Frost flower (sea ice)

This article is about formations on sea ice and lake ice. For extrusions formed on freezing plants, see frost flower.
Frost flowers growing on young sea ice in the Arctic

Frost flowers are ice crystals commonly found growing on young sea ice and thin lake ice in cold, calm conditions. The ice crystals are similar to hoar frost, and are commonly seen to grow in patches around 3–4 cm in diameter. Frost flowers growing on sea ice have extremely high salinities and concentrations of other sea water chemicals and, because of their high surface area, are efficient releasers of these chemicals into the atmosphere.[1][2][3]

Formation

Frost flowers form when a layer of relatively warm ice is exposed to still, cold air that is at least 15 °C colder. For example, this would occur when freshly-formed ice at 0 °C underlies cold air at -30 °C.[4] In this situation, water vapor sublimates from the surface of the ‘warm’ ice. As this moist air rises into the colder overlying air, the temperature drops, and the air becomes supersaturated. The final result is a layer of supersaturated air, lying directly above the ice (just like how steam forms above the surface of a hot mug of water on a cold day). Any protrusions from the ice surface stick up into the supersaturated air, and end up being covered in hoar-frost like crystals (i.e. frost flowers) due to condensation. This only occurs when there is little wind: in high winds the supersaturated layer is scrubbed from the surface and blowing snow obscures the ice surface.[4]

Typically, frost flowers are only found on new ice, when the air temperature is very low. This is because thin, new ice has a temperature close that of the underlying, warm water. As ice thickens, its surface becomes much colder, and it is harder to get the necessary ice/air temperature difference needed for frost flower growth.[4] Over fresh water, these conditions are only found when the air temperature drops dramatically below zero in a short amount of time, leading to a sudden freezing event. Thus, fresh-water frost flowers are relatively rare.[4] By contrast, salt-water frost flowers are more common -- especially in cold, polar regions.[3][5] Sea ice is often broken apart by winds, tides and currents, leading to open water ‘leads’ that are exposed to extremely cold air temperatures. When these leads freeze over, forming thin ice, frost flowers often form in dense concentrations.

Frost flowers on sea ice are extremely saline. When sea water freezes, it forms porous ice that consists of mostly fresh-water ice, run through with brine channels filled with very briny water (containing the salt rejected from the ice during the freezing process). These brine channels extend to the ice surface and coat it with a wet, highly saline surface (‘a surface skim’). This is then wicked up onto frost flowers, increasing their salinity.[6][7] The tips of mature frost flowers are less saline due to vapor deposition and the bulk salinity decreases at night due to hoarfrost accumulation as the temperature drops and new snow (they are very good at collecting snow) which also reduces their bulk salinity over time.[5][7] Studies have been done on frost flowers and in one study in the ocean near Barrow, Alaska Alvarez-Aviles et al. (2008) found that the bulk salinities of the frost flowers ranged from 16 ppt to 105 ppt with an average of about 62 ppt. (approximately three times more salty than sea water).[7][8]

Morphology

Temperature, specifically the temperature at the surface of the ice that is not in the vicinity of the frost flowers, has a direct impact on the morphology as well as the thickness and absorbency of the ice, snow coverage and the blanket of frost flowers.[9] The shape of frost flowers changes when the air temperature or the degree of supersaturation changes during the growth process by changing the crystal tips.[9][10] The level of supersaturation determines the general formation, size and shape of the frost flower. In lower supersaturation, the tip of the frost flower will be faceted and side branches will form creating a branched-like crystal, resembling a tree, where in higher supersaturation the tip shape of the main branch will be rounded forming a star-like crystal without side branches.[7][11] The ice crystals in frost flowers are usually dendritic but similarly to hoar frost can grow in rod-like morphologies. When warm brine is wicked up onto the ice crystals, it can also give the frost flower a 'clumped' appearance as the facets of the ice crystals are partly melted.[12]

Chemistry

Frost flowers are complex in microstructural chemistry due to many different conditions, like air, temperature, chemical concentrations in the water, surface skim, humidity, and precipitation influencing their formation and growth. An important part of their formation is the fractionation of sodium and sulfate in respects to chloride during precipitation of the salts.[7] When the temperature decreases brine rejection increases and the channels become more and more concentrated, especially at the surface. When the salts begin the precipitate out of the ice, it changes the relative ion concentrations available in liquid water and in the frost flowers. Temperatures below -8 °C there is an increase loss of sodium and sulfate in relation to a decreasing temperature resulting in a depletion of aerosol from frost flowers at such temperatures in contrast to other ions.[6][13] Frost flowers aerosol will have a higher sodium to sulfate ratio in comparison to aerosol from seawater because sulfate has a greater proportion being removed than sodium when mirabilite (Na2SO4 · 10H2O) precipitates.[6][13] Frost flowers have a high concentration, typically 2 to 3 times greater, of bromide ions than found in seawater which is proportional to the salinity in the frost flowers. If the temperature were low enough for the sodium chloride that is present in the brine or frost flowers to freeze out, then the bromide may become readily available.[14] Ice surface temperatures below -22 °C start to precipitate out sodium chloride and even lower temperatures other ions will precipitate out, but with surface ice temperature that low frost flowers cannot form, so it is unlikely that there will be depleted sodium chloride.[13]

Aerosol release

Frost flowers have attracted interest as a possible source of polar atmospheric aerosol. High chemical concentrations and the extended surface area may facilitate efficient release into the atmosphere. In particular studies have shown that abundance of frost flowers can be linked to high concentrations of tropospheric bromine monoxide causing tropospheric ozone depletion events, and higher quantities of airborne sea-salt particles.[15] The study Obbard et al. (2009) addressing the concern of bromine, which may be causing the ozone depletion, showed no conclusive evidence that the frost flower aerosol is causing a significant contribution of bromine enrichment into the atmosphere. Furthermore, the study showed that there was bromine depletion as well as enrichment relative to chloride in frost flowers.[16]

Arctic "sea meadows"

On Sept. 2, 2009, a University of Washington biology team sailing back from the North Pole encountered these little flowery things growing on the frozen sea "like a meadow spreading off in all directions. Every available surface was covered with them." When allowed to melt, the one to two milliliters of water recovered was found to hold about a million bacteria. Professor Jody Deming believes that as the poles warm, there will be more and more of these meadows, because there will be more and more open sea that turns to thin ice in winter, and her team is eager to discover what the bacteria living in the frost flowers are doing.[17][18]

See also

References

  1. ^ "Mystery of frost flower growth explained - environment". New Scientist. 20 May 2009. Retrieved 2010-03-28.
  2. ^ "University of Leeds - Christmassy frost flowers - or symbols of climate change?". Leeds.ac.uk. 2009-12-17. Retrieved 2010-03-28.
  3. ^ a b Roscoe, H. K.; Brooks, B.; Jackson, A. V.; Smith, M. H.; Walker, S. J.; Obbard, R. W.; Wolff, E. W. (2011). "Frost flowers in the laboratory: Growth, characteristics, aerosol, and the underlying sea ice". Journal of Geophysical Research. 116 (D12): D12301. Bibcode:2011JGRD..11612301R. doi:10.1029/2010JD015144.
  4. ^ a b c d Style, R. W.; Worster, M. G. (2009). "Frost flower formation on sea ice and lake ice" (PDF). Geophysical Research Letters. 36 (11): L11501. Bibcode:2009GeoRL..3611501S. CiteSeerX 10.1.1.586.182. doi:10.1029/2009GL037304. S2CID 28823749..
  5. ^ a b Perovich, D.K; Richter-Menge, J.A (1994). "Surface Characteristics of lead ice". Geophysical Research Letters. 99 (C8): 16341–16350. Bibcode:1994JGR....9916341P. doi:10.1029/94JC01194.
  6. ^ a b c Rankin, A. M.; Auld, V.; Wolff, E. W. (2000-11-01). "Frost flowers as a source of fractionated sea salt aerosol in the polar regions" (PDF). Geophysical Research Letters. 27 (21): 3469–3472. Bibcode:2000GeoRL..27.3469R. doi:10.1029/2000GL011771. ISSN 1944-8007.
  7. ^ a b c d e Alvarez-Aviles, Laura; Simpson, William R.; Douglas, Thomas A.; Sturm, Matthew; Perovich, Donald; Domine, Florent (2008-11-16). "Frost flower chemical composition during growth and its implications for aerosol production and bromine activation". Journal of Geophysical Research: Atmospheres. 113 (D21): D21304. Bibcode:2008JGRD..11321304A. doi:10.1029/2008JD010277. ISSN 2156-2202.
  8. ^ Martin, S.; Drucker, R.; Fort, M. (1995). "A laboratory study of frost flower growth on the surface of young sea ice". Journal of Geophysical Research. 100 (C4): 7027. Bibcode:1995JGR...100.7027M. doi:10.1029/94JC03243.
  9. ^ a b Martin, Seelye; Yu, Yanling; Drucker, Robert (1996-05-15). "The temperature dependence of frost flower growth on laboratory sea ice and the effect of the flowers on infrared observations of the surface". Journal of Geophysical Research: Oceans. 101 (C5): 12111–12125. Bibcode:1996JGR...10112111M. doi:10.1029/96JC00208. ISSN 2156-2202.
  10. ^ Nelson, J (2001). "Growth mechanisms to explain the primary and secondary habits of snow crystals". Philos. Mag. A. 81 (10): 2337–2373. Bibcode:2001PMagA..81.2337N. doi:10.1080/01418610010030050.
  11. ^ Domine, Florent; Taillandier, Anne Sophie; Simpson, William R.; Severin, Ken (2005-07-01). "Specific surface area, density and microstructure of frost flowers" (PDF). Geophysical Research Letters. 32 (13): L13502. Bibcode:2005GeoRL..3213502D. doi:10.1029/2005GL023245. ISSN 1944-8007. S2CID 140605508.
  12. ^ Perovich, D. K.; Richter-Menge, J. A. (1994). "Surface characteristics of lead ice" (PDF). Journal of Geophysical Research. 99 (C8): 16341. Bibcode:1994JGR....9916341P. doi:10.1029/94JC01194. Archived from the original (PDF) on 2013-02-17. Retrieved 2015-08-29.
  13. ^ a b c Rankin, Andrew M.; Wolff, Eric W.; Martin, Seelye (2002-12-16). "Frost flowers: Implications for tropospheric chemistry and ice core interpretation". Journal of Geophysical Research: Atmospheres. 107 (D23): 4683. Bibcode:2002JGRD..107.4683R. doi:10.1029/2002JD002492. ISSN 2156-2202.
  14. ^ Koop, T; Kapilashrami, A; Molina, L.T.; Molina, M.J. (2000). "Phase transitions of sea-salt/water mixtures at low temperatures: implications for ozone chemistry in the polar marine boundary layers". J. Geophys. Res. 105 (D21): 26393–26402. Bibcode:2000JGR...10526393K. doi:10.1029/2000JD900413.
  15. ^ Kaleschke, L.; Richter, A.; Burrows, J.; Afe, O.; Heygster, G.; Notholt, J.; Rankin, A. M.; Roscoe, H. K.; Hollwedel, J.; Wagner, T.; Jacobi, H.-W. (2004). "Frost flowers on sea ice as a source of sea salt and their influence on tropospheric halogen chemistry" (PDF). Geophysical Research Letters. 31 (16): L16114. Bibcode:2004GeoRL..3116114K. doi:10.1029/2004GL020655. S2CID 51990522.
  16. ^ Obbard, Rachel W.; Roscoe, Howard K.; Wolff, Eric W.; Atkinson, Helen M. (2009-10-27). "Frost flower surface area and chemistry as a function of salinity and temperature". Journal of Geophysical Research: Atmospheres. 114 (D20): D20305. Bibcode:2009JGRD..11420305O. doi:10.1029/2009JD012481. ISSN 2156-2202.
  17. ^ Robert Krulwich (December 19, 2012). "Suddenly There's A Meadow In The Ocean With 'Flowers' Everywhere". NPR. Retrieved December 30, 2012. It was three, maybe four o'clock in the morning when he first saw them. Grad student Jeff Bowman was on the deck of a ship; he and a University of Washington biology team were on their way back from the North Pole.
  18. ^ Jeff S. Bowman and Jody W. Deming (January 21, 2012). "Elevated bacterial abundance in laboratory-grown and naturally occurring frost flowers under late winter conditions" (PDF). University of Washington School of Oceanography and Astrobiology Program. Archived from the original (PDF) on 2013-01-07. Retrieved December 30, 2012. ABSTRACT Sea ice has been identified as an important microbial habitat, with bacteria and other microbes concentrated in the brine inclusions between ice crystals.... The presence of elevated numbers of bacteria in frost flowers may have implications for the previously observed chemical reactions that take place in them, especially if microbial activity can be shown to occur in this unique low temperature, low water

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