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Intensive Sea Surface Microlayer Investigations of Open Leads in the Pack Ice during Arctic Ocean 2001 Expedition Johan C. Knulst,* Dena Rosenberger, Brian Thompson, and Jussi Paatero Evolutionary Biology Centre, Department of Limnology, Uppsala University, Norbyva¨ gen 20, SE-752 36 Uppsala, Sweden Received June 17, 2003. In Final Form: September 30, 2003
During the third Icebreaker ODEN expedition to the North Pole, Arctic Ocean 2001, the surface of open leads between the pack ice was investigated for physical parameters. The major purpose was to evaluate the occurrence of a surfactant microlayer in the leads. This was done by estimating sea-surface tension and collecting sea-surface microlayers in open leads (SMOL). Three collection methods used for SMOL were rotating drums covered with hydrophilic Teflon, thin hydrophobic Teflon sheets, and glass plates. Collections were successfully made at 14 stations between 21 July and 21 August 2001, at geo-positions between 86°29′ N and the geographical North Pole. Surface tensions resembled surfactant-free seawater surfaces during the first 2 weeks of August but were depressed by 6-9 mN m-1 during the third week of August. Variations in SMOL physical properties were large between stations. Weather conditions and collection equipment functionality are discussed in relation to results.
Introduction Sea surfaces are important modulators of the exchange of energy and matter between air and water. The thin film of matter that forms the outer shell of the ocean surface, the sea-surface microlayer, has been shown to affect exchange processes between the two media.1,2 Seasurface microlayers are commonly defined by the way in which they are collected.3-7 Studies of microlayer properties have been conducted on freshwater lakes, seas, and temperate open oceans. The microlayer generally contains substances with surfactant properties, of which the majority have a biological origin in the bulk seawater.8 Surfactants are chemical agents with the ability to lower surface tension. Microorganisms can lower surface tension9 or degrade surfactants.10 Near shore microlayers are commonly heavily enriched by surfactants, due to the abundance of organisms present, and the wind-wave down mixing of bubbles that strip organic substances from the bulk water and move them to the air-water boundary as bubbles rise in the water column.11 Open ocean coverage of surfactants is believed to be low due to low intensities of organic matter present in the bulk water.12 Various * To whom correspondence may be addressed. Present address: IVL Swedish Environmental Research Institute Ltd., Aneboda Research Facility, SE-360 30 Lammhult, Sweden. E-mail: Johan.
[email protected]. (1) Liss, P. S.; Slater, P. G. Nature 1974, 247, 181-184. (2) Liss, P. S.; Watson, A. J.; Bock, E. J.; Jahne, B.; Asher, W. E.; Frew, N. M.; Hasse, L.; Korenowski, G. M.; Merlivat, L.; Phillips, L. F.; Schluessel, P.; Woolf, D. K. Physical Processes in the Microlayer and the Air-Sea Exchange of Trace Gases. In The Sea Surface and Global Change; Liss, P. S., Duce, R. A. Eds.; Cambridge University Press: Cambridge, U.K., 1996; pp 1-34. (3) Norkrans, B. Adv. Microb. Ecol. 1980, 4, 51-85. (4) Huhnerfuss, H. Meerestechnik 1981, 12, 137-142. (5) Hardy, J. T. Prog. Oceanogr. 1982, 11, 307-328. (6) Zuev, B. K.; Chudinova, V. V.; Kovalenko, V. V.; Yagov, V. V. Geochem. Int. 2001, 39 (7), 702-710. (7) Huhnerfuss, H. Meerestechnik 1981, 12, 170-173. (8) Hunter, K. A. Chemistry of the SeasSurface Microlayer. In The Sea Surface and Global Change; Liss, P. S., Duce, R. A., Eds.; Cambridge University Press: Cambridge, UK, 1996; pp 287-319. (9) Maki, J. S.; Hermansson, M. 1994, 2, 161-182. (10) vanGinkel, C. G. Biodegradation 1996, 7, 151-164. (11) Wotton, R. S. J. North American Benthological Soc. 1996, 15, 127-135.
methods have been employed to measure surface tension in marine locations.13-15 Leads are open water channels between ice floes. Studies of sea-surface microlayer in open leads (SMOL) and observations of surfactant presence in the central Arctic Ocean are lacking, and general knowledge about SMOL is mainly based on studies in Antarctica.16,17 Arctic seasurface microlayers have been mentioned as agents for transfer of pollutants to sea ice18 or as a source for airborne particles.19,20 Thus, to know the extent of microlayer presence in open leads and Arctic seas is important. Our working hypothesis is that sparse surfactants are present in SMOL of the central Arctic basin during a few weeks in summer. To resolve this hypothesis, an intensive surface survey was performed during the pack-ice zone (PIZ) part of this expedition. The survey was part of the atmospheric research program of the expedition. The overall objective of that program was to elucidate the biological, chemical, physical, and meteorological processes that control the influence of aerosol particles on climate change in the Arctic region as well as on a global scale.21 This study reports the findings from sea-surface studies inside the PIZ of the central Arctic Ocean, conducted from the Icebreaker ODEN during the Arctic Ocean 2001 expedition at latitudes above 86° N. (12) Williams, P. M.; Carlucci, A. F.; Henrichs, S. M.; Vleet, E. S.; Horrigan, S. G.; Reid, F. M. H.; Robertson, K. J. Mar. Chem. 1986, 19, 17-98. (13) Gericke, A.; Huhnerfuss, H. J. Colloids Interface Sci. 1989, 131, 588-591. (14) Williams, P. M.; VanVleet, E. S.; Booth, C. R. J. Mar. Res. 1980, 38, 193-204. (15) Adam, N. K. Proc. R. Soc. London, Ser. B 1937, 122, 134-139. (16) Loglio, G.; Degli Innocenti, N.; Tesei, U.; Stortini, A. M.; Cini, R. Anal. Chim. Acta 1989, 79, 571-587. (17) Grotti, M.; Soggia, F.; Abelmoschi, M. L.; Rivaro, P.; Magi, E.; Frache, R. Mar. Chem. 2001, 76, 189-209. (18) Pfirman, S. L.; Eicken, H.; Bauch, D.; Weeks, W. F. Sci. Total Environ. 1995, 159, 129-146. (19) Nilsson, E. D.; Rannik, U.; Swietlicki, E.; Leck, C.; Aalto, P. P.; Zhou, J.; Norman, M. J. Geophys. Res. 2001, 106 (D23), 32, 139-32, 154. (20) Leck, C.; Norman, M.; Bigg, E. K.; Hillamo, R. J. Geophys. Res. 2002, 107 (D12), 10, 1029-10, 1046. (21) Leck, C.; Matrai, P.; Swietlicki, E.; Tjernstro¨m, M. Swed. Polar Res. Secretariat Yearbook 2001 2001, 67-75.
10.1021/la035069+ CCC: $25.00 © 2003 American Chemical Society Published on Web 11/01/2003
Surface Monolayers on Arctic Ocean Open Leads
Figure 1. Map of the Arctic Ocean showing the locations of the stations during Arctic Ocean 2001 expedition, at which successful surface microlayer collections were made. Inset shows the geopositions in degrees and minutes longitude/latitude of the drift stations.
Locations and Methods Expedition Route. The expedition with Icebreaker ODEN started from Gothenburg, Sweden, on 26 June 2001 and ended at Longyearbyen, Svalbard, on 29 August 2001. Stations were made in the open ocean, marginal ice zone (82°-86° North) and PIZ (above 86° N). Sea-surface microlayer investigations were only conducted in the PIZ (Figure 1). Most of the atmospheric program was conducted during the drift experiment when the icebreaker was moored to a 1 by 2 km ice floe of 2-3 m thickness for 3 weeks in August. Leads. The leads in this study had dynamic sizes and shapes. At most sites, the open water channel was between a few to 150 m wide, with varying lengths. Within the sampling time of a few hours, the leads frequently either opened up or closed to narrow channels. On some occasions, a thin (a few millimeters to a few centimeters thick) ice sheet covered the lead edges. A few smaller ice floes sometimes floated in the leads. During station stops, the sea-surface microlayer study crew and equipment were driven out by snowmobiles to sites at open leads as far away from the icebreaker as the situation permitted. In some cases the site was reached on foot or skis. Sites were chosen so that they provided relative polar bear safe places, a large enough water surface to run the SMOL collectors, and ice edge conditions safe enough to approach the lead. According to ice coverage surveys during the expedition, the investigated area had an ice coverage of about 90% during August 2001. Sea-Surface Microlayer Collection. Microlayer material was collected by three methods simultaneously. The
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methods were comprised of the rotating drum collection method first described by Harvey,22 the glass plate dipping method by Harvey and Burzell,23 and the Teflon sheet dipping method described by Kjelleberg et al.24 Two battery-operated catamaran type vessels25 were used side by side (Figure 2a) to collect large (several liters) volumes of microlayer material from the open leads at expedition stations (Table 1). Each vessel was fitted with a rotating drum covered with a thin sheet of sodium in liquid ammonia-edged (hydrophilic) Teflon film. One of the catamaran type vessels, the INTERFACE 2 (Figure 2a, foreground, 2.0 m length × 1.4 m width × 0.5 m height), collected micrometeorological data and drum rotation counts during field operations. SMOL collected by the smaller of the two vessels, the INTERFACE 1 (0.9 × 0.3 × 0.3 m), are further referred to as Drum 1 samples. SMOL from the INTERFACE 2 are Drum 2 samples. The catamarans were deployed manually from the ice edge and were navigated via FM bandwidth radio control units. The microlayer collecting vessels were randomly navigated across as much of the open lead as possible to get a representative sample for the lead during that time slot. Care was taken to not run the collector through its own wake in order to reduce possible contamination of the sample. Although problems with the FM radio control devices were anticipated, no problems due to radio interference or weather conditions occurred. At some stations it was difficult to deploy the collectors due to ice edge conditions where a thin sheet of ice at the edge of the lead had to be removed. Samples were collected in 2.5 L borosilicate glass bottles which were transported back to the laboratory on ODEN packed in snow and ice in dark coolers. Operational collecting time varied between 20 min and 2 h per run. In the laboratory, collected volumes were determined by weighing. Glass plate (Plate) collections (Figure 2b) were done by dipping a 25 cm square sheet of clean 3 mm glass into the lead water perpendicular to the water surface and retracting it at constant speed. Clean glass plates were transported in cooler boxes. The plate was attached to a stainless steel (STS) laboratory type handle before being dipped into the lead water. The glass surface on either side of the plate was then wiped off into a borosilicate glass bottle through a glass funnel, using a neoprene lined squeegee. The collected sample was packed in ice and snow in a cooler and transferred to the laboratory on ODEN. In the laboratory, collected volumes were determined by weighing. The Teflon sheet dipping equipment (Figure 2c) was the same as that used in the Kattegatt Sea24 and previously in the Arctic Ocean.26 A thin sheet of untreated, precleaned Teflon was attached to a STS frame. The frame had a surface of 0.0225 m2. This surface was lowered parallel to the water surface just until contact, and subsequently raised after several seconds. The STS frame was then placed in a holder, after which the Teflon sheet was removed by cutting along the edges of the surface with a STS scalpel. The loose Teflon sheet was picked up with STS forceps and placed in a STS rack. The rack had slots (22) Harvey, G. W. Limnol. Oceanogr. 1966, 11, 608-613. (23) Harvey, G. W.; Burzell, L. A. Limnol. Oceanogr. 1972, 17, 156157. (24) Kjelleberg, S.; Stenstrom, T. A.; Odham, G. Mar. Biol. 1979, 53, 21-25. (25) Knulst, J. C. Interfaces in aquatic ecosystems: Implications for transport and impact of anthropogenic compounds. Ph.D. Thesis, Lund University, Lund, Sweden, 1996. (26) Dahlba¨ck, B.; Gunnarsson, L. A° . H.; Hermansson, M.; Kjelleberg, S. Mar. Ecol. (Prog. Ser.) 1982, 9, 101-109.
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Figure 2. (a) Two battery-operated, radio-controlled, rotating drum microlayer collectors skimming an open lead. (b) Glass plate collection by the open lead. (c) Teflon sheet collection. (d) Oil drop surface tension estimation in an open lead during expedition Arctic Ocean 2001.
for 10 sheets and was kept in a STS watertight box. After 10 samples were loaded, the box was closed and brought into the laboratory packed in a cooler with icepacks. Sea-Surface Tension. The sea-surface tension was estimated by the oil drop method (Figure 2d) first described by Adam15 and modified by Barger.27 This test included a set of 2 mL vials containing mixtures of light mineral oil (#M-3516, Sigma Chemicals Co., St. Louis, MO) and lauryl alcohol (1-dodecanol, #L-5375, Sigma Chemicals Co., St. Louis, MO). Calibration was performed in a Langmuir film balance on double distilled water held at +4 °C. Each vial contained a nominal value of 0.5 mN m-1 intervals between 50 and 73.5 mN m-1. A clean wooden toothpick was first dipped in the vial and then dropped onto the water surface. Visual assessment of the spreading was recorded. The spreading indicated oil film surface pressure relative to the ambient water surface pressure. At most of the locations, the oil drop test was performed on the downwind side of the lead, since the wind can push surfactants toward that side. At several stations the drop method was not performed due to ice mush on the water surface. Miscellaneous. Wind speed (m s-1, Z ) 0.8 m, 1 min averages, 40 measurements min-1) and water temperatures (30 s averages, 26 measurements min-1) were logged onto a Campbell Scientific CR10 data logger aboard the surface microlayer collector INTERFACE 2 (Figure 2a). The temperature probes were micro T-type thermocouples (0.3 mm tip) inserted in a Teflon mounting rod. One was placed ahead of the catamaran bow, with the tip extending forward at a depth of 0.2 m. The other probe was inserted in the Teflon microlayer collection tube on top of the scraper. Air temperature, wind speed and direction, (27) Barger, W. R. ORNL Ocean Branch Laboratory, Washington, D.C., 1994, personal communication.
relative humidity, visibility, and atmospheric pressure were monitored continuously aboard ODEN. Wind speed was monitored aboard the INTERFACE 2 during three of the drift stations, using a cup anemometer at Z ) 0.8 m. During two collection periods, at stations SMOL 5-6 and SMOL 5-9, seals surfaced near the drum collectors while they operated. Otherwise, no animals were visible during operations. Data Analysis. Sampled SMOL thickness was calculated by dividing the surface area swept by the drum by the volume of SMOL collected. For glass plates, the plate’s total wetted area was divided by the collected SMOL volume to calculate average SMOL thickness. Temperature and wind records collected by the INTERFACE 2 catamaran were compared with the continuous record from the ODEN weather station (Z ) 36 m). Temperature records were collected by the catamaran only at stations SMOL 5-7, 5-8, and 5-9 (Table 1). Comparison of wind speed, mean air temperature, relative humidity, visibility, SMOL thickness, and surface tension was made by time resolved t-tests where time of collection was matched up. For illustrative purposes the relationship between variables was plotted as X-Y diagrams and checked with least-squares regression analysis. The data discussed in the text refer to the t-test analysis, if not mentioned otherwise. The few wind measurements from the catamaran show strong influences of ice topography on the near surface wind field. The wind factor (Pw) discussed is an approximate measure of the wind power during sampling collection, calculated as Pw ) 1/2ld(U)3, where l is the estimated fetch distance (m), d is air density (approximated from air pressure in hPa), and U the average wind speed (m s-1) measured at Z ) 36 m.
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Table 1. Listing of the Stations at Which Successful Surface Investigations Were Performed during the AO-01 Expedition, Accompanied by the Types of Sample Collected, Thickness of Collected Microlayer (Depth), and Some Physical Dataa date
time in
time out
type
20 Jul 01 21 Jul 01 22 Jul 01
22:37 16:10 16:35 17:35 11:02 13:22 16:48 15:35 16:48 16:50 17:55 14:30 15:38 15:15 15:46 20:18 20:32 9:35 10:16 10:48 11:25 11:30 14:21 14:32 15:25 15:18 14:46 15:22 16:02 14:38 15:30 16:22
23:05 17:05 17:40 18:30 13:18 13:55 17:48 15:48 17:48 17:10 18:35 16:00 16:00 16:38 16:08 21:12 21:20 10:40 11:05 14:08 11:42 11:48 15:12 15:21 16:02 16:12 15:55 16:25 16:32 15:10 16:15 17:15
sheet sheet drum 2 sheet drum 2 sheet drum 1 sheet drum 1 sheet plate drum 1 sheet drum 2 plate drum 1 drum 2 drum 1 drum 2 drum 1 plate sheet drum 2 drum 1 drum 1 drum 2 drum 2 drum 1 drum 2 drum 1 drum 1 drum 1
26 Jul 01 04 Aug 01 05 Aug 01 08 Aug 01 10 Aug 01 12 Aug 01 15 Aug 01
17 Aug 01
19 Aug 01
20 Aug 01
station 2-16 2-18 2-21 4-1 SMOL 5-1 SMOL 5-2 SMOL 5-3 SMOL 5-4 SMOL 5-5 SMOL 5-6 SMOL 5-7
SMOL 5-8
SMOL 5-9
SMOL 5-10
depth
fetch
wind speed
na na 15 na 10 na 23 na 26 na 15 13 na 21 15 21 41 6 7 (12) 24 na 38 23 23 40 36 24 83 48 34 24
10-20 10-20 >100 20-40 10-20 10-20 2-40 na 60-100 60-100 60-100 20-40 2-20 2-20 2-20 20-40 20-40 20-40 20-40 20-40 20-40 20-40 40-60 40-60 20-40 20-40 10-40 40-60 40-60 10-40 20-40 20-40
4.2 4.4 8.4 8.8 5 4.9 3 6.1 6.3 6.3 6.6 2.6 6.5 6.3 6.2 5.1 5.2 3.1 4 5.1 4.7 4.7 4.2 4.3 4.7 4.2 4.1 4.4 4.7 4.1 5.8 6.5
a Time is given in hours:minutes UTC. Wind fetch (fetch, m) estimated visually. Average speeds (wind speed, ms-1) are calculated from measurements aboard ODEN deck 7 (Z ) 36 m). na means not established. Depth in micrometers. Value in parentheses is uncertain.
Results and Discussion Microlayer Thickness. The drum collectors gathered a water layer between 6 and 83 µm thick (Table 1). The glass plate gathered roughly 15-24 µm thickness of water, and Teflon sheets were shown to collect 2-3 µm of water in the laboratory.26 No field measurements were made to confirm the Teflon sheet collected thickness during this expedition. The operation of the sheet collector had several drawbacks for application in open leads. The major problem was that it was difficult to get a sample manually from the floe and free from ice edge effects. The same problem hampered the use of the glass plate. The use of a dinghy was not considered due to swift ice floe dynamics and possible contamination of the lead surface. Field Conditions. Figure 1 shows the locations of the stations at which successful sea-surface investigations were performed. Station time and collected SMOL thickness are listed in Table 1. Weather conditions were dominated by a low cloud base and air temperatures between -6 °C and +1 °C. Average daily temperatures were mostly near 0 °C (Table 2). Lowest temperatures occurred during sunny and calm conditions with good visibility (Table 2), such as on August 8, 15, and 17. When conditions in the field were colder than -5 °C, or snowfall occurred, a slurry would form on the water surface which caused the drum collectors to push aside the upper layer of the lead water. When that happened, collection attempts were abandoned. In some instances the microlayer would freeze on the collector surface, but the produced slurry was collected nevertheless. The wind fetch on the open lead was seldom long, and wind speeds were generally below 6 m s-1 during operation
of the collectors (Table 1). Surface water temperatures during the drift experiment were steady, with a mean water temperature near -1.7 °C at 0.20 m depth recorded at three stations. Surface temperatures, measured in the collected flow of SMOL, were 0.1-0.5 °C lower than the corresponding bulk water temperature at 0.2 m depth at the three stations and varied somewhat over time and between stations. The collected SMOL liquid often showed evidence of supercooling by the instant formation of ice crystals in the drum collection bottles. Note that the collection bottles were in continuous contact with the surface water during sampling operations. Surface Tension, Meteorology, and Surfactant Properties. The oil drop method used to estimate the surface tension of open leads (Table 3) has been used in other marine areas,28 but no evidence was found for it having been used in the Arctic Ocean. Viscosity and spreading behavior of the oil mixture is temperature dependent. Although this batch of oil mixtures had been calibrated at lower than room temperature, it is uncertain how much the ambient water temperature effected the instantaneous spreading behavior. The vials containing the oil were kept in a heated cooler to avoid excessive cooling during field operations. Since the established values (Table 3) covaried with the ability of the collectors to remove a certain volume of SMOL (Figure 4), there appears to be some reliability in our data. Zhang and Pelton13 compared surface tension of polymeric microgels in a Langmuir balance with several factors and found that temperature changes made little difference for (28) Peltzer, R. D.; Griffin, O. M.; Barger, W. R.; Kaiser, J. A. C. J. Geophys. Res., C: Oceans 1992, 97, 5231-5252.
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Table 2. Daily Average Weather Statistics Calculated from the ODEN Weather Station Data during the Surface Microlayer Collection Period
date
air min air max air pressure, temp, temp, temp, % rel visibility, hPa °C °C °C humidity km
20 Jul 01 21 Jul 01 22 Jul 01 23 Jul 01 24 Jul 01 25 Jul 01 26 Jul 01 27 Jul 01 28 Jul 01 29 Jul 01 30 Jul 01 31 Jul 01 01 Aug 01 02 Aug 01 03 Aug 01 04 Aug 01 05 Aug 01 06 Aug 01 07 Aug 01 08 Aug 01 09 Aug 01 10 Aug 01 11 Aug 01 12 Aug 01 13 Aug 01 14 Aug 01 15 Aug 01 16 Aug 01 17 Aug 01 18 Aug 01 19 Aug 01 20 Aug 01
1017 1017 1012 1013 1015 1015 1011 1002 1004 1004 1007 1004 1007 1000 991 999 999 1003 1006 1013 1014 1006 997 1004 1009 1010 1006 999 996 997 997 1010
-0.3 -0.8 0.5 -1.2 -0.9 -1.2 -1.3 -0.9 -1.6 -2.0 -1.6 -1.4 -1.4 -0.6 -0.6 -0.5 -0.2 -0.6 -0.6 -1.7 -0.7 -1.7 -0.2 -2.3 -2.3 -3.0 -4.7 -1.3 -0.4 -1.7 -0.8 -1.8
-1.5 -4.9 -1.2 -4.9 -2.3 -4.5 -2.8 -2.1 -3.5 -3.8 -2.7 -2.8 -3.2 -1.3 -1.7 -1.3 -1.0 -1.7 -2.0 -2.2 -2.5 -2.7 -2.8 -3.6 -3.5 -4.3 -6.3 -3.1 -1.5 -2.6 -2.2 -5.5
1.7 0.6 3.2 -0.5 0.0 0.0 0.0 0.0 0.0 -0.4 -0.7 0.0 0.9 -0.1 0.9 0.6 0.3 0.1 0.8 1.1 0.8 0.9 0.5 0.0 -0.4 -1.7 -2.4 0.1 0.0 0.0 0.3 -0.4
94.0 97.5 96.1 95.7 94.2 89.5 92.1 91.7 88.9 92.2 88.0 89.7 93.7 93.0 93.4 95.5 95.5 92.5 93.8 86.2 93.5 93.7 97.4 93.5 90.5 90.9 90.1 95.4 93.5 87.7 94.3 92.0
48 35 52 3.5 27 49 55 61 44 76 75 36 24 40 19 36 37 58 46 73 63 33 12 57 70 46 58 24 13 71 25 51
Table 3. Surface Tension Estimates with Adam’s Oil Drop Method (mN m-1) for Stations at Which Surface Investigations Were Conducted during AO-2001a surface tension
wind factor
21 Jul 01 22 Jul 01 27 Jul 01 31 Jul 01
Predrift 2-19 02:30 2-21 17:35 3-1 12:55 north pole 09:15
69.0 62.0 73.0 72.5
0.56 375 1.12 4.74
04 Aug 01 05 Aug 01 08 Aug 01 10 Aug 01 12 Aug 01 15 Aug 01 17 Aug 01 19 Aug 01 20 Aug 01
SMOL 5-2 SMOL 5-3 SMOL 5-4 SMOL 5-5 SMOL 5-6 SMOL 5-7 SMOL 5-8 SMOL 5-9 SMOL 5-10
Drift 17:50 17:10 16:10 16:30 20:40 10:25 16:05 16:02 16:40
71.5 73.0 71.5 72.0 71.5 69.0 67.5 65.0 70.0
2.81 10.0 0.27 1.58 2.00 0.70 1.85 1.27 3.61
date
station
timeb
a See text for an explanation of the wind factor. b Time is given in hours:minutes UTC.
obtaining a surface tensional steady state. Under colder than room-temperature conditions, the ambient surface tension is somewhat higher due to slower molecular diffusion.14 Also, the spreading speed of the oil is slowed slightly by low ambient temperatures.13 The first predrift stations (stations 2-x, 3-1, and 4-1, Figure 1) showed evidence of a surfactant film (Table 3), likely due to the influences of the circumpolar currents originating in the marginal ice zone with an expected higher biological activity. Wind factors had a greater influence on the open water surfaces of the marginal ice zone than inside the PIZ. No visible slicks were observed in open leads in the PIZ, although slicks were a common
Figure 3. Temporal variation in surface tension (open squares) and of collected thickness of surface microlayer (solid tilted squares) on open leads in the central Arctic Ocean.
Figure 4. Relationship between estimated surface tension and thickness of the collected sea-surface microlayer from open leads (SMOL) during the drift experiment.
phenomenon in the marginal ice zone (not presented here). Some smooth (dampened short waves) surfaces were present in the leads studied. These smooth areas, however, were located on the leeward side of ice obstructions or under calm conditions. Data from stations during the drift, for which the water analyses reveal low levels of organic matter, imply low surfactant availability by having a surface tension close to that of clean seawater (near 73.0 mN m-1) during the first 2 weeks of August. In the last week of the drift, however, both surface tensions and collected microlayer samples reveal increased presence of surfactants in the open leads by decreasing surface tension (Figure 3). It is hypothesized that the short arctic (biological) summer had begun during the last week of the drift experiment. Only 1 day, 17 August, revealed visible quantities of algae in each of the collected SMOL samples. On that day, the scrapers on both drum collectors were covered with a slimy, brown mush. Williams et al.12 reported reduction of oceanic surface tension in a comprehensive study of surface characteristics in the Pacific Ocean off the coast of California. In that study, near shore surface tension was depressed by 5-6 mN m-1 with lowered wind speeds and during certain times of the day. In open ocean locations, the surface tension depression was more patchy and varied between 1 and 6 mN m-1 during daylight hours. Heavily slicked sea surfaces were found to have surface tensional depressions greater than 16 mN m-1. Our single measurement a day means that we may have missed diurnal events of lowered surface tension in the Arctic. The patchiness and regime of the
Surface Monolayers on Arctic Ocean Open Leads
surface tensional depressions in our study are similar to the ones reported by Williams et al. for the open Pacific Ocean. SMOL enrichment with surfactants is caused either by biological activity in the water column or by the active transfer of surfactants from the water column to the surface by breaking waves and bubbles. In the central Arctic Ocean, however, biological activity is suspected to be low and there is not much wave action inside the PIZ, where wind factors generally are low. Therefore, surfactants are not expected to be present in great amounts in the SMOL. A slight raise in biological activity is thus easily witnessed by the increased presence of surfactants at the lead surface (Figure 3). The relationship between collected microlayer depth and the estimated surface tension is significant (P < 0.05) with a negative coefficient during the drift (Figure 4). Wind factors varied in this study, while the opening and closing of leads is dependent on the wind factor as well as on sea currents. The question remains whether there is extensive near surface turbulence in the lead that can either induce rising bubbles in the water column or cause sudden surface enrichment with particles and associated surfactants by other means. At low wind speeds for which wind waves are not generated (
