In the Taylor Valley of the McMurdo Dry Valleys in Victoria Land, East Antarctica, Bill Falls is the outflow of a plume of seawater contaminated with iron(III) oxide. The plume originates on the ice-covered surface of West Lake Bonney and flows from the tongue of Taylor Glacier.
Small cracks in the ice cascades occasionally release iron-rich hypersaline water. The saltwater source is located several kilometers from its modest outflow at Blood Falls in a subglacial lake of unknown size that is covered by around 400 meters (1,300 feet) of ice.
Australian geologist Thomas Griffith Taylor, who first studied the valley that bears his name, discovered the reddish deposit in 1911. Iron oxides were eventually shown to be the cause of the red tint, which the early explorers of Antarctica had thought was caused by red algae.
Geochemistry with Pat
The ferrous ions in the unfrozen seawater oxidize when they come into contact with ambient oxygen, resulting in the deposition of poorly soluble hydrous ferric oxides at the ice’s surface. Initially, the more soluble ferrous ions dissolve in ancient saltwater that has been trapped in an ancient pocket that was formerly part of the Antarctic Ocean. This occurred when a glacier isolated a fjord during the Miocene epoch, around 5 million years ago, when sea levels were higher than they are now.
The Taylor Glacier is not frozen to the bedrock like most other Antarctic glaciers, most likely due to the concentration of salts from the crystallization of the old saltwater trapped underneath it. The heat exchange of the trapped liquid saltwater with the massive glacier mass caused pure ice to form and release its dissolved salts as it cooled, resulting in salt cryo-concentration in the deep residual seawater.
Consequently, brines with salinities two to three times that of the average ocean water were formed from the concentrated saltwater that had been trapped. The water evaporation of surface lakes in the McMurdo Dry Valleys that are directly exposed to the extremely dry northern environment is a second mechanism that can also account for the production of hypersaline brines.
As long as there is no mixing between brines that have developed differently, it is theoretically possible to discriminate between the two processes using analysis of stable isotopes of water.
Fortunately, a fracture in the ice allowed for the sampling of hypersaline fluid, which was rich in ferrous ions and sulfate and devoid of oxygen. While soluble divalent iron was probably released under reducing circumstances from the subglacial bedrock minerals eroded by microbial action, sulfate is a residual geochemical hallmark of marine settings.
Microbial ecosystem
Autotrophic bacteria that metabolize sulfate and ferric ions have formed in a unique subglacial habitat, according to both chemical and microbiological investigations. Geomicrobiologist Jill Mikucki of the University of Tennessee reports that Blood Falls water samples have nearly no oxygen and at least 17 different kinds of microorganisms.
One possibility might be that the bacteria respire with ferric ions and digest the trace amounts of organic substances they are trapped with by using sulfate. No other metabolic activity of this kind had ever been seen in the natural world.
The coexistence of ferrous and sulfate ions in anoxic environments is a perplexing finding. There are no sulfide anions in the system. This points to a complex and poorly understood relationship between the metabolic cycles of iron and sulfur.
Under Mikucki’s direction, a team of scientists and engineers visited Taylor Glacier again in December 2014, this time using a German-designed probe named IceMole to melt into the glacier and take a direct sample of the brine, or salty water, that feeds Blood Falls.
After samples were examined, it was found that the subglacial brine (8% sodium chloride) was frigid (−7 °C/19 °F), and iron-rich (3.4 mM). From these samples, researchers identified and described a form of heterotrophic, psychrophilic, and halophilic bacterium that grows well in cold environments.
They classified this type of bacteria under the name Marinobacter. At least four gene clusters implicated in secondary metabolism were found, according to DNA bioinformatic analysis. The synthesis of aryl polyenes, which serve as antioxidants to shield bacteria from reactive oxygen species, is associated with two gene clusters.
It appears that another gene cluster contributes to terpene biosynthesis, most likely in the pigment production process. Desulfocapsa sp. and Thiomicrospira sp. were among the other bacteria found.
Implications for the Snowball Earth Hypothesis
The now-inaccessible subglacial pool was shut off 1.5–2 million years ago, according to Mikucki et al. (2009), and became a sort of “time capsule”, isolating the ancient microbial species long enough for it to evolve apart from other related marine creatures. The Snowball Earth theory, explains how other creatures may have lived when the Earth was completely frozen over.
During the Proterozoic eon, some 650–750 million years ago, when the Earth was presumably covered in glaciers at tropical latitudes, ice-covered seas may have been the sole haven for microbial life.
Implications for astrobiology
This unique location provides scientists with a rare chance to investigate deep subterranean microbial life in harsh environments without having to drill deep boreholes in the polar ice cover and risk contaminating the delicate and pristine ecosystem.
Understanding the variety of circumstances to which life may adapt and improving the evaluation of the likelihood of life elsewhere in the solar system, such as on Mars or Europa, Jupiter’s ice-covered moon, are two benefits of studying severe settings on Earth.
According to NASA Astrobiology Institute scientists, these worlds would have subglacial liquid water habitats that are suitable for supporting primitive life because they would be more shielded from cosmic and UV radiation below than they would be above the surface.
