Dethleff, Dirk

Kempema, Ed

OceanRep

1 18Mitteilunaen zur Kieler Polarforschuna Heft 21-2005 1 Dirk Dethleff Institut für Polarökologie der Un1vers1tat Kiel Ed Kempema University of Wyoming, Laramie, USA Particle entrainment into newly forming ice in Lake Hattie, Wyoming {USA) lntroduction The entrainment of particulate material into newly forming ice was investigated on different shallow circum-Arctic Arctic shelves and in Lakes (Osterkamp and Gosink, 1984; Kempema et al., 1989; Reimnitz et al., 1991; Kempema & Reim­nitz, 1991; Reimnitz et al., 1998; Eicken et al., 2000; Dethleff, 2005). Among processes like tidal pumping, wave action, floe-bulldozing, near shore seabed freezing of grounded ice floes, and beach-ice formation, the principle mecha­nism of 'suspension freezing' was proposed to be the most effective mechanism to entrain fine-grained particulate material into newly forming ice (Reimnitz et al., 1992). Accordingly, driven by cold and stormy weather conditions, the water column mixes down to the bottom at shallow locations in open water areas. Su­per cooling of the water leads to frazil ice formation, and subsequent turbulent scavenging and filtration of suspended particulate matter by rising ice crystals as well as uplifting of anchor ice promote the entrainment and enrichment of particles in the ice cover (Osterkamp & Gosink, 1984; Reimnitz et al., 1993). Many of the above studies revealed that the particle content in ice by far ex­ceeded the suspension load of the underlying water column, so that turbulent enrichment processes like Langmuir circulation (Lc) were proposed to form sedi­ment-laden ice rather than simple freezing of the particle laden water (Dethleff, 2005). The aim of our study in Lake Hattie, Wyoming (USA), during fall freeze-up in late November 2004 was to investigate the entrainment of particles into newly forming lake ice in the presence of water column turbulence. Environmental Settings, Material and Methods Lake Hattie (Fig. 1), a shallow reservoir used for local farmland watering, is situated about 25 km west of Laramie at the foothills of the Medicine Bow Moun­tains, western Rocky Mountains, on an altitude of about 2,200m. The Lake covers an area of few square km. The investigations were carried out on the southern shore of the lake. 50° 45° 40° 35° 30° 25° Mitteilun en zur Kieler Polarforschun Heft 21-2005 20°N 120°W 100° 80° During the freeze-up period of Lake Hattie, we conducted two sampling cycles of suspended particulate material (SPM), ice interstitial water (IW) and newly forming lake ice (Fig. 2). The north-easterly wind varied between 3-4 m/s (wave height: -10cm) and 5-6m/s (wave height: -15-30cm). The lake surface water (SPM) was sampled using 1l PE-bottles. Newly forming ice was taken from the ice streaks on the lake surface by a plastic net, and the IW was dropped from the net into a beaker immediately after ice sampling (Fig. 2). Due to temporary snowfall, we sampled slush ice with a roughly 5-10% portion of frazil crystals rather than pure frazil ice (grease ice). Preliminary Results and Discussion During freeze-up of Lake Hattie we observed the development of parallel new ice streaks on the water surface (Fig. 3). According to various investigations (e.g. Langmuir, 1938; Weller et al., 1985; Faller & Auer, 1988; Zedel & Farmer, 1991), wind parallel surface streaks of floating material in lakes and oceans may reflect the convergent zones of counter-rotating pairs of circulation cells (Lc), where the water is transported downward and floating particles are collected on the surface. The downwind propagating cell rotation is induced by a combination of wind, cross-wave action and current (e.g. Uzaki & Matsunaga, 2000). Particle concentrations in the streaks of newly forming lake ice and in the IW exceeded the SPM load of the water column by to as much as 80%. All IW and ice samples contained extremely high percentages of fine-grained particles (silt and clay fractions). Qualitative particle compositions in SPM, IW and ice mostly resemble well in the Lake Hattie samples. 60° Figure 1 Locotion ond photo of Loke Hottie, WY, USA. View is from the southern loke· shore. 19 Figure 2 Sampling in Lake Hat­tie raughly 30m fram the shore in about 1 m water depth. Figure 3 Surfoce streak of slush and frazi/ ice in Lake Hattie indicating a conver­gent pair af Lc vortices in the underlying shallaw water column. 20 Mitteilun en zur Kieler Polarforschun Heft 21-2005 E Langmuir circulation cell newice streak Langmuir circulation cell I Mitteilunaen zur Kieler Polarforschuna Heft 21 -2005 We suggest that the turbulent mechanism of Lc affects the re-suspension, coll­ection and entrainment of particulate material into the newly forming grease ice cover of Lake Hattie. Turbulent convergent water flow through the parallel surface streaks of new ice - consisting of a net of frazil and slush crystals - may particularly promote the filtration enrichment of extremely fine grained particles in the ice. References Dethleff D {2005). Entrainment and expert of Laptev Sea ice sediments, Siberian Arctic. J Geophys Res, C: accepted. Eicken H, Kolatschek J, Freitag J, Lindemann F, Kassens H, Dmitrenko I (2000). A key source area and constraints on entrainment for basin-scale sediment transport by Arctic sea ice. Geophys Res Lett 27(13): 1919-1922. Faller AJ, Auer SJ {1988). The roles of Langmuir circulations in the dispersion of surface tracers. J Phys Oceanogr 18: 1108-1123. Kempema EW, Reimnitz E, Barnes PW {1989). Sea ice sediment entrainment and rafting in the Arctic. J Sed Petr 59, 2: 308-317. Kempema EW, Reimnitz E {1991). Near shore Sediment Transport by Slush/Brash Ice in Southern Lake Michigan. In: Kraus NC, Gingerich KJ, Kriebel DL (Eds.), Coastal Sediments, Proceedings of a Speciality Conference on Quantitative Approaches to Coastal Sediment Processes: 212-219. Langmuir I {1938). Surface motion of water induced by wind. Science 87, 2250: 119-123.Osterkamp TE, Gosink JP {1984). Observations and analysis of sediment Laden sea ice. In: Barnes PW, Schell DM, Reimnitz E, The Alaska Beaufort Sea: Eco­system and Environment, San Francisco, Academic Press: 73-94. Reimnitz E, Hayden E, McCormick M, Barnes PW (1991). Preliminary observations on coastal sediment Loss through ice rafting in Lake Michigan. J Coastal Res 7, 3: 653-664. Reimnitz E, Marincovich jr. L, McCormick M, Briggs WM {1992). Suspension free­zing of bottom sediment and biota in the Northwest Passage and implications for Arctic Ocean sedimentation. Canadian J Earth Sciences 29: 693-703. Reimnitz E, Clayton JR, Kempema EW, Payne JR, Weber WS {1993). Interaction of rising frazil with suspended particles: tank experiments with applications to nature. Cold Region Science and Technology 21: 117-135. Reimnitz E, McCormick M, Bischof J, Darby DA {1998). Comparing sea-ice se­diment Load with Beaufort Sea shelf deposits: Is entrainment selective? J Sediment Res 68: 777-787. 21 l 1 22 Mitteilunqen zur Kieler Polarforschunq Heft 21-2005 1 Uzaki K, Matsunaga N {2000). Wind-driven currents with Langmuir circulation in a shallow water region. Advances in hydro-sciences and -engineering, IV, 95: pp. 10. Weller RA, Dean JP, Marra J, Price JF, Francis EA, Boardman DC {1985). Three­dimensional tlow in the upper ocean. Science 227: 1552-1556. Zedel L, Farmer D {1991). Organized structures in subsurface bubble clouds: Langmuir circulation in the open ocean. J Geophys Res 96: 88,890-88,900. 

Particle entrainment into newly forming ice in Lake Hattie, Wyoming (USA)

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