Blood Falls is a flow of saltwater mixed with iron(III) oxide that comes from the Taylor Glacier and moves onto the ice-covered surface of West Lake Bonney in the Taylor Valley of the McMurdo Dry Valleys in Victoria Land, East Antarctica.

Very salty water rich in iron sometimes flows out of small cracks in the ice. This saltwater comes from a pool of water hidden under about 400 meters (1,300 feet) of ice, located several kilometers away from Blood Falls.

The reddish color of the deposit was first noticed in 1911 by Australian geologist Thomas Griffith Taylor, who explored the valley named after him. Early explorers in Antarctica thought the red color came from red algae, but later studies showed it was caused by iron oxides.

Geochemistry

Poorly soluble iron-rich oxides form on the surface of ice when iron ions in unfrozen saltwater react with oxygen in the air. These iron ions were originally dissolved in ancient seawater trapped in a pocket beneath the glacier when a fjord became isolated by a glacier during the Miocene period, about 5 million years ago, when sea levels were higher than they are today.

Unlike most Antarctic glaciers, the Taylor Glacier is not attached to the bedrock, likely because salts from the trapped ancient seawater are concentrated. As pure ice formed beneath the glacier, it pushed dissolved salts into the remaining liquid seawater, increasing its saltiness. This process, called cryo-concentration, made the trapped seawater two to three times saltier than average ocean water. Another way hypersaline brines form is through the evaporation of water in surface lakes exposed to the extremely dry air in the McMurdo Dry Valleys. Scientists can use stable isotope analysis to tell these two processes apart, as long as the brines formed by different methods do not mix.

A sample of highly salty fluid, found through a crack in the ice, contained no oxygen and had high levels of sulfate and dissolved iron. Sulfate is a sign that the water was once part of the ocean. The dissolved iron likely came from minerals in the bedrock beneath the glacier, which were weathered by microbial activity under conditions with little oxygen.

Microbial ecosystem

Chemical and microbial tests show that a rare ecosystem of bacteria living under a glacier uses sulfate and ferric ions to survive. Scientist Jill Mikucki from the University of Tennessee found that water samples from Blood Falls contain at least 17 types of microbes and very little oxygen. These microbes may use sulfate and ferric ions to produce energy and break down small amounts of organic matter trapped with them. This process has never been seen in nature before.

A puzzling finding is the presence of both ferrous and sulfate ions in the water without oxygen and without sulfide anions. This suggests a complex and not fully understood connection between sulfur and iron in the environment.

In December 2014, scientists and engineers led by Mikucki returned to Taylor Glacier and used a tool called IceMole, created by a German team, to melt through the glacier and collect samples of the salty water (brine) that flows into Blood Falls.

The samples showed the water is very cold (−7°C or 19°F), rich in iron (3.4 mM), and contains 8% sodium chloride. Scientists found bacteria that can live in salty water (halophilic), survive in cold temperatures (psychrophile), and use organic matter for energy (heterotrophic). These bacteria were classified into the genus Marinobacter. DNA analysis showed at least four gene clusters involved in secondary metabolism. Two clusters are linked to making aryl polyenes, which help protect the bacteria from harmful oxygen compounds. Another cluster is likely involved in creating pigments through terpene biosynthesis. Other microbes found include Thiomicrospira sp. and Desulfocapsa sp.

Implications for the Snowball Earth hypothesis

According to Mikucki et al. (2009), a subglacial pool that is now impossible to reach was sealed off 1.5 to 2 million years ago. This event turned the pool into a kind of "time capsule," keeping an ancient group of microbes separate from other similar ocean life for a long enough time to develop on their own. This discovery helps explain how some microorganisms might have survived during the Snowball Earth period, when the Earth (according to the Snowball Earth hypothesis) was completely frozen.

During the Proterozoic eon, about 650 to 750 million years ago, oceans covered by ice may have been the only safe places where microbial life could continue to exist. This was a time when glaciers extended to tropical regions, making much of Earth's surface extremely cold.

Implications for astrobiology

This special location allows scientists to study tiny life forms deep underground in very harsh conditions without drilling deep holes into the polar ice cap, which could risk polluting a fragile and untouched environment.

Studying Earth's tough environments helps scientists understand how life can survive in different conditions and helps them find out if life exists elsewhere in the Solar System, such as on Mars or Europa, a moon covered in ice around Jupiter. Scientists from the NASA Astrobiology Institute think these places might have underground water environments that could support simple life. These environments would be more protected from harmful radiation than life on the surface.