It was probably dammed by ice that filled the northwestern part of Taylor Valley,[1] when the Ross Ice Shelf expanded during the Last Glacial Maximum. Depending on how much the ice shelf expanded, two different sizes of the lakes are possible,[5] the smaller of which would have been limited to the Lake Bonney basin[6] at the western end of the valley.[7] Delta deposits that were reinterpreted as river deposits and salt accumulations below the maximum lake levels point to a small size of Lake Washburn[8] as do the properties of soils around the hypothesized shorelines,[9] while soil carbonates[10] and the presence of unfrozen groundwater in the area that would have been covered by a large Lake Washburn implies a large size.[7] The lake extended into Hjorth Valley.[11]
Water levels reached an elevation of 300–336 metres (984–1,102 ft) and its shores featured deltas and shorelines with algal mats.[5] The Canada Glacier, Commonwealth Glacier and Howard Glacier probably reached its shores, while other glaciers such as Crescent Glacier contributed runoff to Lake Washburn.[12] The Ross Ice Sheet formed a subaqueous moraine in Lake Washburn.[13]Icebergscalving from the Ross Ice Sheet pushed the lake ice away and transported debris into the lake,[14] where it generated ridges and mounds on the now exposed lakefloor.[15] Wind transported aeolian sediments into the lake.[16]
Chronology and disappearance
The ice dam existed between 23,000-8,340 radiocarbon years ago[5] and the extent of the lake fluctuated in response to climatic and glacier changes, but they are poorly documented.[17]Evaporation appears to be responsible for the decrease in water levels at the beginning of the Holocene,[18] after which it split into Lake Fryxell and Lake Hoare.[19] It is possible that leftover ice from the Ross Ice Sheet delayed the decline of the lake for millennia, into the middle Holocene.[6][20]
It left carbonates[21] and lake sediments in the Taylor Valley; organic matter in these sediments constitutes an important component of and organic matter input the present-day soils in the valley[22] while the presence of non-marine diatoms in the sediments demonstrates that they were not deposited by the sea.[23] The Alpine I Drift from later glacial advances overlies the Lake Washburn deposits.[24] Local springtail populations bear imprints of the former existence of the lake.[25] Later water level fluctuations in the valleys could have caused the lakes to dry out and overspill.[26]
Geographical and climatic context
The McMurdo Dry Valleys are presently unglaciated, as the Transantarctic Mountains block glaciers and moisture from reaching the valleys. Despite mean temperatures of −30 – −14.8 °C (−22.0–5.4 °F), meltwater from glaciers supply fresh water to the valleys, where it forms closed lakes (lakes without an outflow) that are perennially ice-covered.[27] During December and January, meltwater flows from the glaciers to the lakes.[5] Occasionally, during summer strong winds blow from the Antarctic Ice Sheet through the valleys to the sea; these winds warm as they descend into the valleys and can melt the ice, causing water levels to rise.[28]
There is evidence that during the last glacial maximum, these lakes expanded in size despite drier and colder conditions; several different mechanisms ranging from more effective melting of glaciers during drought, increased temperatures,[27] supply of subglacial water from the Ross Ice Shelf[29] to more intense winds that warmed the valleys, inducing ice melt, have been proposed.[30] The water in these lakes would have been prevented from discharging to the sea by the expanded Ross Ice Shelf, which at that time penetrated the valleys[31] and blocked the outlet at Explorers Cove.[32] Some of the lakes persisted into the Holocene; presumably their outlets were still blocked by glacier tills even after the ice had retreated at the beginning of the Holocene.[33]
Precise dating of these paleolakes is rendered difficult, however, by the rarity of dateable deposits and the unreliability of radiocarbon dates due to reservoir effects.[8] The existence of Lake Washburn appears to be secure;[5][2] a proposal that a lake also existed in Victoria Valley was contested as being due to landslides rather than from a former lake.[27] Present-day Lake Vanda may resemble the former Lake Washburn[34] although it has a distinct diatom fauna.[34]
Barrett, J. E.; Virginia, Ross A.; Wall, Diana H.; Parsons, Andrew N.; Powers, Laura E.; Burkins, Melody B. (November 2004). "Variation in Biogeochemistry and Soil Biodiversity Across Spatial Scales in a Polar Desert Ecosystem". Ecology. 85 (11): 3105–3118. Bibcode:2004Ecol...85.3105B. doi:10.1890/03-0213.
Hall, Brenda L.; Denton, George H.; Hendy, Chris H. (1 August 2000). "Evidence from taylor valley for a grounded ice sheet in the ross sea, antarctica". Geografiska Annaler: Series A, Physical Geography. 82 (2–3): 275–303. Bibcode:2000GeAnA..82..275H. doi:10.1111/j.0435-3676.2000.00126.x. S2CID129844638.
Konfirst, Matthew A.; Sjunneskog, Charlotte; Scherer, Reed P.; Doran, Peter T. (August 2011). "A diatom record of environmental change in Fryxell Basin, Taylor Valley, Antarctica, late Pleistocene to present". Journal of Paleolimnology. 46 (2): 257–272. Bibcode:2011JPall..46..257K. doi:10.1007/s10933-011-9537-6. S2CID128907135.
Stuiver, M.; Denton, G.H.; Hughes, T.J.; Fastook, J.L. (1981). "History of the marine ice sheet in West Antarctica during the last glaciation: A working hypothesis". In Denton, G.H.; Hughes, T.J. (eds.). The Last Great Ice Sheets.
