شلالات الدم هي الاسم الذي يتم إطلاقه على الشلالات الناتجة عن مادة أكسيد الحديد وقد أطلق عليها هذا الاسم نتيجة للون الأحمر القاتم الذي تتميز به، مما يعطيها مظهراً مشابهاً للدم. تنساب هذه الشلالات من لسان مثلجة تيلر إلى السطح المغطى بالثلج لغرب بحيرة بوني في وادي تيلر في وديان مكموردوالجافة في أرض ڤكتوريا شرق القارة القطبية المتجمدة أنتارتيكا.

Iron-rich hypersaline water sporadically emerges from small fissures in the ice cascades. The saltwater source is a subglacial pool of unknown size overlain by about 400 مترs (1,300 قدم) of ice several kilometers from its tiny outlet at Blood Falls.

يعود فضل هذا الاكتشاف إلى الجيولوجي الأسترالي توماس گرفث تيلر سنة 1911، ومنه أخذ نهر تيلر تسميته. رواد أنتارتيكا الأوائل عزوا اللون الأحمر إلى الطحالب الحمراء، ولاحقاً ثبت أنها بسبب أكسيد الحديد.

يتكون نهر تايلور من خمس طبقات جليدية تنساب ببطء شديد جداً، حيث افترض الفهماء سابقاً حتى سبب اللون الأحمر القاتم عائد إلى تواجد نوع من الطحالب، لكن البحوث المستمرة في هذا الميدان نفت هذه الفرضيات. يعود اللون الأحمر للشلالات لأكسيد الحديد الذي يتكون جراء تفاعلات أكسدة بين عناصر معينة متواجدة في المياه شديدة الملوحة بالمنطقة، وقد كانت هذه العناصر مخزنة ومعزولة في جيوب ينعدم فيها الهواء والضوء منذ خمسة ملايين سنة، حين كانت المياه أعلى من مستوياتها التي هي عليها حالياً. فتح هذا اللغز نافذة للبحوث الكثيرة التي سيحاول الفهماء الإجابة بها عن مختلف التساؤلات المتعلقة بهذا النوع من الحياة المعزولة تماماً عن العالم المحيط بنا.

الكيمياء الأرضية

Poorly soluble hydrous ferric oxides are deposited at the surface of ice after the ferrous ions present in the unfrozen saltwater are oxidized in contact with atmospheric oxygen. The more soluble ferrous ions initially are dissolved in old seawater trapped in an ancient pocket remaining from the Antarctic Ocean when a fjord was isolated by the glacier in its progression during the Miocene period, some 5 million years ago when the sea level was higher than today.

Unlike most Antarctic glaciers, the Taylor glacier is not frozen to the bedrock, probably because of the presence of salts concentrated by the crystallization of the ancient seawater imprisoned below it. Salt cryo-concentration occurred in the deep relict seawater when pure ice crystallised and expelled its dissolved salts as it cooled down because of the heat exchange of the captive liquid seawater with the enormous ice mass of the glacier. As a consequence, the trapped seawater was concentrated in brines with a salinity two to three times that of the mean ocean water. A second mechanism sometimes also explaining the formation of hypersaline brines is the water evaporation of surface lakes directly exposed to the very dry polar atmosphere in the McMurdo Dry Valleys. The analyses of stable isotopes of water allow, in principle, to distinguish between both processes as long as there is no mixing between differently formed brines.

Hypersaline fluid, sampled fortuitously through a crack in the ice, was oxygen-free and rich in sulfate and ferrous ion. Sulfate is a remnant geochemical signature of marine conditions while soluble divalent iron likely was liberated under reducing conditions from the subglacial bedrock minerals weathered by microbial activity.

النظام البيئي الجرثومي

Chemical and microbial analyses both indicate that a rare subglacial ecosystem of autotrophic bacteria developed that metabolizes sulfate and ferric ions. According to geomicrobiologist Jill Mikucki at the University of Tennessee, water samples from Blood Falls contained at least 17 different types of microbes, and almost no oxygen. An explanation may be that the microbes use sulfate as a catalyst to respire with ferric ions and metabolize the trace levels of organic matter trapped with them. Such a metabolic process had never before been observed in nature.

A puzzling observation is the coexistence of [[ferrous ion|نطقب:Chem2]] and [[sulfate|نطقب:Chem2]] ions under anoxic conditions. No sulfide anions ([[sulfide|نطقب:Chem2]]) are found in the system. This suggests an intricate and poorly understood interaction between the sulfur and the iron biochemical cycles.

In December 2014, scientists and engineers led by Mikucki returned to Taylor Glacier and used a probe called IceMole, designed by a German collaboration, to melt into the glacier and directly sample the salty water (brine) that feeds Blood Falls.

Implications for the Snowball Earth hypothesis

According to Mikucki et al. (2009), the now-inaccessible subglacial pool was sealed off 1.5 to 2 million years ago and transformed into a kind of "time capsule", isolating the ancient microbial population for a sufficiently long time to evolve independently of other similar marine organisms. It explains how other microorganisms could have survived when the Earth (according to the Snowball Earth hypothesis) was entirely frozen over.

Ice-covered oceans might have been the only refugia for microbial ecosystems when the Earth apparently was covered by glaciers at tropical latitudes during the Proterozoic eon about 650 to 750 million years ago.

Implications for astrobiology

This unusual place offers scientists a unique opportunity to study deep subsurface microbial life in extreme conditions without the need to drill deep boreholes in the polar ice cap, with the associated contamination risk of a fragile and still-intact environment.

The study of harsh environments on Earth is useful to understand the range of conditions to which life can adapt and to advance assessment of the possibility of life elsewhere in the solar system, in places such as Mars or Europa, an ice-covered moon of Jupiter. Scientists of the NASA Astrobiology Institute speculate that these worlds could contain subglacial liquid water environments favourable to hosting elementary forms of life, which would be better protected at depth from ultraviolet and cosmic radiation than on the surface.

انظر أيضاً

• Extremophiles (organisms resistant to extreme conditions)
  • Psychrophile (bacteria resistant to cold)
• Cryoconcentration of hypersaline brines
  • Eutectic system
  • Freezing-point depression
• Life on Mars

المصادر

للاستزادة

• Lerman, Abraham (February 2009). "Saline Lakes' Response to Global Change". Aquatic Geochemistry. 15 (1–2): 1–5. doi:10.1007/s10498-008-9058-8.
• Green, William J.; Lyons, W. Berry (February 2009). "The Saline Lakes of the McMurdo Dry Valleys, Antarctica". Aquatic Geochemistry. 15 (1–2): 321–348. doi:10.1007/s10498-008-9052-1.
• Kluger, Jeffrey (April 18, 2009). "An Organism Survives Antarctica, and Maybe Mars". Time.
• Tierney, John (April 19, 2009). "The Dark Secret at Blood Falls". The New York Times.
• "Newly Discovered Iron-breathing Species Have Lived In Cold Isolation For Millions Of Years". ScienceDaily. Harvard University. April 17, 2009.
• Badgeley, Jessica A.; Pettit, Erin C.; Carr, Christina G.; et al. (24 April 2017). "An englacial hydrologic system of brine within a cold glacier: Blood Falls, McMurdo Dry Valleys, Antarctica". Journal of Glaciology. 63 (239): 387–400. Bibcode:2017JGlac..63..387B. doi:10.1017/jog.2017.16.

وصلات خارجية

مشاع الفهم فيه ميديا متعلقة بموضوع Taylor Glacier.

• Blood Falls, Antarctica's Dry Valleys by NASA Earth Observatory
• Blood Falls - 100 Wonders at YouTube by Atlas Obscura

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