Langmuir Circulation Driving Sediment Entrainment Into Newly Formed

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, C02004, doi:10.1029/2005JC003259, 2007 Click Here for Full Article

Langmuir circulation driving sediment entrainment into newly formed ice: Tank experiment results with application to nature (Lake Hattie, United States; Kara Sea, Siberia) Dirk Dethleff1 and E. W. Kempema2 Received 30 August 2005; revised 20 July 2006; accepted 5 September 2006; published 3 February 2007.

[1] Langmuir circulation (Lc) was generated under freezing conditions in saltwater tank experiments through surface wind stress and cross-waves interacting with subsurface return flow. Fine-grained sediments distributed in the tank prior to frazil crystal formation were aligned in parallel streaks in Lc bottom convergence zones. Downwelling at Lc surface convergence zones aligned floating frazil in wind-parallel rows, and individual crystals rotated on helical paths down to the tank bottom and up again to the surface. The crystals interacted with suspended particles in the water column, and with sediment on the tank bottom, preferentially collecting fine-grained particles and enhancing their entrainment into new ice. Evidence includes higher sediment concentrations in ice and ice- interstitial water (ice pore water) as compared to the tank water. Both tank ice and ice interstitial water contain more silt-sized particles than tank water suspension load and tank bottom sediment. Sand is reduced in the ice, and clay is about the same concentration in all samples. This points to preferential entrainment of fine particles in newly formed ice supported by Lc-driven circulation. Comparable results of Lc-supported ice particle entrainment were found in Lake Hattie. Comparison of ice sediment from tank experiments run with Kara Sea material to ice particles from the natural Kara setting showed both types of ice sediment have very similar grain size distributions and mineralogical compositions. Results from experiments and nature help to better understand the potentially Lc-driven entrainment of sediment into ice formed in shallow freezing waters. Citation: Dethleff, D., and E. W. Kempema (2007), Langmuir circulation driving sediment entrainment into newly formed ice: Tank experiment results with application to nature (Lake Hattie, United States; Kara Sea, Siberia), J. Geophys. Res., 112, C02004, doi:10.1029/2005JC003259.

1. Background and Introduction and Thorpe [2004]. The downwind propagating, convergent cell rotation characteristic of Lc is generated by the inter- [2] Langmuir circulation (Lc) is a wind and wave driven action of wind-induced Stokes drift and surface gravity helical flow pattern in lakes and seas. Lc consists of parallel, waves with vertical turbulence resulting from wind-driven counter-rotating vortex pairs oriented roughly parallel to the cross-wave trains and surface currents [Craik and wind direction [Langmuir, 1938]. The surface convergence Leibovich, 1976]. Lc cell width (defined here as a pair of zones of the counter-rotating cells, where water and mo- counter rotating vortices) may vary from a few cm to mentum are transported downward into the flow interior are hundreds of m, and the vortices can be many km long marked by long, linear, subparallel surface streaks of [Thorpe, 2004]. Downwelling velocities at surface conver- collected foam, bubbles, seaweed or other floating debris gence zones of Lc cells generally exceed upwelling veloc- [e.g., Weller et al., 1985; Faller and Auer, 1988; Zedel and ities at surface divergences [Weller et al., 1985; Smith et al., Farmer, 1991]. 1987]. Maximum downward velocities in unstratified water [3] The complex origin of Lc has been the subject of under fully developed seas are about 0.8% of the horizontal numerous field studies and tank experiments [Craik and wind speed measured at 10 m altitude. Downwelling Leibovich, 1976; Faller and Caponi, 1978; Veron and 1 velocities of up to 22 cm sÀ have been measured in the Melville, 2001; Matsunaga and Uzaki, 2004]. Comprehen- open ocean [Weller et al., 1985]. Lc cell width tends to be sive reviews are given by Leibovich [1983], Smith [2001], 2–2.5 times the mixed layer depth, so individual roll vortices are roughly circular in cross section and approxi- 1Institute for Polar Ecology, Kiel, Germany. mately represent the water depth penetrated. 2Department of Geology and Geophysics, University of Wyoming, [4] Lc has been generated in laboratory tanks by surface Laramie, Wyoming, USA. shear stress and cross wave action induced by wind only, or by the combination of wind driven surface currents and Copyright 2007 by the American Geophysical Union. mechanically produced waves [e.g., Faller and Caponi, 0148-0227/07/2005JC003259$09.00

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crystals. Dethleff [2005] concludes that fine-grained sediment may be preferentially entrained into newly formed sea ice by convective, turbulent Lc on the broad Laptev Sea shelf. [7] The entrainment of sediment into newly forming ice has been investigated and modeled on different shallow circum-Arctic shelves and in lakes [Osterkamp and Gosink, 1984; Reimnitz et al., 1987; Kempema et al., 1989; Reimnitz et al., 1998; Lindemann, 1998; Dethleff et al., 2000; Eicken et al., 2000; Stierle and Eicken, 2002; Darby, 2003; Smedsrud, 2003; Dethleff, 2005; Eicken et al., 2005]. Many of these studies report that the sediment concentration in ice far exceeds the concentration of suspended particulate Figure 1. Sketch of Langmuir circulation in tank experi- matter (SPM) in the water column when the ice formed, ments showing counter rotating Lc roll vortices, surface so various processes of enrichment (enhancement of the convergence zones marked by streaks of floating frazil (grey sediment content in ice compared to the water column) are row), and the downwind surface flow and upwind return proposed to form sediment-laden ice. Sherwood [2000] flow of water in the tank. An Lc cell consists of two counter modeled coconcentrations of suspended particles and frazil rotating rolls. ice formed in freezing water, which were in the range of sea ice sediment concentrations found in turbid Arctic sea ice, but were lower than maximum concentrations observed. He 1978; Faller and Cartwright, 1983]. Because of the limited concluded that no enrichment processes are necessary to size of laboratory tanks, wind- and wave-induced surface generate sediment concentrations observed in turbid Arctic currents deflected at the downwind tank boundary generate sea ice. a subsurface, upwind return flow near the tank bottom [8] Processes including tidal pumping, wave action, floe- [Mizuno and Cheng, 1992]. The primary flow consisting bulldozing, nearshore seabed freezing of grounded ice floes, of surface downwind and bottom upwind current compo- and beach-ice formation have been invoked as mechanisms nents finally results in a circular, wind-parallel helical of sediment entrainment into sea ice [e.g., Reimnitz et al., secondary flow [Mizuno et al., 1998; Uzaki and Matsunaga, 1987; Rearic et al., 1990]. Campbell and Collin [1958] 2000], which is generally accepted as Lc in tank experi- introduced the term ‘suspension freezing’ for the process ments (Figure 1). Lc in natural water bodies with no where frazil and sediment suspended in the turbulent water boundaries (where most natural Lc has been studied) form column interact, leading to the formation of sediment-laden long downwind roll vortices, with a different net circulation sea ice. The concept of ‘suspension freezing’ was further pattern than observed in tanks. However, in both tank and developed by Reimnitz et al. [1992], and by Reimnitz et al. natural waters, the primary Lc characteristic is the series of [1993a] to include suspended frazil entraining sediment parallel-oriented convergent rolls in the cross-wind/vertical directly from the shallow sea bed into the growing ice plane (Figure 1). cover. [5] Many studies document that Lc vertically distributes [9] During fall freezeup storms, suspension freezing can gas, heat, and momentum throughout the oceanic surface occur over large areas of the Arctic continental shelf. mixed layer or even down to the bottom of shallow water Supercooling of the water leads to frazil formation. Subse- bodies [Faller, 1969; Scott et al., 1969; Weller et al., 1985]. quent scavenging and/or filtration of suspended particulate Bees [1997] investigated the impact of Lc on the advection matter by buoyant rising ice crystals as well as resuspension of planktonic communities. Uzaki and Matsunaga [2000] of bottom material by uplifting anchor ice promote the suggested that Lc plays a role in near coastal sediment entrainment of fine-grained particles into the ice cover transport processes, and Gargett et al. [2004] showed that [Osterkamp and Gosink,1984;Reimnitz et al., 1993b, Langmuir cells penetrating a shallow water column down to Dethleff, 2005]. Once the annual ice cover forms, the the seabed are an important mechanism for major sediment process of suspension freezing is restricted to flaw leads resuspension (redistribution of bottom sediment throughout (long extended narrow bands of open water between fast ice the water column) and sediment transport events on mid- and drifting ice) and polynyas. latitude continental shelves. [10] The term ‘suspension freezing’ is used in the litera- [6] The role that Lc may play in bringing suspended frazil ture to imply turbulent entrainment of sediment into ice, and sediment into contact in the water column, leading to though the details of this process remained unclear. The sediment entrainment into seasonal ice covers, has received purpose of this study is to examine the potential importance little study. Recent research suggests Lc as a mechanism for of Lc on the process of suspension freezing, with the goal of suspending frazil in the water column [Drucker et al., progressing to a better understanding the mechanisms that 2003], or for driving the entrainment of sediment into newly lead to sediment-laden sea ice. Freezing tank experiments formed ice [Dethleff, 2005]. Drucker et al. [2003] used were conducted to examine the hypothesis that Lc promotes satellite observations and upward looking sonar in the fine-grained sediment entrainment into new ice in open Bering Sea to document that streaks of frazil on the water. The purpose of the tank experiments was to produce water surface were correlated with increased particles in Lc under controlled freezing conditions and to document the water column. Based on several lines of evidence, sediment entrainment into newly formed ice. We then they conclude that the particles were suspended frazil compare our tank results with field observations in Lake

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Figure 2. Location map (a) and individual charts (b) of Wyoming (United States), Albany County (bold lined area in midpanel), and Lake Hattie, and (c) the Kara Sea (Siberia, Russia). Black dot in the Lake Hattie sketch denotes the experiment site. Black dots in the Kara Sea chart display sampling sites. The stippled line along the Cape Yugorskyi coast shows the winter position of the Amderma flaw lead. Note the wind parallel surface frazil ice streaks in the flaw lead displayed on the photo in the Kara chart. Hamburg (Germany) is also indicated as the location of the tank experiments.

Hattie, Wyoming, and with data from shallow flaw lead Germany, in December 1998. The experiments were carried sites in the Kara Sea (Figure 2). out in a styrofoam-insulated tank measuring 3.2 m long by 0.5 m wide by 0.12 m deep (Figure 3). The tank was 2. Material and Methods installed in a 2 m by 5 m wind tunnel that was constructed in a walk-in freezer. The tank was filled with artificial salt 2.1. Setup of Freezing Tank Experiments 1 water (salinity: 33.5 psu), and winds of 5 to 7 m sÀ were [11] Five freezing-tank experiments were conducted at blown down the long axis of the tank by fans (Table 1). Air the Hamburg Ship Model Basin (HSVA) in Hamburg, temperatures during the experiments varied between À10°C

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Amderma region) and consisted of 13% sand, 59% silt, and 28% clay. In experiments 4 and 5 the sediment was a mixture of two shelf samples from Abrasimov Bay located at the eastern coast of Novaya Zemlya (Figure 2c). This composite sample contained 3% sand, 72% silt, and 25% clay. All the sediment is from areas of the Kara Sea where there is continuous ice growth throughout the winter [Martin and Cavalieri, 1989], and where the entrainment of fine-grained sediment into newly forming sea ice may occur in local flaw leads [Dethleff et al., 1998]. [14] Sediment was distributed on the tank floor test section (Figure 4) when the water temperature was still slightly above freezing. The sediment was allowed to settle Figure 3. Side view sketch of the freezing tank in the for several minutes in calm water before the fans were wind tunnel. started. After the fans started, the water supercooled and frazil formed. This frazil quickly accumulated on the surface at the downwind end of the tank, forming a layer of floating and À12.5°C in experiments 1, 2 and 3, and between grease ice that rapidly grew upwind. The grease ice was À3.5°CtoÀ6°C in experiments 4 and 5. Experiment 1 sampled with a small dip net (mesh size: 1.5 mm). Intersti- was performed with 8 cm deep water in order to document tial water (IW) was separated from the grease ice by tapping the formation and movement of individual frazil crystals in the dip net on the edge of a sample beaker. Water samples the tank. Experiments 2 to 5 were conducted with 4 cm were collected from under the grease ice layer with a 100 ml water depth to study the entrainment processes. syringe to determine SPM concentrations. The ice was [12] Parallel patterns of sediment on the bottom and frazil melted and all samples were filtered through preweighed on the surface highlighted Lc cells in the tank. Frazil and 0.45 mm Millipore filters. The filters were oven-dried at sediment distributions were recorded with video and still 63°C and weighed, and sample weights and water volumes cameras mounted above the tank. In addition, individual were used to calculate particle concentrations per liter ice, frazil crystals suspended in the water column showed the IW, and tank water (SPM). Grain-size distributions and flow structure in the water column. Formation and move- particle compositions of the samples were determined ment of such frazil crystals were recorded with an under- semiquantitatively in area percentage by examining the water video camera and a still camera positioned at the tank filter surface with a binocular microscope at 40-fold mag- sidewall. nification. The particles were classified in seven groups: [13] The sediment used in experiments 1 through 3 was quartz and feldspar, rock fragments, dark minerals, mica, obtained from the southwestern Kara Sea shelf (Figure 2c; biogenic material, plant debris and other material.

Table 1. Environmental Parameters and Particle Load in Suspension (SPM), Interstitial Water (IW), and Ice From the Freezing Tank Experiments, the Lake Hattie Investigations, and From the Kara Seaa Experiment/ Air Wind Water Sample Water Station Temperature, Speed, Depth, Salt Sample Weight, Volume, Particles, Enrichment Location Number °C msÀ1 m Content Type mg ml mg/l Ratio Tank 2 grease ice 4.28 146 29.32 2.18 Tank 2 À10 to À12.5 5–7 0.04 33.5 IWb 1.15 52 22.11 1.65 Tank 2 SPMc 1.41 105 13.43 - Tank 3 grease ice 2.03 97 20.93 1.43 Tank 3 À10 to À12.5 5–7 0.04 33.5 IW 0.63 32 19.69 1.34 Tank 3 SPM 1.54 105 14.67 - Tank 4 grease ice 4.23 115 36.78 1.53 Tank 4 À3.5 to À6 5–7 0.04 33.5 IW 1.30 42 30.95 1.28 Tank 4 SPM 2.53 105 24.10 - Tank 5 grease ice 4.00 128 31.25 1.59 Tank 5 À3.5 to À6 5–7 0.04 33.5 IW 0.86 39 22.05 1.12 Tank 5 SPM 2.07 105 19.71 - Lake Hattie 1 slush/frazil ice 3.45 620 5.56 1.81 Lake Hattie 1 À8 3–4 1 0 IW 2.67 495 5.39 1.76 Lake Hattie 1 SPM 2.86 933 3.07 - Lake Hattie 2 slush/frazil ice 2.77 470 5.89 1.12 Lake Hattie 2 À8 5–6 1 0 IW 3.45 455 7.58 1.44 Lake Hattie 2 SPM 4.87 925 5.26 - Kara Sea Amderma 1 - 5 11 - unconsolidated new ice - - visible - Kara Sea Amderma 2 3.6 11 40 - ice core 0–17cm 13.62 1968 6.92d - aThe ‘enrichment ratio’ values represent the excess particle load in IW and ice as compared to the suspension load. bIce interstitial water. cSuspended particulate matter. dFew g lÀ1 sediment in adjacent ice ridge.

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2.2. Lake Hattie day before complete lake freezeup in late November 2004. [15] Lake Hattie (Figure 2b) is a shallow fresh water Air temperature was about À8°C during the morning reservoir located 25 km west of Laramie, Wyoming, United observations. We collected two suites of SPM, IW, and States, at an altitude of about 2,200 m. The lake has an area ice samples using the same techniques as in the laboratory. of nine square kilometers. The ice and Lc investigations [16] The sampling sites were at 0.9–1.0 m water depth, were carried out on the southern shore of the lake during the 20 m from the shore with a 1.7 km upwind fetch. The northeast, obliquely onshore wind had a speed of 3–4 m sÀ1 duringthefirstsampling(waveheight:10cm),and5–6msÀ1 during the second sample collection (wave height: 15– 30 cm). Lake water was sampled for SPM using 1 liter plastic bottles.Newlyformedfloatingicewascollectedfrom streaks at Lc convergence zones on the lake surface with a dip net, and the IW was separated from the ice by tapping the net on the edge of a beaker immediately after sampling. Due to falling snow, we sampled ice that was an unknown mixture of frazil and snow, rather than the pure frazil we sampled in the tank experiments. The Lake Hattie sediment samples were processed in the same way as the tank samples. [17] Lake Hattie bottom sediment was sampled using a 60 ml syringe. The bottom is composed mainly of sand and gravel (90%), with 10% silt and clay. The bottom material was freeze-dried, embedded in resin on a smear slide and investigated under the microscope for further comparison with the SPM, IW, and ice samples. 2.3. Kara Sea Settings [18] Semienclosed between Novaya Zemlya in the west, Cape Yugorskyi in the south, and Yamal Peninsula to the east (Figure 2c), the shallow SW Kara Sea shelf is generally ice covered from November through June [Pavlov et al., 1994]. Initial new ice is produced across the entire region during fall freezeup. New ice production continues throughout the winter in shallow, nearshore polynyas and flaw leads [Martin and Cavalieri, 1989; Cavalieri and Martin, 1994]. The annually recurring Amderma flaw lead (Figure 2c, photo inset) borders a narrow band of shorefast ice off the coast of the Yugorskyi Peninsula. The lead forms widely over water 10 to 20 m deep, though deeper sections to as much as 40 m also occur. The entrainment of sediment into newly forming sea ice is thought to be most effective in such water depths on shallow Arctic shelves [Reimnitz et al., 1993b; Harms et al., 2000; Dethleff, 2005]. [19] Ice and seabed samples from the Amderma flaw lead were obtained at the same locations in late winter 1997. We collected wet, visibly turbid, unconsolidated ice attached to the fast ice edge over 11 m deep water at station Amderma 1 in the eastern flaw lead section (Table 1 and Figure 2c).

Figure 4. (a) Top view of tank test section (with rectangular 10 cm grid), showing streaks of upwind- migrating bottom sediment highlighted by thin dotted and solid black lines (surface frazil streaks not visible in this reproduction). (b) Sketch of alternating surface frazil ice streaks (oval dots) developing down wind, and bands of bottom sediment (stipples) seen in the tank experiments. Note the development of herringbone-like pattern on the sediment streaks initiated by the helical Lc flow (white- lined boxes in Figures 4a and 4b). Tank cross section (c) displays the cell rotation sense as well as the alternating frazil streaks and sediment bands in surface and bottom Lc convergent zones, respectively. Wind direction in Figure 4c is into the sheet.

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primary flow consisting of a surface downwind and a bottom upwind directed component was present in all tank experiments. Surface waves superimposed with the primary flow patterns thereby making the tank water column unsta- ble for the origin of helical Lc. Due to the strong upwind bottom component of the primary flow, the resulting subsurface Lc flow was characterized by upwind directed roll vortices. [23] When the cold air was blown across the tank surface, a series of parallel-crested, interfering waves propagated down the tank. In experiment 1 (8 cm water depth), the wave height was 1–2 cm, the wavelength was 8 cm, and the wave frequency was 5 sÀ1. In 4 cm water depth (experiment 2–5), the wave height was roughly the same, while the wavelength was 5–7 cm, and the wave frequency was 7–9 sÀ1 with a more irregular and interference pattern. Primary flow and subsequent Lc were present in all experiments within a few seconds after the wind-induced wave patterns were established. [24] The occurrence of Lc in our experiments could be Figure 5. Trajectory of one frazil ice crystal in the wind- firstly determined by the development of parallel streaks of parallel/vertical plane in the tank as traced from 11 fine-grained sediments on the tank floor in Lc bottom individual frame captures from a 10 s video record. The convergence zones (Figure 4a). At the time of bottom streak starting point is indicated by the frazil in the black ‘O’; development, the water cooled to freezing point. Frazil was subsequent positions are superimposed on the initial frame. visible throughout the entire water column after 12 minutes The counter rotating rolls characteristic of Lc are in the in experiment 2, and after 4 to 7 minutes in experiment 3–5. cross wind/vertical plane, perpendicular to the plane of this The frazil probably originated from seed crystals that picture. These rolls are sketched in the cartoon in the bottom dropped into the tank from the surrounding air, and grew left of the figure, indicating the three-dimensionality of the rapidly to crystals of about 10 mm in diameter. Some vortices. Direction of frazil movement on the inset is into individual crystals agglomerated to form flocs up to the paper plane analogous to the left-to-right propagation 15 mm in diameter. (along the tank long-axis) of the crystal traced on the tank [25] The frazil crystals were aligned in windrows in the photo. Wind direction is from right to left. Black triangles downwelling surface convergent zones of Lc cells indicate further crystals in the water column. Note vertical (Figure 4b). Some frazil remained permanently in the centimeter scale in the left background (8 cm water depth) surface windrows, but many crystals were advected down- and the thin layer of sediment on the shiny tank bottom. ward into the flow interior (Figure 5). These ice crystals and frazil aggregates were forced by Lc against buoyancy into Additionally, one ice core was collected close to the fast ice the tank water column, and moved downward with a edge at station Amderma 2 (Figure 2c) over 40 m deep water. velocity of 2–2.5 cmsÀ1 (Figure 5). The individual crystals We sampled the uppermost 17 cm of the core, which described a helical path (see inset in Figure 5) all the way consisted of granular ice formed under turbulent open water down to the bottom, where some of the crystals rested for conditions. At the same site we also collected sediment laden few seconds, and then lifted up again to the surface in up- ice from a pressure ridge. The seabed sediment samples at welling Lc bottom convergence zones. Upward-directed both stations were obtained with an Ekman-Birge grab. velocities in Lc bottom convergence were slower than [20] Seabed sediment samples from Abrasimov Bay downward velocities at surface convergences. In the cross- (Figure 2c) were kindly provided by the Norwegian Radi- wind/vertical plane, frazil traversed counter-rotating circular ation Protection Authority. Abrasimov Bay is a coastal inlet paths (Figure 5) and followed the upwind helical Lc flow. (with water <17 m deep) that was used as dump site for As frazil traversed the shear zone separating surface down- radioactive waste by the former Soviet Union authorities wind and bottom upwind flow, they changed direction from until the late 1980s [Yablokov et al., 1993]. downwind to upwind while maintaining their circular orbits [21] Grain size distributions of the sea ice and seabed perpendicular to the wind direction. sediment samples were determined by sieving and gravita- [26] As visible from the top view (Figure 4b) and in the tional separation, and by smear slide analysis. The compo- cross section (Figure 4c), tank floor sediment streaks in sitions of all samples were determined by semiquantitative bottom Lc convergent zones alternate with frazil crystal estimation of area percentage under the microscope. rows in surface Lc convergent zones. During a maximum experiment run of 20 minutes, four fully developed Lc cells 3. Results (8 individual vortices) were produced in the tank. Lc cells 3.1. Freezing Tank Experiments were outlined by frazil rows at the surface and sediment streaks at the tank bottom (Figures 4a–4c). The outermost 3.1.1. Langmuir Circulation and Ice Formation Lc cell on each side of the tank converged with the wall, and [22] We routinely reproduced regular and steady Lc by thus surface convergence zones formed along each tank blowing wind across the tank surface (Figures 1 and 4). The wall. Both the spacing of surface ice rows and the bottom

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Figure 6. Frazil crystal trajectories under the thickening grease ice edge, and generation of subsurface return flow at the ice edge (see text for more details). Note the black/white centimeter scale in the left background on the tank sidewall, and on the sediment-covered bottom in the left foreground. sediment streaks represented the horizontal diameter of one concentration in the IW varied from 19.69 to 30.95 mg lÀ1, Lc cell (8–10 cm). The diameter of each individual roll while even higher sediment loads were measured in the ice vortex was therefore approximately the water depth (4– samples (20.93 to 36.78 mg lÀ1). In all four experiments, 5 cm), and the factor between the dimension of one Lc cell the particle load in the IW and the ice far exceeded that of and the tank water depth was 2–2.5. Individual Lc rolls the underlying water column. The enrichment ratios were nearly circular in the cross wind/vertical plane (Table 1) in IW varied between 1.12 and 1.65, while the (Figures 4c and 5). ratios in the ice were 1.43 to 2.18. Higher ice and IW [27] Due to the wind-driven surface flow, frazil was particle loads were associated with enhanced SPM advected downwind to the end of the tank forming a concentrations. continuous grease-ice zone of floating frazil crystals. The [30] Sand percentage in ice, SPM, and IW samples was windward edge of the grease-ice zone migrated upwind as mostly depleted relative to the original bottom material, more frazil formed and was advected downwind (Figures 4b while silt concentration was similar or even clearly in- and 6). Surface waves were damped and no evidence of Lc creased (Table 2). The clay fraction was abundant, ranging was observed in the water column underlying the grease-ice from 10 to 30%. zone. Lc occurred only in the upwind open water area. The [31] The samples were composed to as much as 75–90% presence of a continuous layer of mobile grease ice on the of quartz and feldspar particles, while other components like water surface thus inhibited the transfer of momentum from rock fragments, dark minerals and biogenic material were the atmosphere to the water, and stopped Lc. less common or of minor abundance. While the SPM mostly [28] As the grease ice edge migrated upwind and thick- contained no lithic coarse grains, IW and ice showed ened in experiments 2–5, the surface flow was deflected slightly enhanced abundances of coarser grains or aggre- downward at the grease ice edge, and then separated into a gates of fine particles. branch of subsurface return flow and a branch of subice flow (Figure 6). The latter flow advected frazil crystals 3.2. Lake Hattie below the edge of grease ice, so that the ice edge piled up to 3.2.1. Ice Formation and Lc a thickness of several cm. (Figure 6; stage I + II). As the ice [32] Snow was falling during the night preceding the edge grew downward into the water column, both flow experiments and throughout the morning of the field stud- branches - and the associated frazil transport - steepened. In ies. As a result, there was a combination of snow and frazil stage III of the subice flow, larger frazil and flocs were floating on the water surface. This floating slush ice was transported beyond the ice edge and accumulated further aligned in well-defined and regular surface windrows in- downwind on the underside of the grease ice cover. In stage dicative of active Lc. IV, individual frazil were transported down to the tank [33] Lc streaks were continuously during the experiments. bottom and then floated up again in a u-shape path towards These surface streaks formed far upwind of our sampling the ice underside. Due to the decreasing cross sectional area area, had lengths of several tens to hundreds of m, and had of the tank below the grease ice layer, the flow velocity cross-wind spacing of about 2 m close to the shore. Thus, under the accumulating ice edge increased, and many frazil one roll vortex (1/2 of an Lc cell) was approximately crystals struck the bottom as they passed under the thick- circular in the cross wind/vertical plane, with a diameter ening ice edge. roughly equal to the local water depth. At Lc divergence 3.1.2. Sedimentology zones between surface streaks we saw individual ice crystals [29] The SPM load in tank experiments 2–5 varied and aggregates rise to the water surface and move obliquely between 13.43 and 24.10 mg lÀ1 (Table 1). The particle downwind into surface frazil streaks. Frazil and ice flocs

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Table 2. Grain Size Distribution (Sand/Silt/Clay) Determined by Sieving and Gravitational Separation, and Estimated From Binocular Filter Scansa Sample Composition, %

Experiment/ Quartz SEDIMENTS ICE AND CIRCULATION LANGMUIR KEMPEMA: AND DETHLEFF Station Sample Sand, Silt, Clay, and Rock Dark Plant Location Number Type % % % Feldspar Fragments Minerals Mica Biogenic Debris Others Remarks Tank 2 grease ice <1 70 30 85 151 206 high abundance of fine silt; angular grains Tank 2 IW 5 80 15 80 1510 3 10 angular grains Tank 2 SPM 10 60 30 75 21520 3 3 angular grains Tank 2 Amderma bottom 12.7b 59.1b 28.2b 75 5 1 15 1 12 high abundance of fine silt; angular grains Tank 3 grease ice <1 85 15 90 1 3 10 1 4 abundance of coarse silt; angular grains Tank 3 IW <1 90 10 90 1 2 10 1 5 angular grains Tank 3 SPM 5 80 15 90 2 2 10 05 angular grains Tank 3 Amderma bottom 12.7b 59.1b 28.2b 75 5 1 15 1 12 high abundance of fine silt; angular grains Tank 4 grease ice <1 70 30 90 0 5 10 1 3 fine grained dark minerals; fine silt; aggregates of fine particles (300–400mm) Tank 4 IW 5 75 20 85 2510 1 6 lithic coarse fraction abundant Tank 4 SPM <1 70 30 85 1510 1 7 no lithic coarse fraction Tank 4 Abrasimov bottom 3b 72b 25b 80 10 5 1 1 1 2 high abundance of medium-fine silt;

8of15 angular-subrounded particles, angular grains Tank 5 grease ice 5 75 20 85 1510 1 7 fine silt highly abundant; extremely fine grained dark minerals; coarse lithic particles Tank 5 IW 5 80 15 85 1510 1 7 angular grains Tank 5 SPM 5 70 25 85 1510 1 7 angular grains Tank 5 Abrasimov bottom 3b 72b 25b 80 10 5 1 1 1 2 high abundance of medium-fine silt; angular-subrounded particles; angular grains Lake Hattie 1 slush ice 5c 65 30 80 3 30 5 1 8 less fine; angular sand-sized particles Lake Hattie 1 IW <1 60 40 80 5 302 2 8 less fine; angular sand-sized particles Lake Hattie 1 SPM <1c 30 70 85 1 202 2 8 finest; rounded particles Lake Hattie 1 bottom 90d 5 5855 3 1 2 2 2 sand particles angular to subrounded Lake Hattie 2 slush ice 5c 35 60 80 1 10 5 51 coarsest; sand particles angular to subrounded Lake Hattie 2 IW <1 50 50 95 1 101 1 1 fine grained Lake Hattie 2 SPM <1c 50 50 90 2 101 1 5 coarse; subrounded coarse silt particles Lake Hattie 2 bottom 90d 5 5855 3 1 2 2 2 sand particles angular to subrounded Kara Sea Amderma 1 sea ice sed. 1.4b 60.6b 38.0b 75 15551 8 high abundance of medium to coarse silt Kara Sea Amderma 1 bottom sed. 98.2b 0.7b 1.1b 75 5 1 15 1 12 - Kara Sea Amderma 2 sea ice sed. 1.7b 72.7b 25.6b 75 1 1 15 1 16 high abundance of fine silt Kara Sea Amderma 2 bottom sed. 12.7b 59.1b 28.2b 75 5 1 15 1 12 high abundance of fine silt aBulk particle compositions were estimated from filter and smear slide scans. Please note that lowest percentages (1; 2) represent traces of the respective particle groups. bDetermined by wet sieving, Atterberg separation and dry weighing of sand, silt, and clay from ice ridge sediment. cOccurrence of water fleas (Cladocerans) and copepods; medium to large and many in SPM, smaller and lesser in slush ice. dSand and gravel. C02004 C02004 DETHLEFF AND KEMPEMA: LANGMUIR CIRCULATION AND ICE SEDIMENTS C02004 were also observed traveling downward into the water [40] The ice core, the ridge-ice, and the bottom sediments column at surface convergence zones. at station Amderma 2 (40 m water depth) were sampled two 3.2.2. Sedimentology days after the storm event. The uppermost section (17 cm) À1 [34] The SPM load in the lake water varied between 3.07 of the core contained about 7 mg l of sediment (Table 1), and 5.26 mg lÀ1 (Table 1). The sediment concentration in while the ice obtained from the nearby pressure ridge the IW was 5.39 (first sampling) and 7.58 mg lÀ1 (second contained several grams of sediment per liter. sampling), while the sediment loads determined in the ice 3.3.2. Sedimentology À1 were 5.56 and 5.89 mg l . As in the tank experiments, the [41] Bottom sediment from the Amderma 1 station con- sediment load in Lake Hattie ice and IW exceeded that of sisted to more than 98% of sand with minor abundances of the underlying water column. The enrichment ratios in the silt and clay. The sea ice sediment contained 1.4% sand, IW varied between 1.44 and 1.76, while the ratios in the ice 60.6% silt and 38% clay. Bottom sediment from the were 1.12 and 1.81 (Table 1). Sediment concentrations in Amderma 2 station (also used in tank experiments 2 lake suspension, in the IW and in the ice were higher during and 3) contained 12.7% sand, 59.1% silt and 28.2% clay the second sampling period when wind and waves were (Table 2). The sea ice sediment consisted of 1.7% sand, higher. Higher IW and ice sediment concentrations were 72.7% silt and 25.6% clay. The Amderma bottom and ice related to higher water column SPM loads. samples were all composed to as much as 75–90% of quartz [35] Lake Hattie bottom sediment is about 85% quartz and feldspar particles; rock fragments, dark minerals and and feldspar particles. Other particle groups like rock frag- biogenic material were minor constituents. The sea ice ments, dark minerals and biogenic material were less sediment contained less sand and more silt than the bottom common or of minor abundance. sediment. [36] The sand/silt/clay percentages in SPM, IW and ice [42] The mixed sample from the Abrasimov Bay (used in samples showed a rather uniform distribution, with very low tank experiments 4 and 5) was composed of 3% sand, 72% percentages of sand and strongly enriched percentages of silt, and 25% clay. The samples are 80–90% quartz and fine-grained particles (Table 2). The samples consist of up feldspar with minor amounts of rock fragments, dark to 85–90% quartz and feldspar particles, while other minerals and biogenic material. components like rock fragments, dark minerals and biogenic material are less common or of minor abundance. While the 4. Discussion SPM contained rounded to subrounded lithic particles, 4.1. Lc in Freezing Tank Experiments and Cold IW and ice showed enhanced abundances of angular to Aquatic Environments subrounded particles in the coarse silt to sand sized material. 4.1.1. Tank Experiments [43] The routinely generation of Lc cells under freezing 3.3. Kara Sea conditions in our laboratory tank experiments confirm 3.3.1. Ice Formation and Lc findings from laboratory studies under non-freezing con- [37] During fieldwork in late winter 1997 (April 13 to ditions [e.g., Mizuno and Cheng, 1992; Uzaki and 20), the fast ice canopy in the Amderma region was Matsunaga, 2000]. The occurrence of Lc cells was shown extremely narrow with a width of partly only 500 m. The by the distribution of frazil crystals, which described com- edge of fast ice coincided roughly with the 10–15 m plex, three-dimensional flow paths through the water col- isobath, though deeper sections to as much as 30–40 m umn due to interaction with the primary surface-downwind/ also occurred (see Table 1). Open water (i.e., the Amderma bottom-upwind flow in the tank. The Langmuir nature of flaw lead) occurred between the fast ice and drifting ice due the flow was evidenced by the counter rotating rolls in the to offshore winds. cross wind/vertical plane. Lc was further demonstrated by [38] New frazil ice formed in the flaw lead during stormy surface frazil crystal rows, and bottom sediment streaks events (wind velocity: 5–8 m sÀ1; air temperature: À4to (Figure 4). The surface frazil rows and the bottom sediment À11.5°C) throughout the days prior to our sampling. The streaks compare to lines of surface floaters (e.g., seaweed, frazil ice collected in wind parallel surface streaks and was computer cards, paper dots) and bottom dye streaks (e.g., advected offshore towards the drift ice (Figure 2c). The potassium permanganate, condensed milk) that developed surface frazil ice streaks indicated active Lc in the shallow in the downward and upward convergence zones of Lc cells water column. in various field and tank studies [Faller and Caponi, 1978; [39] The day after the stormy period, we sampled fresh, Faller and Cartwright, 1983; Weller et al., 1985; Uzaki and unconsolidated and visibly sediment-laden grease ice com- Matsunaga, 2000]. pressed to the fast ice edge at the 11 m deep site in the [44] Measured downward and upwind velocity compo- eastern part of the Amderma flaw lead (station Amderma 1; nents of 1.5–2.5 cmÀ1 for individual frazil crystals in our Table 1 and Figure 2c). On contrary, the directly adjacent tank experiments caused by Lc flow under wind velocities fast ice at station Amderma 1 contained less visible sedi- of 5–7 m sÀ1 well compare to Langmuir’s [1938] obser- ment. The new, unconsolidated sediment-laden ice was vations. In tank experiments, Langmuir found that fluores- formed under stormy, turbulent freezing conditions in cein dye moved downward in Lc surface convergent zones extremely shallow water, and was accumulated at the fast- with a velocity of about 1.6 cm sÀ1 when wind speed was ice edge prior to our observations. The presence of this fresh 6msÀ1. Measured Lc downward velocities also agree with turbid ice is strong evidence that sediment laden ice forms those given by Filatov et al. [1981], who found Lc down- in the Amderma flaw lead, regardless of the mechanism of welling velocities of 1.8–2.5 cm sÀ1 associated with wind formation. speeds of about 5–7 m sÀ1 in shallow Lake Ladoga. The

9of15 C02004 DETHLEFF AND KEMPEMA: LANGMUIR CIRCULATION AND ICE SEDIMENTS C02004 general relationship between wind speed and Lc downwel- the water mechanically and enhances atmospheric heat ling velocities was found to be linear [Leibovich, 1983]. transfer, and may intensify frazil formation under cold, [45] Our observed maximum downward Lc velocities of turbulent conditions. À1 À1 2.5 cm s are lower than those (4–5 cm s ) predicted [49] Our studies at Lake Hattie and in the Siberian Kara following Gargett et al.’s [2004] calculation for the same Sea show that Lc (as indicated by frazil streaks) can be wind conditions. The difference may be explained by the readily observed in the field under wind and temperature fact that fully developed Lc in natural waters with depths of conditions similar to those in the tank experiments, though about 15–20 m or more (as in the Gargett et al. study) may the flow of natural, unconstrained Lc differs from the produce stronger downward velocity components than Lc complex Lc flow in tank experiments. Convergent Lc flow generated in very shallow lakes or tanks with smaller was indicated in both the Lake Hattie study and in the Kara horizontal and vertical scales. It is also possible that frazil Sea flaw lead observations by frazil streaks aligned roughly velocities are less than the surrounding water velocity due to parallel to the wind direction (Figure 7), and by clearly the buoyancy of the ice. visible downwelling and up-rising frazil at Lake Hattie. [46] Inertial forces, shear stress, drag, gravity forces, and However, we were unable to determine if the Lc roll buoyancy determines the motion of frazil in water [Daly, vortices in the shallow Lake Hattie and Kara Sea settings 1984, 1994]. Daly [1984] calculated a theoretical rise were influenced by a larger scale ‘primary flow’ necessary velocity of 2cmsÀ1 for 1 mm diameter frazil disks (the to balance the net surface downwind flux towards the lake size we observed in the tank). Reimnitz et al. [1993a] found shore and the lead ice edge. that larger frazil crystals and flocs rose faster than smaller [50] Dmitrenko et al. [2005] raise the question if convec- frazil crystals, with a positive, linear relationship between tive mixing in Siberian shelf poylnyas can penetrate to the crystal size and rise velocity in static tank experiments. Our sea floor and may play a role in seafloor sediment dynam- tank experiments show that even a relatively weak wind ics, particularly in sediment resuspension. The authors blowing across a short fetch can create Lc flow that is found that convective penetration to the seafloor may capable of entraining relatively large frazil crystals against exceed a probability of 70% in a shallow, 10 to 40 m deep buoyancy from the water surface into the flow interior, western Laptev Sea flaw lead during winter. Pavlov et al. where frazil and sediment interact to form incipient sedi- [1994] also reported on convective penetration for the ment-laden ice. This tends to reinforce our argument that shallow sea along the eastern coast of Novaya Zemlya advection by helical Lc dominated the vertical distribution located north of the Amderma flaw lead region in the of ice and sediment in our tanks experiments. western Kara Sea. The authors concluded that rapid cooling [47] We followed the method of Faller and Caponi and ice formation result in convection penetrating to depths [1978, p. 3620] to determine the number of Lc cells in of 50 to 70 m. our tank experiments. Various studies propose that the [51] Gargett et al. [2004] introduced the term of ‘Lang- number of Lc cells is controlled by basin geometry, partic- muir supercells’ for Lc cells that penetrate to the seabed. ularly by tank width and water depth [Uzaki and From the scaling arguments of Lc cell depth to width we Matsunaga, 2000; Matsunaga and Uzaki, 2004], and pre- conclude that Lc supercells were present at Lake Hattie dict that the ratio of cell width to water depth should be in when we made our observations. Based on our observations the range of 2–2.5, which was the case in our experiments. of frazil streak spacing in the Amderma flaw lead and the Using the formula lc/H = 4.8 [1 À exp (À0.5lw/H)] from general knowledge on convective mixing in shallow Arctic Faller and Caponi [1978], where lc is the Lc cell width, coastal seas and polynyas [Pavlov et al., 1994; Dmitrenko et lw is the surface water wavelength and H is water depth, we al., 2005] we suspect that Lc cells could have penetrated investigated the relationship of lc/H upon lw/H - or in other deep into the local water column. We further assume that the words, we tested the influence of the surface wavelength on probability of winter convective mixing on the shallow the Lc cell width in dependency to the water depth - for Siberian Arctic shelves is even higher when Lc is present wind-wave stressed water bodies at freezing. In experiment 5, like shown for the St. Laurence Island polynya by Drucker which had the most regular streak pattern and a surface et al. [2003], and that Lc may contribute to the role of wavelength of 9 cm, we calculated a value of 3.24 for convective mixing in the process of bottom and sea ice lc/H. This value is higher than the value of 2–2.5 derived sediment dynamics. from the observed Lc cell width and the water depth of our tank experiments. Following our calculation (i.e., the 4.2. Lc and Sediment Entrainment Into Ice 52 imbalance between the value for lc/H calculated from the [ ] Wind-wave driven Lc develops under the same Faller and Caponi formula and the value derived from Lc conditions (strong winds, well mixed shallow water col- cell width and tank water depth) and Matsunaga and Uzaki umn) optimal for suspension freezing. Though the contem- [2004] we conclude that Lc spacing in the tank was influ- poraneous observation of Lc and suspension freezing does enced by water depth more than by surface wave length. not necessarily prove that Lc plays a role in sediment 4.1.2. Extrapolation to Natural Freezing Waters entrainment into newly formed ice, Lc appears to be more [48] Veron and Melville [2001] point to the direct com- effective in bringing ice and sediment into contact in the parability of Lc phenomena observed in tank experiments water column (Figure 7) than simple dispersion and settling and field studies. They showed that the transitional processes in a turbulent water column as proposed by Sherwood of Lc generation by current and waves documented in the [2000]. laboratory could also be observed in the field under similar [53] Lc provides coherent vertical velocities to transport wind conditions. They also found that heat exchange across ice down and sediment up into the water column, thus the surface was significantly increased by Lc, which mixes enhancing the probability of ice/sediment interactions.

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Figure 7. Photos of Lc induced surface frazil windrows in Lake Hattie (top left) and in Arctic open water (top right). Schematized Lc-driven entrainment of fine-grained sediment into newly forming ice in shallow Lake Hattie, and potentially in Arctic open water (polynyas) is shown in the lower part of the figure. (a) The sprinkle of settling sediment particles over buoyant uprising frazil injected from below, (b) a Lc surface convergent zone with filtration trapping of particles in the frazil ice, and (c) a bottom convergent zone with scavenging of sediment particles from the water column by uprising frazil (see text for more details).

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Though the down welling and upwelling limbs of Lc may ice, as was found in the tank studies by Reimnitz et al. contain mixed populations of ice and sediment, respectively, [1993a] and also observed in natural shallow environments the flotation of ice and sinking of sediment from the in the Laptev Sea [Dethleff, 2005]. These findings suggest horizontal components of Lc separates the trajectories of that most frazil/sediment interactions take place in the water both types of particles from the water (Lc) trajectories column, where silt and clay are suspended in higher con- (Figure 7a). As both particle types traverse the horizontal centrations than they occur on the bottom. limbs of Lc rolls, sediment will settle downward and frazil [58] Though our tank ice sediments consist mainly of silt will rise towards the surface. This separation of water and clay, they also contain as much as 5% sand. Based on and particle trajectories helps to sprinkle sediment over our observations that (1) sand and aggregates of fine the rising ice, and injects ice crystals under the settling particles are present in tank IW and ice (particularly in sediment. Besides mechanisms like filtration trapping experiment 4; see Table 2), (2) these particle groups are [Osterkamp and Gosink, 1984] in descending Lc currents absent in the SPM samples, and (3) the frazil touched the under surface convergences where frazil concentrations bottom and moved up again, we conclude that sand attached are highest (Figure 7b), and the joint rise of sediment to frazil can be transported from the bottom to the water particles and frazil (Figure 7c) in upwelling bottom con- surface and entrained into the growing surface ice cover. vergences, this process may lead to higher coconcentrations Both Reimnitz et al. [1993a] and Ackermann et al. [1994] of sediment particles and frazil in the water column as discussed the potential of rising frazil to transport sand modeled by Sherwood [2000] and increase the probability upward into the water column. Our tank observations of sediment/ice interactions leading finally to sediment- reinforce this idea that frazil striking the bottom entrains laden ice formation. coarse bottom sediment up into the ice cover, and that the 4.2.1. Tank Experiments process of suspension freezing may involve interactions [54] The increased particle content in IW and ice versus between ice crystals and sediment both in the water column SPM documented in the tank experiments (Table 2) shows and on the bottom. that the sediment entrainment into newly formed ice is [59] Component analysis reveals that all sedimentary enhanced relative to the water column. Particularly, the high compartments (bottom, SPM, IW, ice) in the tank experi- sediment content in IW and ice in experiment 2 (enrichment ments are dominated by angular clastic particles consisting ratios: 1.65–2.18) points to very active, potentially Lc- primarily of quartz and feldspar. As in field investigations supported enrichment of fine clasts from water with low [Dethleff, 2005], our grain size and component analysis SPM concentrations (13.43 mg lÀ1). Smedsrud [2001] shows that in tank experiments angular, lithic, mainly silt- reports on sediment enrichment in ice by ratios of 2–4 in sized particles are preferentially entrained over sand-sized tank experiments associated with high heat fluxes, corre- material into newly formed ice by turbulent processes. In spondingly high rates of frazil formation, SPM concentra- general, we saw no sorting or preferential entrainment tions of 10–20 mg lÀ1, and impeller-induced turbulence. between compartments, though fine-grained dark minerals [55] The tank SPM loads in our experiments 4 and 5 were were slightly more obvious in the ice sediment samples in higher than in experiments 2 and 3. This may be due to the experiments 4 and 5, as compared to the bottom sediment. fact that the bottom sediment used in experiments 4 and 5 is 4.2.2. Comparison to Natural Conditions slightly finer than that used in experiments 2 and 3, and thus [60] Extremely little attention has been given to Lc in easier to suspend. Higher sediment concentrations in ice and shallow coastal seas, where Lc cells may penetrate the full IW are associated with higher initial SPM concentrations, water depth, and thus help to distribute suspended sediment which is in agreement to findings from Reimnitz et al. delivered by the wave boundary layer throughout the full [1993a]. These authors also found that with low to medium extent of the water column [Gargett et al.,2004].We initial SPM, the difference between SPM and sediment in suggest that such processes may also play a substantial role the ice is higher. Our studies support this conclusion in for the interaction of sediment and frazil in the water experiment 2 (lowest SPM concentration; Table 1), where column, ultimately resulting in the formation of sediment- the ice sediment concentration had an enrichment ratio of laden sea ice. 2.18. In experiment 4 (with the highest SPM concentration) [61] The increased sediment concentrations in IW and ice the enrichment ratio was only 1.53, one of the lowest of all compared to SPM found in the tank experiments were also experiments. observed in the Lake Hattie study. Sediment enrichment [56] Reimnitz et al. [1993a] also showed that under low ratios in ice varied between 1.1 and 1.8, and were similar to to medium SPM conditions the ratio of the particle the enrichment seen in our tank experiments and also content between grease ice (high) and interstitial water documented in literature [Smedsrud, 2001]. Higher ice (low) was high, while under medium to high or high tank and IW sediment concentrations in the Lake Hattie study SPM concentrations the ratio was small, or even less than were related to increased SPM load in the water column one. These conclusions are reinforced by our observations (compare Hattie 1 and 2, Table 2), which was also found in of an enhanced ratio (1.33) between IW particle concen- the Lc tank experiments (experiment 4) and described by tration and grease ice particle content in our low SPM Reimnitz et al. [1993a]. experiment 2, and by a lower ratio (1.19) in our high [62] Sediment contents in Lake Hattie ice and Kara Sea SPM experiment 4. ice vary between 6 and 37 mg lÀ1 (Table 1), and thus [57] In general, the tank ice and IW sediments are finer compare well with the lower range of sediment-laden ice thantheoriginalsedimentplacedinthetank.Thesilt cores from the Beaufort Sea (0.54–920 mg lÀ1), the content is as much as 50% higher. This points to a Chukchi Sea (24–1470 mg lÀ1) and the Laptev Sea (3.5– preferential entrainment of silt and clay into newly forming 490 mg lÀ1)[Reimnitz et al., 1993b, 1993c; Stierle and

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Eicken, 2002; Dethleff, 2005]. As in our tank study and in underneath an existing ice cover in refreezing leads and Laptev Sea observations [Dethleff, 2005], the preferential polynyas may produce stratified layers of sediment-laden entrainment of silt-sized sediment into ice was also found in ice in the annual sea ice cover. Lake Hattie (experiment 1) and in the Kara Sea (see Table 2). The ice sediment in Lake Hattie experiment 2, 5. Conclusions however, was dominated by silt and clay. The increased clay fraction in Lake Hattie experiment 2 (Table 2) may result [67] From our studies we draw the following conclusions: from very fine particles being the predominant sediment in [68] 1. Regularly spaced and steady Langmuir circulation the water column further offshore where the frazil formed. (Lc) was routinely formed in shallow freezing-tank experi- [63] As in the tank experiments, we observed preferential ments by the superimposition of (1) wind and cross-wave entrainment of fine grained, mostly silt-sized, angular, action with (2) the subsurface return flow deflected from the clastic particles consisting predominantly of quartz and downwind basin wall. Langmuir circulation was identified feldspar into ice or IW in the Lake Hattie and the Amderma by the circular path of frazil ice crystals in the water column, lead studies. The resemblance of sea ice sediment and wind-parallel surface rows of frazil ice crystals, and bottom bottom material both in our tank experiments 2 and 3 (run streaks of sediment that formed in the experiments. with Amderma material) and at the natural Kara Sea setting [69] 2. The spacing of one paired Lc cell was about twice (Amderma lead) shows that turbulent entrainment conditions the tank water depth (H), showing that in well-mixed may be very similar even at extremely different size scales. shallow water bodies at freezing temperatures the diameter [64] The abundance of coarse, angular lithic particles in of one Lc roll vortex is controlled by the water depth. We Lake Hattie ice, compared to SPM, suggests entrainment of supported this observation by demonstrating the imbalance sand sized material into ice crystals directly from - or from of the Faller and Caponi [1978] formula lc/H = 4.8 [1 À close to - the bottom. Under the stormier conditions during exp (À0.5lw/H)] for our experiment design, which shows the second Lake Hattie sampling, the SPM, IW and ice that the water depth influences the Lc cell width (lc) rather sediment were all coarser than in the first experiment. This than the surface wavelength (lw). When the roll vortex is is consistent with the argument that wind and waves under controlled by water depth, the conditions for Lc supercell freezing conditions are the driving forces for turbulence- formation are met, and there is an increased probability of supported entrainment of particles into newly forming ice. frazil/sediment interactions in the water column leading to Based on our tank observations, along with the observed sediment-laden sea ice formation. evidence of Lc in nature, we conclude that turbulent [70] 3. Individual ice crystals and aggregates traversed processes of sediment entrainment and enrichment in ice complex, circular, three-dimensional flow paths through the formed in shallow freezing water may be supported by Lc. water column due to the interaction of Lc and primary 4.2.3. Entrainment Processes downwind/upwind flow in the tank experiments. As frazil [65] Based on observations from Laptev Sea flaw leads, circulated through the water column, they entrained sedi- Dethleff [2005] proposed that vertical trapping by rising ment suspended in the water column and sediment resting frazil and filtration of fine-grained sediment in convergent on the tank floor. In particular fine-grained, angular silt was surface ice rows can both be enhanced by Lc. Drucker et al. entrained into newly forming tank ice. Particle enrichment [2003] concluded from remotely sensed data, measurements ratios in ice and IW (interstitial water) versus the SPM of supercooling, and upward-looking sonar soundings col- (suspended particulate matter) load of the underlying water lected in a Bering Sea polynya, that during freezing events column varied both in tank and lake experiments between frazil either nucleates at up to 20 m depth (44 m total depth) 1.12 and 2.18. Qualitative particle composition in bottom or are transported to depth by Lc. The Lc penetration depth sediments, IW and ice shows that no mineral component is is consistent with the depths at which turbulent ice sediment preferentially entrained into the ice. entrainment processes are believed to occur on shallow [71] 4. The similarity of sea ice sediment and bottom Arctic shelves [Reimnitz et al., 1993b; Sherwood, 2000; material both at the Amderma lead station and in our tank Dethleff, 2005]. Relying on (1) our tank and Lake Hattie experiments 2 and 3 (run with Amderma sediment) shows observations of Lc, (2) the sedimentological data from tank that sea ice sediment entrainment conditions may be very experiments and the lake, and (3) the studies referred to in this similar on different scales in the tank and at natural settings. paragraph we suggest that Lc maybe an important mecha- We suggest Langmuir supercells support ice sediment nism for bringing frazil and fine-grained sediment together in entrainment in the tank experiments, while the role of Lc the water column where suspension freezing occurs. on sea ice sediment entrainment in the Amderma lead yet [66] From the arrangement of sediment layers in thin remains unclear. level ice floes from coastal areas of the Beaufort and [72] 5. Finally, we found that natural processes of sedi- Chukchi Seas Eicken et al. [2005] deduced that the entrainment entrainment and enrichment into newly forming ice ment of particles occurred after the initial ice growth. They can be reproduced in laboratory experiments. Observations proposed that sediment-enriched frazil accumulated under and sedimentological results from our tank investigations an already existing ice cover, a mechanism they assumed to and the Lake Hattie study suggest that Lc-related flow be characteristic of coastal polynyas. We observed this enhances the entrainment of silt and clay into newly form- subice flow of frazil and water in our tank experiments ing ice in experimental and natural settings. Lc appears to (Figure 6), which piled newly formed, sediment-enriched enhancing contact between suspended sediment and ice frazil under the upwind-propagating ice edge. Based on the crystals during ice formation in various aquatic environ- Eicken et al. [2005] findings and our tank observations we ments, and should be regarded as an important contributor suggest that Lc-driven subice flow of sediment-laden frazil to the process of suspension freezing.

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[73] Acknowledgments. This study was financially supported by the Gargett, A., J. Wells, A. E. Tejada-Mart´nez, and C. E. Grosch (2004), German Bundesminister fu¨r Umwelt, Naturschutz und Reaktorsicherheit Langmuir supercells: A mechanism for sediment resuspension and trans- (BMU, project StSch 4101; 1996–1999) and the United States Navy Office port in shallow seas, Science, 306(5703), 1925–1928, doi:10.1126/ of Naval Research (grant N00014-02-1-0650). The content of this paper science.1100849. does not necessarily reflect the opinion of the above German ministry. Per Harms, I. H., M. J. Karcher, and D. Dethleff (2000), Modeling Siberian Strand and Bjørn Lindt (Norwegian Radiation Protection Authority, Nor- river runoff—Implications for contaminant transport in the Arctic Ocean, way) kindly provided bottom samples from the Abrasimov Bay, Kara Sea. J. Mar. Syst., 27, 95–115. We are grateful to the European Commission DG XII-G-2 for financially Kempema, E. W., E. Reimnitz, and P. W. Barnes (1989), Sea ice sediment supporting the TMR-Programme ‘‘Access to Large Scale Facilities’’ in entrainment and rafting in the Arctic, J. Sediment. Petrol., 59, 308–317. 1998 (INTERICE II). The crew of the Hamburg Ship Model Basin (HSVA), Langmuir, I. (1938), Surface motion of water induced by wind, Science, 87, particularly the members of the ice tank group, earned our highest respect 119–123. for providing perfectly prepared ice tank facilities, numerous electronic Leibovich, S. (1983), The form and dynamics of Langmuir Circulation, devices, and so many helping hands during our experiments. We are Annu. Rev. Fluid. Mech., 15, 391–427. indebted to Erk Reimnitz, Peter Loewe, and Gesa Kuhlmann for their help Lindemann, F. (1998), Sedimente im arktischen Meereis—Eintrag, Char- during the experiments. We further thank Peter Loewe (again), Dominik akterisierung und Quantifizierung, Rep. 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