Blood Falls, the Red Stain on Taylor Glacier

Blood Falls spills from the snout of Taylor Glacier in the McMurdo Dry Valleys of Victoria Land, Antarctica, staining the ice a deep red-orange where it meets the frozen surface of West Lake Bonney. The Australian geologist Thomas Griffith Taylor found the stain in 1911 while surveying the valley that now carries his name, and early expedition members first guessed the colour came from red algae. It took decades of further study to settle on the real cause: iron dissolved in the water turning to rust-coloured iron oxide the moment it reaches open air. In this photograph the falls run down a dark outcrop of exposed rock at the glacier's edge, the only patch of colour in a frame otherwise made up of white ice above and pale blue-grey lake ice below; a single small tent near the shoreline, part of a field research camp, gives a sense of just how large the ice wall really is.

The red water does not come from inside the glacier itself but from a reservoir of ancient seawater trapped beneath roughly 400 metres of ice, several kilometres back from the visible outlet. Geologists trace that water to the Miocene epoch, around 5 million years ago, when sea levels were higher and a fjord here was sealed off as Taylor Glacier advanced over it. Cut off from the ocean and slowly freezing from the outside in, the trapped water concentrated into a brine two to three times saltier than ordinary seawater — salty enough to stay liquid at temperatures that would freeze fresh water solid, and salty enough that Taylor Glacier is thought to be one of the few Antarctic glaciers not frozen directly to the bedrock beneath it. When this iron-rich brine finally reaches a crack in the ice and meets the atmosphere, the iron oxidizes almost instantly, the same chemistry that turns exposed metal to rust. Researchers who have sampled the brine have also found it hosts a community of microbes that respire by shuttling electrons between sulfate and iron rather than using oxygen, living in permanent cold and total darkness — a metabolism not previously known to occur in nature when scientists first described it.

As a jigsaw, the photograph divides into three very different textures, which makes it easier to sort pieces before trying to connect them. The red-orange stain is the smallest area in the frame but by far the easiest to place, since nothing else in the photo repeats that colour; start there and work outward. The glacier above it is a much larger field of white and pale blue-grey, textured by vertical streaks of dirt and shadow running down the ice — sort those pieces by the direction and spacing of the streaking rather than by colour alone, since most of them are close to the same shade. The dark rock and pale lake ice at the bottom of the frame are the simplest pieces to place last, acting as a stable anchor once the stain and the ice above it are already positioned.