio mass spectrometer with an automatic CO2-H2-H20 equilibration unit at the International Arctic Research Center, University of Alaska, Fairbanks, USA. The analytical results are presented as per mils deviation of H218O/H216O ratio from the International Standard Mean Ocean Water (VSMOW). Typical analytical error was 0.04 per mils for d18O analysis.

Yearly biochemical oxygen demand (BOD): For measuring BOD samples, 100 ml oxygen glasses were used. The glass sample containers were sealed with Parafilm to exclude atmospheric oxygen. Samples were kept at a temperature of about 0 °C. These samples will be analyzed for BOD at the Otto Schmidt Laboratory for Marine and Polar Research by using a modified Winkler titration method.

7.2.3. Preliminary Results

Analysis for stable isotope and nutrients has not yet been completed. Below, dissolved oxygen results are shown (Fig. 7.25). To compare methods of measuring dissolved oxygen by DO sensor and by titration, DO values were measured by a DO sensor at each seawater sampling, and the DO value of the collected seawater was also obtained by the titration method. In Figure 7.25, differences in DO sensor values from the correspondence titration values are plotted.

Figure 7.25: Difference in DO values between CTD/DO sensor measurements and chemical titration analysis(DOtitration - DOsensor) The difference in surface layer (0-100m) results ranges from -1.2 to 0.2 and the difference in deeper layer ranges from -0.3 to 0.3. The data are from both descending and ascending casts. The DO sensor tends to underestimate by 0.2 ml/kg. Solid black line represents linear regression of the data.



The west to east cross-sections of DOtitration and DOsensor along cross-section B are shown in Figure 7.26. The general pattern of DO distribution obtained by both methods is comparable, although some details are different. DO was high at the surface and the steep negative gradient in the subsurface pycnocline, because of the salinity gradient in the subsurface water. At the core depth of AW at about 300 m, there exists a clear DO concentration gradient. In addition, the west side of the cross-section is distinctively lower than the east side. DOtitration values along the north-south cross-section A are given in Figure 7.27.

Figure 7.26: DO distribution based on (a) titration analysis (ml/l) and (b) measurements by the DO sensor installed on the CTD system (mmol/kg) along cross-section B.

Figure 7.27: Cross-section A of DO titration.

At first glance, a startlingly complex structure of DO distribution is revealed in the AW layers, suggesting various AW masses, which are subjected to a variety of influences after they enter the Arctic Ocean. The cause for distinctively high DO contents in the deeper water near the slope has not yet been identified.

The density of Atlantic water and T, S and DO distribution

In Figure 7.28, Sigma 0 vs in situ seawater temperature is given. The subsurface temperature maxima is the core part of the AW mass always located at the density value (sigma0) = 27.9. This density value for the AW with subsurface maxima in temperature seems universal anywhere in the Arctic Ocean.

Figure 7.28: Vertical distribution of salinity and density vs temperature at all NABOS-03 hydrographic stations.

Isopycnal distributions of in situ seawater temperature, salinity and DOsensor are shown in Figure 7.29. Both temperature and salinity show west-east contrast in distribution. The western side is occupied by relatively warm and more saline AW, presumably a young AW mass. The east side shows relatively cold and less saline AW. Interestingly, the east side does not have the significant intrusion structures which are common in the Laptev Sea as well as in the Kara and Barents Seas. Those are quite different AW masses, which may be closer to that frequently observed in the Canada Basin, Chukchi Sea, and the Beaufort Sea. Co-existence of AW masses with distinctively different water properties in the Laptev Sea is clearly revealed in the data. Taking a closer look at DO distribution reveals an additional feature. In the southern part of the observation area, moderate temperature water with depleted DO can be found. This water mass should be formed by strong interaction of the AW mass with the continental margin. Thus, a third type of AW mass can be identified in the observed data set.


Figure 7.29: Isopycnal distributions of in situ seawater temperature, salinity and DOsensor . Fluorometer result (fls) was not calibrated well and the observed values are almost at the detection limit. Therefore, this result should not be interpreted as reliable.

Closing remarks

Preliminary analysis of dissolved oxygen data revealed three distinctive AW masses in the Laptev Sea, which could have different ages and interaction processes with the continental margin. Nutrients data are expected to allow us to construct a more detailed picture of the water mass structures in this region, and also to yield some insights into the controlling factors for biological activities in this region. Once stable isotope analyses are completed, freshwater dispersion patterns and possible mechanisms controlling dispersion can be discussed in more quantitative terms.

7.2. Ice buoys deployments (I.Dmitrenko, IARC)

Ice buoys have been used extensively in Arctic and Antarctic regions to track ice movement and are available commercially for deployment by ships or aircraft. Such buoys are equipped with low temperature electronics and lithium batteries that can operate at temperatures down to -50°C. The Mass Balance Buoy (also called the ice-temperature buoy, the PMEL/CRREL buoy, Figure 7.30 A) includes an acoustic pinger that measures the depth of the snow on top of the sea ice. It also includes a chain of thermistors which measure temperatures from the air down through the snow cover, through the sea ice and into the sea water below the ice. The chain is several meters long, and has temperature sensors every 5-10 cm. Data are transmitted by the NOAA Argos satellite. The description of AARI (Figure 7.30 C) and AWI (Figure 7.30 B) buoys can be found on http://iabp.apl.washington.edu/gpsbuoy.gif and http://iabp.apl.washington.edu/awiaari.gif respectively.

Information about buoy deployments is presented in Table 7.1 and Appendix 1. The stations where the buoys were deployed are referenced as ice stations (ICE##03) in the chronological order (see also Figure 7.31).

All buoys were deployed in the vicinity of the oceanographic stations on at least 1 m thick two-year old ice. Distance from ice floe edges was at least 400-500 m. Five buoys were deployed along the oceanographic transect B (ARGOS ID # 2735, 10120, 2736, 25526, 10812) and other two buoys were deployed along the transect D (# 2737 and 10810, Figure 7.31).

Figure 7.30: Mass Balance (A), AWI (B) and AARI (C) construction ARGOS ice buoys deployed during NABOS-03 cruise.

Figure 7.31: Positions of deployment and drift from September 3-10 to December 1, 2003 of ARGOS ice buoys. The buoy numbers are presented according to ARGOS ID. From December 1, 2003 the buoy positions have been taken from http://iabp.apl.washington.edu.

 

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