The Lost City Hydrothermal Field

The steep, white pinnacles at Lost City, which stand like old growth Sequoia trees in the forests of California, are in stark contrast to the black smokers that typify hydrothermal systems hosted by basaltic volcanoes. The cream to white colored chimneys are composed mostly of carbonate (CaCO₃) minerals. Carbonate is the same mineral group that forms cave deposits made of limestone, and it is also what corals are composed of.

Gently flowing fluids that exit the pinnacles are relatively free of particulates, and therefore lack easily observable plumes such as the dark plumes of fine-grained metal sulfides that characterize black smokers. A second important difference is that, although microorganisms are abundant both on and within the Lost City chimneys, larger macrofaunal communities are sparse. We do not yet know why this is, but hypothesis that it may be do to the different chemistry of the vent fluids.

The ghost-like structures that appear out of the darkness upon being lit by the lights of Alvin, are reminiscent of some of the most spectacular cave deposits on land. Tall, composite pinnacles rise from concave ledges that protrude several meters from the sides of the chimneys. These ledges, or flanges, are similar to upside down bowls that trap pools of warm hydrothermal fluids, which leak out the sides of the chimneys. These pools eventually fill the bowls, and the buoyant fluids waft up over the edges of the flanges and percolate through their roofs, causing spectacular stalagmites to form that are as high as three story buildings. Many of the chimneys reach 20-30 m in height. The tallest chimney called 'Poseidon', the famous god of the sea, stands nearly 200 feet above the surrounding seafloor. Temperatures of the vent fluids at the top of this chimney were 75°C.

In addition to the chimneys, carbonate precipitates in a variety of forms. Immediately below the field, steep overhanging cliffs host active and inactive white hydrothermal veins and partially filled open fissures from which grow delicate, finger-like growths of carbonate crystals. In addition to the vein deposits, hundreds of large, overlapping, carbonate flanges extend from the cliff face. These protrude as much as 2 m and show evidence of multiple stages of growth. Their shingled appearance is reminiscent of travertine deposits in hydrothermal systems at Yellowstone National Park and other active hydrothermal sites.