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hydrological system is channelized or distributed. Channelized systems are an efficient form of drainage as they are able to rapidly move water out of the glacier, reducing water pressure in the system. By decreasing water pressure underneath the glacier, friction between the glacier ice and the bedrock below increases, slowing the movement of the glacier. Distributed flow systems, contrastingly, are characterized by slow-moving water in small cavities and passages; when water flux into the system increases, such as during periods of high melt, the system is unable to compensate, resulting in large increases in basal water pressure. As a result, friction between the glacier and the bed is reduced, and glacial sliding speed increases. Glacial motion can also cause changes in subglacial stream systems, and there are feedbacks present between the two. As subglacial water pressure increases, the speed of glacial sliding increases. The glacier encounters bumps in the bedrock as it slides: as a result, cavities are created between the ice and the bed. The glacier encounters more bumps due to its higher speed and, since ice moving at a higher speed is less able to maintain connection with the bedrock, faster moving glaciers are more likely to form cavities when passing over bumps. This increases the subglacial space which can be filled with water, decreasing basal water pressure. The interaction between glacial motion and subglacial hydrology creates a negative feedback loop, in which increased water pressure below the glacier increases glacial sliding speed, which in turn decreases pressure and, consequently, sliding speed. Through this mechanism, the effects of speedup events can decay over time. Another control on glacial sliding speed is the process of channelization. Sustained high levels of meltwater input result in a shift from a distributed network of subglacial streams to a more channelized system as larger passages through the ice develop. As larger channels are able to more efficiently remove water from the subglacial system, water pressure decreases, increasing friction between the glacier and the bedrock and decreasing sliding speed. Channelization is the most significant process in terminating speedup events, and is responsible for the slowdown in glacial speed at the end of summer following the speedup commonly observed as meltwater flow increases in spring.
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all meltwater is derived from basal melting and the release of stored meltwater. Both of these sources involve small amounts of water released relatively uniformly throughout the bed of the glacier, making them unlikely to form large drainage channels. Some major tunnels remain in the system year-round, and are the main points of discharge during the winter, but the system at large is characterized by distributed drainage. As temperatures rise and surface melting increases water flux to the bed in late spring, the winter stream system is disrupted. Distributed flow channels, lacking the capacity for increased volumes of meltwater, experience a rise in water pressure and are destabilized. High water pressures lead to the formation of larger tunnels—a process known as channelization—that have a greater capacity for meltwater and allow for pressures to fall. This change can happen gradually or can be triggered by events that rapidly increase meltwater flow, such as consecutive days of high melting or a large rainstorm. The now-channelized system grows in extent throughout the summer as meltwater input continues to increase, with new passages forming and growing in size. In autumn, surface melting decreases, and the volume of meltwater is no longer sufficient to maintain the newly formed channels; deformation of the surrounding ice slowly closes channels that do not generate enough frictional melting along their walls to offset the closure. Eventually, a distributed stream system again becomes dominant. Some perennial channels remain throughout the winter season, but the channels formed in spring disappear—when new tunnels form again the next year, they do not form in the same locations as the ones that closed.
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glacier, which can serve as heat sources for glacial melt. Ice melt due to discharge plumes has a significant impact in areas in which discharge rates exceed 100 m/s; with lesser discharge rates, plume-associated heat is insignificant compared to the effects of ocean mixing. Seasonal variability plays an important role in the way that subglacial streams influence glacial melting. During the summer, subglacial stream output is much greater, resulting in plumes that are larger, faster, and more buoyant than during the winter. In addition to the greater volume of discharge increasing glacial melt, the increased buoyancy of the plume results in more turbulence and, consequently, more heat transfer to the glacier, further increasing melt. The effect subglacial stream discharge has on glacial melt is also influenced by the type of subglacial drainage system; distributed subglacial streams result in an output of meltwater uniformly across the grounding line (where the glacier transitions from grounded to floating ice), whereas channelized drainage results in individual, large outlets. Distributed discharge results in glacial melt volumes up to five times greater than that of channelized drainage, as individual strong plumes of meltwater are not as capable of inducing widespread melting as a much greater number of smaller outputs.
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terminus. Water is fast-moving in these systems, and pressure inside the channels is relatively low compared to pressure in the ice around them. Turbulence in the rapid flow produces heat, which is able to melt the ice walls of the tunnels. While the total water added to the system by this process is insignificant compared to water from the surface and from basal melting, the melting of the channel walls allows the channel to remain open even when the ice pressures surrounding it are much greater than the pressure of the water inside. The constant erosion of the tunnel walls is able to offset the narrowing of the tunnel caused by deformation of the ice. Depending on the water supply and the characteristics of the bed, the tunnels can take different forms, including semicircular tunnels cutting into the ice, broad and low tunnels, and tunnels that cut into the bed rather than the ice. Broad and low tunnels form in channels with variable amounts of meltwater, as melting is concentrated on the tunnel walls rather than the ceiling when the tunnel is not completely full of water. Channels that maintain long-term stability in water flow and location can erode the bedrock over time, resulting in tunnels that cut into the bed rather than the ice above.
122:. In cases in which glaciers are above porous, unconsolidated sediment, some water can flow through the sediment; like film flow, porous flow does not account for much of the water flux in the system. When the bed is deformable, wide, shallow canals up to 10 cm in width can form in the surface of the sediment, topped by the glacial ice. In glaciers with steep slopes, canal systems are unstable, as they can be easily absorbed by channels above the sediment. As glaciers move over bumps in the bedrock, differences in pressure can separate the ice from the bed behind the bump if the glacier is moving fast enough. This creates cavities between the glacier and the bed, which fill with water. If the water pressure is high enough, the cavity expands, and the water can cause more separation between the ice and bed surrounding the cavity. With sustained water supply, small passages form between cavities, creating a large network of linked cavities which water flows between. Water in linked cavity systems flows, on average, in a direction normal to the equipotential surfaces of pressure in the glacier. However, the path taken is long and indirect, and at times water can be flowing nearly parallel to the equipotential surfaces.
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hydrology of subglacial systems, changing the timing of seasonal variations. As a result of climate change-induced increases in meltwater, greater volumes of water are likely to reach the bed earlier in the year. This would cause the transition from winter distributed subglacial drainage to summer channelized streams to occur earlier in the year. Glacial motion could also be affected: since glaciers dominated by channelized systems have lesser sliding speeds, the earlier transition to this system could result in slower moving glaciers. However, short-term fluctuations in meltwater volume and pressure, which may become more intense as runoff increases, could offset this decrease in sliding by causing localized speedups. Increases in the volume of discharge from subglacial streams are likely to increase the melting of marine-terminating glaciers, as submarine melt rates are highly sensitive to the amount of subglacial discharge.
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180:, or DOC, flows into glaciers, eventually reaching subglacial stream systems, which carry the organic matter out of the glacier. This source of DOC is supplemented by organic matter produced within subglacial ecosystems, where there are diverse microbial communities. Though the concentration of dissolved organic matter in glacial meltwater is low, the sheer amount of freshwater discharge from glaciers makes glacially-sourced DOC an important source of bioavailable carbon to marine ecosystems. In the
210:: an elongated ridge of sediment that fills the channel of the subglacial stream in which it forms. These eskers can be temporary, lasting only until increasing water pressure during the next melt season flushes out the sediment, or they could be permanent. The permanent formation of eskers is more common in retreating glaciers and ice sheets, as their termini are thinning, which favors the deposition of sediment. Advancing glaciers and ice sheets exhibit steepening termini, which increases
66:(the combination of temperature and pressure at which ice melts). This liquid water at the bed—present in temperate but not polar glaciers—provides a constant input of water to subglacial stream systems. Water from these two sources meets and is concentrated at the bedrock base of the glacier, where pressure from the ice above forces it to move towards the glacial terminus, creating a network of passageways as it works its way out of the glacier.
62:(large, vertical shafts up to ten meters wide that range from the surface to a lower elevation, sometimes all the way to the glacial bed). Fractures, crevasses, and cavities between glaciers and valley walls can also provide pathways for water to reach the bed. While surface meltwater can be seasonally dependent, the beds of temperate glaciers are maintained at the
206:, and studies modeling subglacial channels suggest that seasonal meltwater flow alone can erode bedrock and transport sediment as large as boulders. Contrastingly, when water pressure is low, such as at the end of a melt season, sediment is deposited. When sediment supply is high enough, the sediment deposition can form an
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The discharge of subglacial stream systems of marine-terminating glaciers into the ocean has a significant impact on the volume and distribution of glacial melt at the terminus. The discharge of glacial streams into the ocean emerges as plumes that travel up to the ocean surface along the face of the
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The structure of subglacial stream systems changes significantly over time as a result of seasonal changes in the volume and source of meltwater input. During the winter, subglacial stream systems are dominated by distributed streams. As there is very little surface melting during this season, nearly
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Subglacial streams can transport, deposit, and remove sediment from the glacier bed; this process is influenced by water supply and the amount and characteristics of the available sediment. The size of sediment particles, the slope of the subglacial stream’s channel, and the roughness of the bed all
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Water in subglacial streams is subject to large amounts of pressure from the mass of ice above; as a result, the direction of water flow cannot be explained in the same way as typical surface streams. Subglacial water flow is, to a large extent, determined by pressure gradients created by the weight
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Anthropogenic climate change is likely to cause significant changes in subglacial stream systems. As glacial melting increases as a result of rising global temperatures, water flux into and discharge from subglacial streams increases as well. Greater water input from surface melting may affect the
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alone, glacial runoff provides 0.13 Tg of organic carbon per year, much of which travels through subglacial streams. Subglacial streams also transport various other important nutrients. Geological processes, including the grinding of glaciers on the bedrock below and water-rock interaction, ensure
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and canals in the sediment, and a thin film between the ice and bed. Films of water between the ice and bedrock form are rarely thicker than tens of micrometers, and form in areas that are isolated from channels and cavities, maintained at the pressure melting point, and are above impermeable bed.
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Channelized drainage systems are characterized by water flowing predominantly through tunnels along the bed of the glacier that take meltwater rapidly and directly to the glacial terminus. These tunnels are arranged in a network of tributaries, joining together and growing in size as they near the
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In temperate glaciers, which are characterized by the presence of liquid water at their base and are able to slide, subglacial streams have a significant impact on glacial movement. The water pressure and friction experienced at the base of a glacier depends in part on whether the subglacial
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Subglacial streams derive their water from two sources: meltwater transported from the top of the glacier and meltwater from the glacial bed. When temperatures are high enough to induce melting on the surface of the glacier, typically during summer, water flows down into the glacier. Surface
41:, where it exits the glacier. Discharge from subglacial streams can have a significant impact on local, and in some cases global, environmental and geological conditions. Sediments, nutrients, and organic matter contained in the meltwater can all influence downstream and marine conditions.
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below, where they form subglacial streams. Subglacial streams form a system of tunnels and interlinked cavities and conduits, with water flowing under extreme pressures from the ice above; as a result, flow direction is determined by the pressure gradient from the ice and the
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of the bed rather than gravity. Subglacial streams form a dynamic system that is responsive to changing conditions, and the system can change significantly in response to seasonal variation in meltwater and temperature. Water from subglacial streams is routed towards the
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Beaud, Flavien, Gwenn E. Flowers, and Jeremy G. Venditti. “Modeling
Sediment Transport in Ice-Walled Subglacial Channels and Its Implications for Esker Formation and Proglacial Sediment Yields.” Journal of Geophysical Research: Earth Surface 123, no. 12 (2018): 3206–27.
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Bendtsen, Jørgen, John
Mortensen, Kunuk Lennert, and Søren Rysgaard. “Heat Sources for Glacial Ice Melt in a West Greenland Tidewater Outlet Glacier Fjord: The Role of Subglacial Freshwater Discharge.” Geophysical Research Letters 42, no. 10 (2015): 4089–95.
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Mayaud, Jerome R., Alison F. Banwell, Neil S. Arnold, and Ian C. Willis. “Modeling the
Response of Subglacial Drainage at Paakitsoq, West Greenland, to 21st Century Climate Change.” Journal of Geophysical Research: Earth Surface 119, no. 12 (2014): 2619–34.
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Hood, Eran, Jason
Fellman, Robert G. M. Spencer, Peter J. Hernes, Rick Edwards, David D’Amore, and Durelle Scott. “Glaciers as a Source of Ancient and Labile Organic Matter to the Marine Environment.” Nature 462, no. 7276 (December 2009): 1044–47.
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Boulton, G. S., R. Lunn, P. Vidstrand, and S. Zatsepin. “Subglacial
Drainage by Groundwater-Channel Coupling, and the Origin of Esker Systems: Part 1—Glaciological Observations.” Quaternary Science Reviews 26, no. 7 (April 1, 2007): 1067–90.
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that minerals are continuously fed into the subglacial system. Iron transported by subglacial streams, for example, is mostly sourced from subglacial weathering, and may be responsible for an Fe flux large enough to significantly influence
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Stevenson, E. I., M. S. Fantle, S. B. Das, H. M. Williams, and S. M. Aciego. “The Iron
Isotopic Composition of Subglacial Streams Draining the Greenland Ice Sheet.” Geochimica et Cosmochimica Acta 213 (September 15, 2017): 237–54.
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Subglacial streams carry a significant amount of organic matter and nutrients, originating both from supraglacial meltwater and subglacial processes. Meltwater from supraglacial environments containing microbially-produced
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Slater, D. A., P. W. Nienow, T. R. Cowton, D. N. Goldberg, and A. J. Sole. “Effect of Near-Terminus
Subglacial Hydrology on Tidewater Glacier Submarine Melt Rates.” Geophysical Research Letters 42, no. 8 (2015): 2861–68.
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meltwater flows downward through millimeter-sized channels that join together in a network of tributaries, growing in size until reaching the bedrock. Additionally, some water is transported to the surface by
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and movement of the glacier. As a result, instead of following the slope of the bed, streams can flow up and across slopes. This behavior can be described by viewing the pressure inside glaciers as
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Ren, Ze, Nicolas
Martyniuk, Isabella A. Oleksy, Anshuman Swain, and Scott Hotaling. “Ecological Stoichiometry of the Mountain Cryosphere.” Frontiers in Ecology and Evolution 7 (2019).
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Hoffman, Matthew, and
Stephen Price. “Feedbacks between Coupled Subglacial Hydrology and Glacier Dynamics.” Journal of Geophysical Research: Earth Surface 119, no. 3 (2014): 414–36.
554:, and P. Heimbach. “Seasonal Variability of Submarine Melt Rate and Circulation in an East Greenland Fjord.” Journal of Geophysical Research: Oceans 118, no. 5 (2013): 2492–2506.
27:. Meltwater from the glacial surface travels downward throughout the glacier, forming an englacial drainage system consisting of a network of passages that eventually reach the
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Subglacial stream systems can be placed in two categories based upon the arrangement and type of passages that make up the system: channelized and distributed.
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may have a significant impact on subglacial stream systems, increasing the volume of meltwater entering subglacial drainage systems and influencing their
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Walder, Joseph S., and Andrew Fowler. “Channelized
Subglacial Drainage over a Deformable Bed.” Journal of Glaciology 40, no. 134 (ed 1994): 3–15.
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Hooke, Roger LeB. “Englacial and Subglacial Hydrology: A Qualitative Review.” Arctic and Alpine Research 21, no. 3 (August 1, 1989): 221–33.
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Hewitt, Ian J., and Timothy T. Creyts. “A Model for the Formation of Eskers.” Geophysical Research Letters 46, no. 12 (2019): 6673–80.
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Hubbard, Bryn, and Peter Nienow. “Alpine Subglacial Hydrology.” Quaternary Science Reviews 16, no. 9 (January 1, 1997): 939–55.
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Flow in films does not account for a large amount of the total meltwater flux out of the glacier, but may be important in the
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contribute to whether sediment is mobilized or deposited. Subglacial flooding events can result in significant erosion and
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1197:Glaciofluvial
1194:
1188:
1187:Veiki moraine
1185:
1183:
1180:
1178:
1175:
1173:
1170:
1168:
1167:Rogen moraine
1165:
1163:
1162:Pulju moraine
1160:
1158:
1155:
1153:
1150:
1148:
1147:Erratic block
1145:
1143:
1140:
1139:
1137:
1133:
1127:
1126:
1122:
1120:
1117:
1115:
1112:
1110:
1109:Tunnel valley
1107:
1105:
1104:Trough valley
1102:
1100:
1097:
1095:
1092:
1090:
1087:
1085:
1082:
1080:
1077:
1075:
1072:
1070:
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1062:
1060:
1057:
1055:
1052:
1050:
1047:
1045:
1042:
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1037:
1035:
1034:Crag and tail
1032:
1030:
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1025:
1022:
1020:
1017:
1016:
1014:
1010:
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985:
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964:
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949:
943:
940:
938:
935:
933:
930:
928:
925:
923:
920:
918:
917:Overdeepening
915:
913:
910:
908:
905:
903:
900:
898:
895:
893:
892:Basal sliding
890:
888:
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883:
880:
879:
877:
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867:
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849:
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844:
842:
839:
837:
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829:
827:
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822:
819:
817:
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812:
809:
807:
804:
802:
799:
797:
794:
792:
789:
787:
784:
782:
779:
777:
774:
772:
771:Ablation zone
769:
768:
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762:
756:
753:
751:
748:
746:
743:
741:
738:
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733:
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249:
247:
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243:
241:
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237:
232:
224:
215:
213:
209:
205:
194:
192:
191:nitrification
188:
183:
179:
163:
154:
146:
132:
123:
121:
116:
106:
97:
89:
87:
83:
82:equipotential
67:
65:
61:
50:
48:
44:
40:
35:
30:
26:
22:
18:
1135:Depositional
1125:Zungenbecken
1123:
1054:Glacial lake
1049:Glacial horn
963:Mass balance
951:Measurements
887:Accumulation
811:Glacier head
806:Glacier cave
750:Rock glacier
613:
221:
200:
174:
160:
151:
129:
112:
103:
95:
78:
56:
16:
15:
1240:Outwash fan
1235:Kettle hole
1119:Valley step
1099:Trough lake
1079:Ribbon lake
1039:Finger lake
781:Bergschrund
552:C. Cenedese
115:porous flow
109:Distributed
100:Channelized
1297:Categories
1267:Glaciology
1250:Urstromtal
1230:Kame delta
1182:Till plain
979:Jökulhlaup
937:Starvation
816:Ice divide
730:Ice stream
227:References
34:topography
1012:Erosional
1004:Landforms
875:Processes
856:Randkluft
851:Penitente
796:Dirt cone
725:Ice shelf
720:Ice sheet
715:Ice field
70:Hydrology
53:Formation
47:hydrology
1303:Glaciers
1272:Category
1210:Diluvium
1205:Alpentor
1114:U-valley
958:Ice core
927:Plucking
882:Ablation
866:Terminus
791:Crevasse
786:Blue ice
686:Glaciers
197:Sediment
25:ice caps
21:glaciers
1152:Moraine
1142:Drumlin
1069:Nunatak
932:Retreat
897:Calving
841:Moraine
826:Icefall
764:Anatomy
735:Ledoyom
710:Ice cap
60:moulins
29:bedrock
1245:Sandur
1089:Suncup
1074:P-form
1024:Cirque
907:Motion
846:Moulin
705:Cirque
700:Aufeis
86:normal
1215:Esker
1044:Fjord
1019:ArĂŞte
942:Surge
902:Creep
861:SĂ©rac
693:Types
208:esker
1277:List
1225:Kame
1094:Tarn
994:Tuya
801:Firn
23:and
1299::
628:^
584:^
563:^
537:^
518:^
495:^
463:^
445:^
392:^
366:^
345:^
275:^
235:^
49:.
678:e
671:t
664:v
648:.
623:.
608:.
579:.
558:.
532:.
513:.
490:.
458:.
440:.
387:.
361:.
340:.
270:.
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