Blood Falls is a rare geological feature in the McMurdo Dry Valleys of Antarctica. At the terminus of the Taylor Glacier, a flow of dark red liquid moves down the ice wall onto the surrounding white surface and dry stones. The red color originates from iron oxides but researchers struggled to identify how the water moved and where it started for many years. To solve this, scientists published findings in journals including Science, Applied & Environmental Microbiology, Frontiers in Microbiology and the Journal of Glaciology. The liquid is actually a highly concentrated salt solution that stayed under the ice for up to about 2 million years. And it contains a dense community of microbes that differs from most other microbial communities.
Not Algae, Ancient Sea
The Australian geologist Thomas Griffith Taylor first recorded the red marks in 1911. Both Taylor Glacier and Taylor Valley are named after him. Early explorers in Antarctica thought that red algae created the color because microbes often change the appearance of water in cold regions. It was a logical hypothesis but the evidence proved it was incorrect. As for its location, Blood Falls flows out at the end of the Taylor Glacier near Lake Bonney. Its salt water lies under 400 to 500 meters of ice, about 3 to 6 kilometers from the glacier terminus. Taylor Glacier is an outlet glacier of the East Antarctic Ice Sheet, but the waterfall lies outside the ice sheet. The site is important to researchers and lies inside the McMurdo Dry Valleys; this area is under the Antarctic Treaty system, where rules limit how much researchers disturb or pollute the environment. In 2009 Jill Mikucki led research showing that Blood Falls is fed by a very old, iron-rich reservoir of salty water beneath the glacier. The liquid is not water that comes from melting ice. Its chemistry shows it began as seawater that became stuck, grew thick with minerals, and lost contact with the air. It stayed behind when the region was an arm of the sea. Researchers link its isolation to the Taylor Valley floor moving upward or the Ross Sea Embayment moving back, though they are still working out when this happened. As the water grew cold, clear ice formed and pushed out salts in a process that researchers call cryoconcentration, which left a liquid much saltier than the ocean.
Liquid Until It Rusts
Liquid water can exist in this cold environment partly because of its high salt concentration. As salt lowers the temperature at which water freezes, the brine stays liquid at temperatures lower than 0 °C. The salt does not function alone to maintain the liquid state. When some brine freezes, it releases heat. This heat and the high salt levels ensure the remaining brine stays liquid inside the glacier. The brine stays under the glacier and moves through cracks until it arrives at the glacier snout. It escapes at Blood Falls. The liquid contains a high amount of dissolved iron that comes from contact between the old water and the rock beneath the glacier. Under the ice the water lacks oxygen and the iron stays in a dissolved state. When the pressurized brine reaches the surface, atmospheric oxygen reacts with the dissolved iron to create ferric iron compounds that do not dissolve easily. Those compounds are essentially rust. The hydrous ferric oxides fall onto the ice and turn the glacier a dark crimson color. The falls contain neither blood nor algae; the brine oxidizes and leaves a layer of rust on the ice.
Microbes in the Dark
Scientists identified microbes living in the highly concentrated salt water. A library of 16S rRNA gene clones from the site is dominated by a phylotype that shares 99 percent sequence identity with Thiomicrospira arctica. The organism is an autotrophic sulfur oxidizer that grows in cold marine environments, and it represents approximately 46 percent of the library. Those microbes cannot use light to create energy because ice and rock cut them off from the sun and the atmosphere for up to about 2 million years. To survive they use sulfur and iron compounds from their surroundings to generate energy. The study from 2009 shows that the microbes do not reduce sulfate completely to sulfide. The sulfur compounds move through intermediate stages and the ferric iron receives the final electrons in the process. Iron isotopes support this model: the signature of ferrous iron in the salt water matches patterns produced when microbes reduce iron. In December 2014, Mikucki and a group of researchers traveled to the Taylor Glacier during the Antarctic field season, where they used the IceMole probe to melt through the ice and collect samples of the salt water that flows to Blood Falls. The water is approximately −7 °C and contains roughly 8 percent sodium chloride and 3.4 millimolar iron. The researchers isolated a halophilic, psychrophilic and heterotrophic bacterium from the collected samples. They assigned the organism to the genus Marinobacter.
How the Brine Travels
And Blood Falls assists scientists who study how liquid brine flows through a polar glacier. In 2015 researchers used airborne transient electromagnetic sensors to detect sediments saturated with brine, extending from at least 6 kilometers upstream of the Taylor Glacier terminus into Lake Bonney. Later studies using radio echo sounding showed a zone rich in brine inside the glacier. To explain how the subglacial brine moves toward the glacier terminus, the researchers used hydraulic potential modeling, which indicates that pressurized saline water moves through cracks and channels before discharging at the surface.
Life at the Limits
Blood Falls matters beyond Antarctica. Subglacial environments like this one may be models for life on other planets or moons. If microbes survive for long periods in cold, dark, oxygen-free, iron-rich brine, similar chemical environments are possible elsewhere. Such conditions might exist beneath the icy surfaces of Europa or Enceladus. They might also exist in frozen areas of Mars. Because of this, Blood Falls is a site for the study of astrobiology. It shows that organisms can survive without warmth, sunlight or an atmosphere when the chemistry supports it. Blood Falls is not a bleeding glacier but the outflow of a subglacial reservoir: it is brine, trapped by ice long ago, whose iron turns into iron oxide on contact with air. Microbes live in this liquid, using minerals and water to survive in an environment once thought unable to support life. In this red water, researchers observe how life exists in extreme conditions. The site shows both geology and biology at once. The iron in the liquid turns red when it meets the air, which makes the site easy to see.