A geological chimney
Off the coast of Western Australia, the seafloor is not as quiet as it seems. Here, a geological feature known as the Giralia Anticline is a natural conduit for methane gas. An anticline is an arch-like fold of stratified rock, and the Giralia structure is a massive one, extending about 80 miles. This formation has trapped ancient hydrocarbon deposits deep beneath the seabed. The gas originates from source rocks of the Devonian and Early Carboniferous periods, making the methane itself hundreds of millions of years old.
Over geologic time, faults and fractures within the anticline have allowed this fossil gas to migrate upwards, seeping directly into the cold waters of the deep sea. This feature is a cold seep, where hydrocarbons and sulfides emerge from the ocean floor at temperatures close to that of the surrounding seawater. These seeps provide a rare energy source in the deep ocean, an environment without sunlight for photosynthesis. The Giralia Anticline is part of the larger Carnarvon Basin, a vast geological province covering approximately 115,000 square kilometers onshore and another 535,000 square kilometers offshore.
Life without the sun
The constant flow of methane and hydrogen sulfide from the Giralia seep supports a unique ecosystem based on chemosynthesis. Specialized microbes harness the chemical energy stored in the seeping gas to produce organic matter. These bacteria and archaea form the foundation of a complex food web. They often grow in thick layers, known as microbial mats, on the seafloor.
This microbial life supports a variety of larger organisms that have adapted to this unusual environment. Mussels, such as those from the genus Bathymodiolus, harbor chemosynthetic bacteria within their tissues in a symbiotic relationship. Bacteria produce food for the mussels, and the mussels provide a safe habitat for the bacteria. Other common inhabitants of cold seeps include siboglinid tubeworms, clams, and crabs, all thriving on an energy source independent of the sunlit world above. The microbial activity also triggers chemical reactions that precipitate carbonate minerals, creating hardgrounds and rock structures directly on the seafloor over long periods.