Determination of the Area of Generation of Big Icebergs in the Barents Sea Temperature Distribution Analysis

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1 Proceedings of the Sixteenth (6) International Offshore and Polar Engineering Conference San Francisco, California, USA, May 28-June 2, 6 Copyright 6 by The International Society of Offshore and Polar Engineers ISBN (Set); ISSN (Set) Determination of the Area of Generation of Big Icebergs in the Barents Sea Temperature Distribution Analysis N.V. Kubyshkin 1, I.V. Buzin 1, A.F. Glazovsky 2, A.A. Skutin 1 1 «Arctic-shelf» laboratory, State Institution Arctic and Antarctic Research Institute 2 -Institute of Gegraphy of Russian Academy of Science 1 -Saint-Petersburg, 2 -Moscow, Russia. ABSTRACT Based on the analysis of published materials and field data on the temperature distribution in icebergs and iceberg-producing glaciers of Franz-Josef Land and Novaya Zemlya obtained by Arctic and Antarctic Research Institute (SI AARI) in 3 and 5, the most possible areas of formation of some large icebergs are determined. In particular, the conclusion that the big tabular berg detected in the Shtokman Gas Condensed Field (Shtokman GCF) in 3, most likely originated from one of the outlet glaciers of Franz-Josef Land is made. Earlier, the glaciers of Novaya Zemlya were pointed out as the source of this iceberg. KEY WORDS: Barents Sea, icebergs, glaciers, temperature profiles, thermistor chain, calving. INTRODUCTION The problem of safety of offshore structures to be located on the shelf of the Barents Sea (primarily of the ice resistant platform in the Shtokman GCF) requires a detailed study of the areas of generation of large icebergs and the conditions under which they can move from the northern part of the sea to the latitude of the Shtokman GCF. The importance of icebergs studies has sharply increased after the AARI expedition in 3 detected an anomalously large number of icebergs in the Shtokman GCF area including very large samples of tabular bergs, which were repeatedly reported in different presentations and publications (Naumov et al., 3; Buzin, 4; Zubakin et al, 5; Kubyshkin, 5). For organizing the iceberg monitoring in the Shtokman GCF area, the objectives to be addressed should include the most probable sources of icebergs that could impact the area of Shtokman GCF. Immediately after detecting the iceberg anomaly of 3, it was suggested that it was of Novaya Zemlya origin. One of the main arguments in favor of this suggestion was that glaciers of Novaya Zemlya were the closest source of iceberg generation to the sighted iceberg population. In 4-5, a group of authors investigated this problem including field experiments with deployment of thermistor chains on icebergs and outlet glaciers in the Barents Sea and study of literature sources. Some results of the studies (which have not yet been completed) are presented in this paper. GENERATION AND SPREADING OF ICEBERGS IN THE BARENTS SEA The sources of icebergs observed in the Barents Sea area are the arctic archipelagos - Spitsbergen, Franz Josef Land (FJL), Novaya Zemlya (Severny Island) and some arctic islands (Ushakov and Victoria). It is possible that icebergs of Severnaya Zemlya archipelago might also appear in the Barents Sea (Sanford, 1955; Koryakin, 1988). The aforementioned geographical objects significantly differ by the iceberg discharge value, which depends on the glaciation scales, length of sea boundaries of glaciers, ice motion rate, etc. Spitsbergen has the largest glaciation area 35.1 thousand km 2 (Atlas of the Arctic, 1985). The latest assessment of iceberg formation on Spitsbergen was performed by Hagen et al (3). In accordance with this study, the ice flow at the producing fronts is estimated as 3±1 km 3 a year. In addition, the annual recession of glaciers is estimated as 1 km 3. Thus the full ice loss due to iceberg production comprises 4±1 km 3 /year. This value is close to the data published in (World water balance, 1974) and is much greater than the estimates of km 3 /year, given by Løset (1993) and Zubakin et al (5). The iceberg discharge of Franz Josef Land in the data published in Grossvald et al. (1973) and World water balance (1974) comprises 2.3 km 3 /year. According to the latest unpublished estimates of А.F. Glazovsky (Glazovsky, personal communication, 5), the ice flow at the producing glacial fronts is about 4. km 3 a year; an additional annual retreat of glaciers is estimated as.4 km 3 /year. In the end, the total ice losses due to iceberg generation at the FJL are 4.4 km 3 /year. The commonly accepted estimates of the iceberg discharge of Novaya Zemlya are within km 3 /year (Chizhov et al., 1968; World water balance, 1974). The last unpublished estimates of А.F. Glazovsky for Novaya Zemlya (Glazovsky, personal communication, 5) are as follows: the ice flow at the producing fronts comprises about 1. km 3 /year; the annual retreat adds about.5 km 3 /year. Thus, the total ice losses for generation of Novaya Zemlya icebergs are about 1.5 km 3 /year. The last assessment of the iceberg discharge for Severnaya Zemlya was performed only relative to the largest glacial complex of the archipelago Akademii Nauk (Dowdeswell et al, 2). According to this study, a complete iceberg discharge from the ice sheet comprises.65 km 3 /year. In general based on this estimate, Severnaya Zemlya can 634

2 produce at least.7 km 3 icebergs a year. Earlier estimates give the iceberg discharge from Severnaya Zemlya within.4.8 km 3 /year (World water balance, 1974; Krenke, 1982; Govorukha, 1988). For Ushakov and Victoria Islands, the iceberg discharge estimates equal to.1 and.1 km 3 /year, respectively, are given in (Govorukha, 1988). The generation of icebergs occurs non-uniformly in pulses under the influence of different factors, connected both with the glaciers itself (flow, glacial structure, non-uniform ablation at the sea boundary), and with the action of external conditions (hydrological etc.). A detailed review of the mechanisms and conditions of the generation of icebergs is presented in the studies (Voyevodin, 1996; Van der Veen, 2). During a year, the number of icebergs calved from the glaciers vary. The increased iceberg discharge for glaciers of the Barents Sea is observed from June through September with a maximum in August. The cause for this increase is clearly explained by the results of ice flow observations at Shokalsky glacier on Novaya Zemlya, made in at a distance of 8 km from the glacial front (Chizhov et al., 1968). The average monthly ice flow rate during the coldest month (February) was about 4.5 m/month, and in the warmest month (July) 1 m/month. Fig. 1 presents the distribution of the frequency of occurrence of different shapes of the Barents Sea icebergs constructed using the database on icebergs of the Barents Sea, which contains in general more than records for the period 1928 to 1991 (It should be specified that the characteristics of the shape of icebergs in this database are contained only from the 197s). The majority of records of the Barents Sea icebergs with a specific shape is comprised of ice fragments of small size: bergy bits and growlers (about 77% of the entire data volume). It is interesting that the group with the second highest frequency is comprised of the largest icebergs tabular bergs (about 15%) and only 8% of the records are comprised of the icebergs of other shapes: glacier bergs, sloping icebergs, aged icebergs, etc. frequency, % form 1 bergy bits; 2 tabular berg; 3 growler; 4 glacier berg; 5 other shapes of icebergs Fig. 1. Distribution of the frequency of occurrence of the shapes of icebergs in the Barents Sea from data of (Zubakin et al, 5) It is common knowledge that large tabular bergs are often observed in the water areas adjoining the Spitsbergen, Franz Josef Land, Novaya Zemlya (northern part) archipelagos and between them. Fig. 2 presents isolines of the probability of iceberg occurrence in the Barents Sea based on all records contained in the database with addition of the AARI expedition data of 1-5. Same figure also shows the records of tabular bergs the majority of them are really concentrated in the aforementioned regions with the Franz Josef Land and the adjoining sea areas being obvious leaders by the quantity of tabular bergs. At the same time, dozens of single tabular bergs and groups of tabular bergs are scattered over the entire Barents Sea, where the total probability of iceberg occurrence is greater than 1%. Latitude, N Longitude, E Fig.2. Isolines of the probability of occurrence of all forms of iceberg in the Barents Sea. Symbols (2) mark the detection of tabular bergs. TEMPERATURE A CONSERVATIVE CHARACTERISTIC ALLOWING DETERMINING THE AREA OF GENERATION OF LARGE ICEBERGS Large icebergs with a mass of hundred thousands and even million tons are capable to preserve the temperature of their internal part practically unchanged for a long time. Only a comparatively small surface iceberg layer responds to the environmental temperature fluctuations. It was shown in (Chikovsky, 197) on the basis of general understanding of the regularities of temperature wave propagating in a semi-infinite medium relative to icebergs and glaciers that day-to-day variations of the surface temperature penetrate to the ice strata by.8 m, semi-annual by 1.5 m and annual by 15.4 m. Diemand (1984) also informs about the prolonged preservation of the cold core in the strata of icebergs due to low heat conductivity of ice, which is not influenced even by the accelerated ablation process at iceberg getting to warmer waters. The author presents data on the temperature change in the strata of icebergs in the area of Newfoundland to the depths of m from the surface, its values comprising -6-13ºС. Løset (1993) was probably the first who suggested the possibility of determining the area of generation of big tabular bergs from the temperature in their strata. In this study, Løset presents several temperature profiles in icebergs detected not far from Spitsbergen in The temperature was measured from the iceberg surface to the levels of 5 m (1988), 6-11 m (1989) and 14 m (199). The author considered one of these icebergs in the assumption that it was formed in the FJL area, modeled numerically its drift from the FJL towards Spitsbergen and calculated the temperature evolution in its entire strata using meteo-data of Hopen station. Løset justified his assumption about the FJL as the place of origin of the iceberg under consideration by presenting the temperature profiles obtained at different time at the Ciurlionis (FJL) and Austfonna (East Spitsbergen) ice caps. At the first of them the temperature at the 1 m depth from the daytime surface was about -1 С and did not practically experience seasonal changes. At the second ice cap, the ice temperature in the same level was about - 1 С. This served as the main argument in favor of the hypothesis about the formation of all surveyed icebergs at the glaciers of FJL (the ice temperature in the lower levels of measurements was within С for all icebergs). 635

3 TEMPERATURE AT THE FRONTS OF ICEBERG- PRODUCING GLACIERS IN THE EURASIAN SECTOR OF THE ARCTIC For the practical use of method of determination of iceberg s generation region using the data on iceberg s internal temperature, reference estimates of temperature within the iceberg-producing glaciers are necessary. In glaciological literature, one can find a lot of evidence about the temperature profiles in the strata of glaciers on Spitsbergen, FJL, Novaya Zemlya and Severnaya Zemlya, however, most of it was obtained in the boreholes located in the central (nearapical) parts of the glaciers with large absolute heights in the accumulation area (Kotlyakov et al, 4). But the temperature conditions of this part of glaciers differ significantly from the temperature conditions of the ablation area, to which the near-frontal areas of outlet glaciers refer where calving occurs. As an example, Fig.3 presents three temperature profiles obtained in April 1959 by the glaciological expedition of the Institute of Geography of the USSR Academy of Science (IG AN USSR) in the area of Russkaya Gavan (Novaya Zemlya archipelago). Curves 1 and 2 reflect the results of temperature measurements in the ablation area on Shokalsky glacier. The first of them was obtained at a distance of 8 km from the glacier front and the second only in 3 m from the front. Both curves are quite close to each other especially in the horizons below the 5-m mark. The borehole in which profile 3 was obtained was located at Ledorazdelnaya station in the accumulation area at the absolute height of 795 m. Curve 3 considerably differs from the first two profiles both in the surface layer (towards lower temperatures), and in the deeper layers (on the contrary towards higher temperature). The presented differences in the temperature profiles in the accumulation and ablation areas show that one has to be very careful in choosing the reference temperature profiles of the Arctic glaciers for determining the iceberg formation region. The temperature measurements in the glacier front area would be ideal for this purpose. In the absence of such measurements one can use the data obtained at some distance from the front. It is extremely desirable for the boreholes where temperature profiles are measured, to be at least in the ablation area if not at one absolute height with the glacier front. The most convenient (and unique in our opinion) data on the temperature of glaciers meeting this requirement, are presented in the materials of glaciological expeditions of IG AN USSR, which worked in at FJL and Novaya Zemlya under the International Geophysical Year Program. These are data of temperature measurements on Sedov glacier, Guker Island of Franz Josef Land (Razumeiko, 1963) and at Barier Somneniy station on Shokalsky Glacier, Novaya Zemlya (Khmelevskoy, 1963). Figs. 4 and 5 present mean monthly ice temperature profiles for most indicative months for both glaciers. Seasonal temperature fluctuations in both cases completely attenuate at a depth of 15 m, being in good agreement with the aforementioned estimates of Chikovsky. In the 15-m depth, the temperature differences between the Sedov and Shokalsky glaciers are quite clearly manifested and comprise about 4ºС. Unfortunately, the authors do not have available data on ice temperature measurements in the areas of the glacier fronts of Spitsbergen and Severnaya Zemlya and of the Ushakov and Victoria Islands. At the same time due to the aforementioned causes, we have intentionally refused from using the available data on the temperature of ice caps both at Spitsbergen and Severnaya Zemlya. According to preliminary considerations based on mean multiyear data on air temperatures at some polar stations of the Barents Sea (Table 1), the coldest should be icebergs formed at FJL, then on Novaya Zemlya and finally the warmest can be icebergs of Spitsbergen. Table 1. Mean annual air temperatures at some polar stations of the Barents Sea Station Air temperature, Observation period ºС Barentsburg (Spitsbergen) Hopen (Spitsbergen) Mys Zhelaniya (Nov. Zemlya) Rus. Gavan (Nov. Zemlya) Tikhaya & Hayes (combined series) (FJL) In addition to differences determined by belonging to some or other group of the Arctic Islands, the individual glaciers at one and the same archipelago can also significantly differ by temperature. In April 5, the AARI expedition onboard the research vessel Mikhail Somov carried out the temperature measurements on the glacier of Salm Island of Franz Josef Land using a 16-m thermistor chain at the point located near the glacier front. The obtained temperature profiles are presented in Fig. 5 together with the results of measurements on the Sedov glacier. One should anticipate the significant differences by temperature near the glaciers of Severny Island of Novaya Zemlya, which has a large meridional length, however there are no available data allowing us to estimate these differences Fig.3. Temperature profiles on Severny Island of Novaya Zemlya in the Russkaya Gavan area (Khmelevskoy, 1963): 1 stationary Barier Somneniy station, ablation zone, absolute height of 29 m, (T air -18.1, snow 74 cm); 2 thermal sounding along the route, ablation zone, absolute height of 17 m, (T air -18.7, snow 45 cm); 3 thermal sounding along the route, ablation zone, absolute height of 795 m, (T air -18.5, snow 115 cm). 636

4 5 1 II V VI X VIII icebergs: 1 in the FJL area and 2 near the shores of Novaya Zemlya. All 8 profiles are presented in Fig. 6. All measurements made by different expeditions in different sea regions and in different years were conducted during April and first ten-day period of May. Nevertheless, the air temperature differences both at the moment of measurements and in the preceding periods determined a significant scattering of surface temperature values of icebergs and of the entire surface layer with a thickness of 4-6 m Fig. 4. Mean monthly vertical temperature profiles on Shokalsky glacier, Novaya Zemlya ( ) 5 II V X VI Novaya Zemlya, Novaya Zemlya, FJL, Shtokman GCF, Løset, Løset, Løset, Løset, Fig. 5. Vertical temperature profiles on FJL glaciers: 1 mean monthly, Sedov glacier ( ); 2 measured on glacier of Salm Island (18.4.5). TEMPERATURE DISTRIBUTION IN THE STRATA OF TABULAR BERGS OF THE BARENTS SEA At the present time we have data on deep (more than 6 m) temperature profiles of 8 large icebergs detected at different time in different regions of the Barents Sea. The temperature profiles of 4 icebergs detected in east and southeast of Spitsbergen are presented in (Løset, 1993); in 3, a thermistor chain was deployed on a tabular berg sighted in the Shtokman GCF area (Kubyshkin, 5); and finally in 5, the AARI expedition deployed thermistor chains on three Fig.6. Vertical temperature profiles in large tabular bergs in the Barents Sea (see the text for explanations) Beginning from the 6 m depth and lower, one can trace a division of the temperature profiles into 3 groups. The warmest group is represented by two icebergs detected in 5 near Severny Island of Novaya Zemlya (curves 1 and 2). One of these icebergs was detected in Inostrantsev Bay and the second in 54 miles to the west of it and in 22 miles to the NNW of Russkaya Gavan. In our opinion, this is already sufficient to consider the outlet glaciers extending along the northern part of the west shore of Severny Island (from Shokalsky to the Petersen glacier) as the origins of these icebergs. The ice temperature values at the lowest points of profiles (at a depth of m) of Novaya Zemlya icebergs were within -1-2ºС. This is by 2-3 degrees higher than the temperature at the same depth from the surface of the Shokalsky glacier according to the data half a century old presented above (Khmelevskoy, 1963). The second group of the temperature profiles in Fig. 6 refers to the icebergs, which were at the time of their detection so far from each other (3 4 nautical miles) that the distances between them are comparable with the horizontal dimensions of the Barents Sea. Profile 3 was obtained on the iceberg near Salm Island (FJL) in 5, profile 4 on the iceberg in the vicinity of the Shtokman GCF, profile 5 was adopted from (Løset, 1993) and belongs to the iceberg sighted in 199 east of Spitsbergen. The temperature values in the m horizons at profiles 3 and 5 comprise about -8ºС, which is very close to data obtained on the Sedov glacier and by 3 degrees higher than on the glacier of Salm Island. The lower temperature measurement point at profile 4 was located at a depth of only 6.4 m. However, based on the values of two parameters the temperature and the gradient 637

5 temperature in this horizon, this profile can be referred to the same group, which includes curves 3 and 5. From our viewpoint, the second group of icebergs can be with sufficient certainty referred by origin to the FJL icebergs. The third the coldest group of temperature profiles (curves 6-8) refers to the icebergs, observed not far from Spitsbergen in The temperature values at the low points of these profiles (at 6-11 m) are within ºС. Closeness of these icebergs to Spitsbergen (three of them simultaneously) suggests their origin at this archipelago. However, as noted above, Løset (1993) suggested that the icebergs under consideration were formed at the FJL, although he recognized that there was no reliable evidence of this. Of all available temperature measurements on glaciers, the most close to profiles 6-8 is the temperature profile obtained in 5 on Salm Island. The temperature values in the 6-11 m layer on this glacier were ºС. DISCUSSION AND CONCLUSIONS The results of comparing the available temperature profiles of several icebergs and glaciers suggest that most of icebergs considered here were generated on the FJL glaciers (excluding the two Novaya Zemlya icebergs surveyed in 5). One can speak with a large degree of certainty about the FJL origin of three of them, scattered over the entire Barents Sea area (regions of FJL 5; Spitsbergen 199 and Shtokman GCF 3). The suggestion about the origin at FJL of three icebergs detected in 1989 requires in our opinion additional investigations. In principle having available a very small volume of information presented in the paper, one can already now with some degree of certainty identify the region of generation of icebergs. For more reliable estimates, it is necessary to expand the data volume on temperature of icebergs and iceberg-producing glaciers. At present, this is one of the objectives of the work of a group of authors. Interesting results can be obtained by a combined use of thermistor chains and drifting buoys for investigating the prevailing motion directions of icebergs in the Barents Sea. A buoy deployed on the iceberg allows determining the drift trajectory and characteristics, but it cannot determine the iceberg history to the moment of its detection. The answer to this question can be obtained from the temperature profile measured by a thermistor chain. This is especially important for the study of icebergs sighted in the critical regions as in the case of detecting icebergs in the Shtokman GCF area in 3. Then the question where this iceberg came from becomes probably even more important than where to and with what speed it will further go. ACKNOWLEDGEMENT The study was made with the support of the RFFI grant No The deployment of thermistor chains on icebergs and glaciers in 3 and 5 was carried out in the AARI expeditions under the projects of comprehensive ice studies for the purpose of the Shtokman GCF development by the order of the CJSC Sevmorneftegas and some other oil and gas producing companies. We are also grateful to Dr. G.K. Zubakin and personnel of Arctic-shelf Laboratory of SI AARI for the discussion and valuable notes on the results of this study. REFERENCES Atlas of the Arctic (1985). Main Administration of Geodesy and Cartography under the USSR Council of Ministers, Moscow, 4 p. Buzin, I.V. (4). Monitoring of ice and icebergs as applicable to the objectives of development of the Shtokman gas-condensate field. Proc. of AARI, v.449, P Chikovsky S.S. (197). On the thermal influence of land ice on supercooling of seawater. Problems of the Arctic and the Antarctic, issue 33, p Chizhov, О.O. et al. (1968). 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