Blood Falls is an outflow of aniron(III) oxide–taintedplume of saltwater, flowing from the tongue ofTaylor Glacier onto the ice-covered surface ofWest Lake Bonney in theTaylor Valley of theMcMurdo Dry Valleys inVictoria Land,East Antarctica.
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 metres (1,300 ft) of ice, several kilometers from its tiny outlet at Blood Falls.
The reddish deposit was found in 1911 by the Australian geologistThomas Griffith Taylor, who first explored the valley that bears his name.[1] The Antarctica pioneers first attributed the red color tored algae, but later it was proven to be due to iron oxides.
Poorly solublehydrous ferric oxides are deposited at the surface of ice after theferrous ions present in the unfrozen saltwater areoxidized in contact with atmosphericoxygen. The more soluble ferrous ions initially are dissolved in old seawater trapped in an ancient pocket remaining from theAntarctic Ocean when afjord was isolated by the glacier in its progression during theMiocene 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 thebedrock, probably because of the presence of salts concentrated by thecrystallization of the ancientseawater imprisoned below it. Saltcryo-concentration occurred in the deep relict seawater when pure ice crystallized and expelled its dissolved salts as it cooled down because of theheat exchange of the captive liquid seawater with the enormous ice mass of the glacier. As a consequence, the trapped seawater was concentrated inbrines with asalinity two to three times that of themean 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.[2]
Hypersaline fluid, sampled fortuitously through a crack in the ice, was oxygen-free and rich insulfate and ferrous ion. Sulfate is a remnant geochemical signature of marine conditions, while soluble divalent iron was likely liberated under reducing conditions from the subglacial bedrock minerals weathered by microbial activity.
Chemical and microbial analyses both indicate that a rare subglacialecosystem ofautotrophicbacteria developed that metabolizes sulfate andferric ions.[3][4] According togeomicrobiologistJill Mikucki at theUniversity of Tennessee, water samples from Blood Falls contained at least 17 different types ofmicrobes, and almost no oxygen.[3] An explanation may be that the microbes use sulfate to respire with ferric ions and metabolize the trace levels oforganic matter trapped with them. Such a metabolic process had never before been observed in nature.[3]
A puzzling observation is the coexistence of ferrous and sulfate ions underanoxic conditions. Nosulfide anions are found in the system. This suggests an intricate and poorly understood interaction between thesulfur and theiron biochemical cycles.
In December 2014, scientists and engineers led by Mikucki returned to Taylor Glacier and used a probe calledIceMole, designed by a German collaboration, to melt into the glacier and directly sample the salty water (brine) that feeds Blood Falls.[5]
Samples were analyzed, and revealed a cold (−7 °C (19 °F)), iron-rich (3.4 mM) subglacial brine (8%sodium chloride). From these samples, scientists isolated and characterized a type of bacteria capable of growing in salty water (halophilic), that thrives in the cold (psychrophile), and isheterotrophic, which they assigned to the genusMarinobacter.[6] DNAbioinformatic analysis indicated the presence of at least four gene clusters involved in secondary metabolism. Two gene clusters are related to the production ofarylpolyenes, which function asantioxidants that protect the bacteria fromreactive oxygen species.[6] Another gene cluster seems to be involved interpene biosynthesis, most likely to producepigments.[6] Other bacteria identified areThiomicrospira sp., andDesulfocapsa sp.
According to Mikuckiet al. (2009), the now-inaccessible subglacial pool was sealed off1.5 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 theSnowball Earth hypothesis) was entirely frozen over.
Ice-covered oceans might have been the onlyrefugium for microbial ecosystems when the Earth apparently was covered by glaciers at tropical latitudes during theProterozoiceon about650 to 750 million years ago.
This unusual place offers scientists a unique opportunity to study deep subsurface microbial life in extreme conditions without the need to drill deepboreholes in the polarice 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 theSolar System, in places such asMars orEuropa, an ice-covered moon ofJupiter. Scientists of theNASA Astrobiology Institute speculate that these worlds could contain subglacial liquid water environments favorable to hosting elementary forms of life, which would be better protected at depth fromultraviolet andcosmic radiation than on the surface.[7][8]